Absolute and Functional Iron Deficiency in Heart Failure – Novel Definitions And Therapeutic Approach – Review of Literature

Б  том ХXXI, 2025, № 2 ДРУЖЕСТВО НА КАРДИОЛОЗИТЕ В БЪЛГАРИЯ ОБЗОРИ REVIEWS ABSOLUTE AND FUNCTIONAL IRON DEFICIENCY IN HEART FAILURE – NOVEL ABSOLUTE AND FUNCTIONAL IRON DEFICIENCY IN HEART FAILURE – NOVEL DEFINITIONS AND THERAPEUTIC APPROACH – REVIEW OF LITERATURE DEFINITIONS AND THERAPEUTIC APPROACH – REVIEW OF LITERATURE G. Goranov1, V. Goranova2 1Section of Cardiology, First Department of Internal Medicine, Medical University – Plovdiv 2Section of Haematology, First Department of Internal Medicine, Medical University – Plovdiv АБСОЛЮТЕН И ФУНКЦИОНАЛЕН ЖЕЛЕЗЕН ДЕФИЦИТ ПРИ СЪРДЕЧНА АБСОЛЮТЕН И ФУНКЦИОНАЛЕН ЖЕЛЕЗЕН ДЕФИЦИТ ПРИ СЪРДЕЧНА НЕДОСТАТЪЧНОСТ – СЪВРЕМЕННИ ДЕФИНИЦИЯ И ТЕРАПЕВТИЧЕН ПОДХОД. НЕДОСТАТЪЧНОСТ – СЪВРЕМЕННИ ДЕФИНИЦИЯ И ТЕРАПЕВТИЧЕН ПОДХОД. ЛИТЕРАТУРЕН ОБЗОР ЛИТЕРАТУРЕН ОБЗОР Г. Горанов1, В. Горанова2 1Секция по кардиология, Първа катедра по вътрешни болести, Медицински университет – Пловдив 2Секция по хематология, Първа катедра по вътрешни болести, Медицински университет – Пловдив Abstract. Absolute and functional iron defi ciency (ID) is a critical factor contributing to various cardiac issues, including mitochondrial dysfunction, myocardial remodeling, and signifi cant systolic and diastolic impairments. These conditions increase the risk of left ventricular hypertrophy, pulmonary congestion, cardiac fi brosis, and decreased exercise tolerance in patients with heart failure (HF). Therefore, timely and accurate diagnosis of ID is essential, as it can be a primary or secondary treatment goal. Current diagnostic criteria often rely on serum ferritin levels, which can be misleading and are derived from previous clinical trial eligibility criteria. This review examines multiple randomized clinical trials, studies, and meta-analyses that advocate for a redefi nition of ID. It proposes new diagnostic criteria based primarily on transferrin saturation (TSAT) levels below 20%, which demonstrate high specifi city and sensitivity for identifying candidates for treatment. These criteria correlate signifi cantly with survival rates and cardiovascular outcomes. Additionally, emerging indicators of ID, such as serum soluble transferrin receptor (sTfR) and the sTfR-ferritin index, are discussed. These indicators provide insights into mitochondrial iron status and help differentiate between absolute iron defi ciency (AID) and functional iron defi ciency (FID). Key words: absolute/functional iron defi ciency, diagnostic criteria, heart failure, treatment Address for correspondence: Georgi Goranov, MD, e-mail: [email protected] Резюме.Абсолютният (АЖ) и функционалният железен дефицит (ЖД) са независими патогенетични фактори за нарушена митохондриална структура, миокардно ремоделиране, тежка систолна и диастолна дисфункция, повишен риск от хипертрофия и дилатация на лявата камера, белодробен застой, сърдечна фиброза и ограничен физически капацитет. ЖД е успоредна и самостоятелна терапевтична цел при пациенти със СН, която изисква ранно и точно диагностициране. Съществуващите традиционни показатели за доказване на ЖД се базират на спорни и подвеждащи серумни нива на феритина (най-често в референтния интервал), механично взети от критериите за допустимост в клинични проучвания (КП). В литературния обзор се анализират редица КП, научни разработки и метаанализи, които обосновават необходимостта от редефиниция на ЖД. Посочени са предложенията за нови критерии за ЖД, основани предимно на TSAT < 20%. Подобен диагностичен подход притежава висока специфичност и чувствителност, идентифицира по-точно подходящите за лечение пациенти и притежава значимо прогностично и предиктивно значение, т.е. корелира с преживяемостта и сърдечните събития, както и с честотата и качеството на терапевтичния отговор. Разгледани са и някои нови диагностични показатели за ЖД – разтворим феритинов рецептор в серума (sTfR), съотношенията sTfR mg/L и log на феритин в μg/L (феритинов индекс) и sTfR:хепцидин, отразяващи митохондриалния железен статус и разграничаващи АЖД от ФЖД. Ключови думи:абсолютен/функционален железен дефицит, диагностични критерии, сърдечна недостатъчност, лечение Адрес за кореспонденция:д-р Георги Горанов, e-mail: [email protected] This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. doi: 10.3897/bgcardio.31.e150462 G. Goranov, V. Goranova 36 I Heart failure (HF) is a widespread condition and presents a major healthcare challenge worldwide. It is the leading cause of hospitalization in patients above 65 years of age and has a 75% mortality rate at 5 years of diagnosis. Given the aging population in western countries, HF is not only becoming more common but also presents a progressively increasing fi nancial burden on healthcare systems. One of the main characteristics of HF patients is the high incidence of comorbidities. About half of the adult patients with HF have at least 5 or more comorbidities such as overweight, metabolic syndrome, diabetes mellitus, arterial hypertension, cerebrovascular disease, COPD, PAD, CKD, degenerative-infl ammatory joint diseases, anaemia, cancer, etc. The above have a negative impact on HF and, by modulating the therapeutic response, worsen the prognosis and survival of patients [1]. Among all comorbidities, iron defi ciency both absolute and functional (AID/FID) and iron defi ciency anaemia (IDA) have a disturbingly high relative incidence. The true incidence of ID in general population is unknown, which is evident from the very wide range reported in the literature: 4-18% in Western Europe and 9-50% in Eastern European countries. Manifest IDA is signifi - cantly less common – 2.6-5%. In general, ID is much more common than IDA. In patients with chronic and acute HF it occurs in 50-57% of men, and in about 80% of women, proportional to their NYHA class [2]. ID in HF has been widely discussed in specialized literature for decades. Numerous randomized clinical trials (RCTs) and publications have demonstrated that ID irrespective of haemoglobin (Hb) levels is an independent prognostic indicator for reduced exercise capacity, increased risk of death, readmission, impaired quality of life, and identifi es patients with the highest risk of death after an episode of acute heart failure [3]. Because of the above ID is to be viewed not only as a concomitant condition but also as a signifi cant pathogenetic factor in the evolution of HF, subject to mandatory therapeutic correction. B  Iron (Fe) is a microelement of great importance for the gas exchange. It is a structural element of Hb and a co-factor of over 30 key enzyme systems responsible for energy production in all human cells and for the synthesis of important organic compounds (Table 1) [1]. Enzymatic Fe is of key importance for normal mitochondrial function, especially for cells with high energy needs, high metabolic activity and mitogenic potential – cardiomyocytes, skeletal muscles, neurons, hepatocytes, kidney cells, hematopoietic and immune cells [4]. On the other hand, quantitative deviations of Fe in ID and iron overload, especially in cardiomyocytes could cause serious dysfunction posing a therapeutic challenge. Fe is an example of the well-known “physiological paradox” – on the one hand, it is vital for gas transport and energy production in the cell, on the other – It catalyses the formation of toxic radicals and oxidative cell damage. M       F In practice, Fe has a “closed” metabolism and utilises almost the same iron atoms, due to the numerous processes of reuse of “aged” cells and iron accumulation sites. Approximately 90% the Fe required daily is obtained through recycling and only about 10% through intestinal absorption [5]. The toxicity of Fe++/Fe+++ ions is physiologically minimized by their almost complete binding to the intracellular protein ferritin or to the extracellular transferrin. The mechanisms and main participants in the resorption, transport, storage and utilization of Fe have been well studied (Fig. 1 and Fig. 2). In the epithelial cells of the duodenum and the initial part of the small intestine, Fe binds to a specifi c protein – apoferritin and ferritin is formed. Fe is stored in macrophages and hepatocytes in this form. Table. 1. Biological signifi cance of iron Participates in the synthesis of: hem lipids and cholesterol catecholamines and neurotransmitters collagen, carnitine thyroid hormones prostaglandins, thromboxane purines RNA, DNA replication and regeneration Transport and storage of О2 Hypoxia regulation Infl ammatory reactions Immune reactions (proliferation of immune cells and immune response) Antioxidant defence Oxidative stress Drug inactivation Apoptosis О2 transport in CNS (neuroglobin) (adaptation of Alnuwaysir RIS and co-authors, BioRender.com) 37 Absolute and functional iron defi ciency in heart failure... Iron passes from enterocyte mucosa into the blood through a special transport protein – ferroportin. The transport of inactive Fe3+ into the circulation is carried out by the glycoprotein transferrin, which binds Fe absorbed and released from macrophages and hepatocytes with high affi nity. A cell membrane complex of transferrin and transferrin receptor protein (TfR1) ensures, the conversion of Fe3+ into biologically active cytosolic Fe2+ through endocytosis. The above is necessary for the synthesis of ATP and hem from erythroid precursors. Excess cytosolic Fe2+ catalyses the oxidation of membrane phospholipids with subsequent cell death by ferroptosis. Ferritin-mediated regulation of cytosolic Fe2+ is crucial in the balance between ferritinophagy and ferroptosis [6]. In healthy adults, serum ferritin levels are typically maintained at ≈ 20-300 μg/L in men and ≈ 20-200 μg/L in women. Fig. 1. (Adapted from Andrews NC. New Engl J Med, 1999) Fig. 2. Systemic iron homeostasis (Ganz T, Physiological Reviews. 2013) [7] G. Goranov, V. Goranova 38 H –       The export of Fe via ferriportin from the basal side of the enterocyte into the blood is regulated by the circulating polypeptide hormone hepcidin. It is synthesized mainly by hepatocytes and is the fi rst hormone proven to regulate Fe homeostasis. At high levels of Fe in transferrin or increased stores in ferritin, hepcidin blocks the “gate” in the membranes of enterocytes and macrophages, respectively the transport function of ferroportin, which limits the mobilization of Fe from enterocytes, hepatocytes and macrophages [7] (Fig. 2). The same mechanism suppresses the entry of alimentary and medicinal Fe. Conversely, when serum Fe decreases, hepcidin production is suppressed, which stimulates Fe resorption and release. Proinfl ammatory cytokines (IL-6, IL-1) have a signifi cant negative impact on Fe homeostasis: they increase hepcidin secretion, decrease the response to erythropoietin (EPO), induce apoptosis of erythroid precursors, decrease transferrin expression and suppress ferroportin mRNA [6, 7]. The importance of infl ammatory processes has been demonstrated in the studies of Jankowska E. A et al., Zaritsky J. et al., Mecklenburg I et al., who found increased levels of hepcidin: up to 98 ng/mL in patients with mild CKD 270 ng/ml; 218 ng/mL in CKD stages 2-4 and 577 ng/mL in active infl ammatory bowel disease [6, 8, 9]. This is 10-20 fold higher than the normal range of 11.4 ng/mL in premenopausal women and 21.8 ng/ mL in men, and shows some correlation with CRP [10]. I     Intracellular iron status and transport – uptake, utilization, storage and excretion by cardiomyocytes is regulated by specifi c mechanisms that ensure suffi cient cytosolic Fe, limit the release of active Fe2+ and lipid peroxidation [1, 11]. Iron homeostasis in the myocardium is governed by a transcriptional system for the iron regulatory protein (IRP-1/-2) and corresponding changes in iron transporters – transferrin receptor-1 (TFR-1), divalent metal transporter-1 (DMT-1) and ferroportin (FPN) [11]. The transferrin – transferrin receptor protein (TfR1) complex undergoes endocytosis and intracellular Fe3+ is converted to biologically active cytosolic Fe2+, which in excess is sequestered in the ferritin nanocage in the unreactive Fe3+ form. If cardiac ferritin is overexpressed, Fe remains trapped in ferritin and HF occurs. Excess cytosolic Fe2+ is sequestered in the ferritin nanostructure in the unreactive Fe3+ form, from where it is released as needed by ferritinophagy. When cytosolic Fe2+ is low, ferritinophagy is enhanced, and conversely, when Fe2+ is elevated, cellular ferroptosis occurs [6]. When ferritin is overexpressed, Fe remains sequestered in the ferritin nanoshell, causing cellular HF. Cardiac hepcidin is regulated intracrinely by damage or infl ammation of cardiomyocytes, and its expression is increased in cases of dietary Fe defi - ciency (the opposite of systemic hepcidin). It should be noted that in HF, myocardial ID does not correlate with standard biomarkers assessing systemic iron homeostasis. Another mechanism for ID-induced HF is neurohormonal activation, which reduces myocardial Fe. In an experimental study, it was found that decreased TFR1 expression was associated with increased activation of the neuroendocrine system, especially aldosterone and norepinephrine [12]. C       As already indicated, after entering the cell, Fe3+ is converted into a highly reactive Fe2+ ion, necessary for the synthesis of haemoglobin or as a major component in Fe-S clusters and the electron transport chain (ETC). Hao Zhang and co-workers identifi ed a number of mitochondrial metabolic dysfunctions and disturbances in the enzymatic activity of the respiratory chain in HF. The enzymatic activity of ETC correlates with the levels of Fe in the myocardium, and in ID remodelling is impaired and characterised by increased oxidative stress. Dilated cardiomyopathy for example is characterised by subcellular dysregulation in Fe metabolism: reduced membrane and cytosolic levels of transporters for Fe uptake and increased levels of the Fe exporter in the sarcolemma. In ID, regardless of erythropoiesis and systemic Fe status, cardiomyocytes have impaired systolic and diastolic function due to structural changes with improper organization of the sarcolemma, mitochondrial swelling, increased anaerobic processes and glycolysis with accumulation of lactic acid, impaired oxidative phosphorylation, decreased antioxidant activity, ultimately resulting in hypertrophy and dilatation of the left ventricle, pulmonary congestion and cardiac fi brosis [12]. The damaging eff ect of the excess Fe should not be underestimated as well - permanent or transient excesses of cytosolic Fe (after boluses of intravenous Fe) increase the levels of malondialdehyde (lipid peroxidation) and troponin [13]. C       The main mechanisms of iron defi ciency are in general: 1) reduced iron intake, 2) reduced absorption and 3) increased loss. In HF, the most common causes of iron defi ciency are more peculiar. According to literature, 35% to 78% of patients with HF suff er from malnutrition, associated with fatigue, dyspnoea, swal- 39 Absolute and functional iron defi ciency in heart failure... lowing disorders, nausea, anxiety, monotonous food, reduced appetite, dyspepsia after eating and early satiety, and frequent use of antacid medications worsening absorption [14]. Consumption of lighter or plant-based food is not a solution – non-hem iron is reabsorbed more diffi cultly, and studies have shown that a vegetarian diet leads to 2.5-fold increase in the incidence of iron defi ciency. Secondly, the infamous gastrointestinal congestion in CHF is associated with non-occlusive intestinal ischemia, mucosal oedema, increased permeability, altered microbiome, and impaired absorption [14]. Another important cause of ID is occult blood loss from the congestive and vulnerable mucosa of the gastrointestinal tract in the presence of antiplatelet and anticoagulant treatment (often in combination) in patients with CHF [12]. C     ID is a condition in which the Fe available in the body is insuffi cient, both for the enzyme systems and for normal erythropoiesis. ID can lead to a variety of clinical manifestations, aff ecting multiple systems. The severity of symptoms often correlates with the degree of defi ciency and the individual’s overall health. The primary clinical manifestations associated with iron defi ciency are: ID anemia, cognitive impairments, dermatological changes (hair loss, brittle nails, glossitis), cardiovascular symptoms (tachycardia, fatigue and weakness) and gastrointestinal symptoms (dysphagia, abdominal discomfort). According to the European Society of Cardiology ID is defi ned as serum ferritin < 100 ng/mL, or between 100-299 ng/mL, but with TSAT < 20%. Currently, ID is classifi ed into 2 forms based on its specifi c pathophysiological role and independently from anemia – absolute (AID) and functional (FID). AID is characterized by reduced iron stores in bone marrow macrophages, liver and spleen, respectively with low levels of serum ferritin, serum Fe and TSAT < 20%. The gold standard for identifying AID is serum ferritin level < 15-20 μg/L, TSAT < 20%, elevated soluble transferrin receptor (sTfR) levels, and decreased hepcidin secretion. Conversely, in FID, the levels of stored iron are normal or elevated (normal or elevated ferritin), but Fe cannot be released due to blockage of ferroportin by hepcidin, i.e. suboptimal transport. Such a constellation is most commonly observed in infl ammatory, infectious, and neoplastic conditions [15] Due to low TSAT, ferritinophagy and Fe release from the ferritin nanostructure decrease in parallel with cytosolic iron levels, while intracellular and serum ferritin levels increase. FID is usually characterised by normal serum Fe, TSAT < 20% and ferritin levels of at least > 15-20 μg/L, but in reality they are often higher and out the reference range, which excludes AID [13]. In an analysis by Grote Beverborg N et al of the EFINEHF and BIOSTAT-CHF studies including 2357 patients, the two forms of ID were defi ned as LIS (Low Iron Storage) – low storage of Fe (transferrin saturation below 20% and serum ferritin concentration ≤ 128 ng/mL) and DIU (Defective Iron Utilization) - defective utilization of Fe (transferrin saturation below 20% and serum ferritin concentration above 128 ng/mL). LIS correlates with a higher incidence of IDA and a worse quality of life, and DIU with higher levels of infl ammatory markers. Both forms of ID showed an abnormal 6-minute walk test, but only LIS was an independent predictor of all-cause mortality or (re)hospitalizations for worsening HF (hazard ratio, 1.47; 95% CI, 1.26-1.71; P < 0.001) [16]. Evidently, both defi nitions leave room for possible misinterpretations. They are both based on ferritin levels within a wide reference range. Furthermore, ferritin is not only an indicator of iron stores, but also an acute phase protein. Unlike ferritin, TSAT increases to a lesser degree in response to infl ammation and correlates well with available Fe [17]. Anemia in iron defi ciency is also diff erentiated depending on the preceding etiopathogenetic mechanisms resulting in ID. It is classifi ed into 2 forms: classic ID and the so-called anaemia secondary to chronic disease. The well-known classic ID anaemia develops as a result of persistent iron defi ciency. It is characterised by low serum Fe, ferritin, erythrocyte indices (microcytosis, hypochromia) and high IBC. Anemia in chronic diseases is most often the result of FID, in which Fe is unavailable for use, and a low level of erythropoietin are often observed; The erythrocyte indices are usually unchanged (normocytosis with normochromemia), and the serum Fe and TSH are low, while ferritin level is most often normal or elevated. It is evident that both forms of ID, as well as both types of anaemia pathogenetically associated with them, have a diff erent clinical profi le and hence require a diff erent therapeutic approach [15]. The standard blood count tests and erythrocyte indices are insuffi cient for the diagnosis of ID. P    ID    Most defi nitions of ID in HF utilise ferritin and transferrin saturation (TSAT). According to the European Society of Cardiology ID is defi ned as serum ferritin < 100 ng/mL, or between 100-299 ng/mL, but with TSAT < 20%. It is evident that such a defi nition off ers inaccurate and misleading information, due to the use of ferritin, which is most often within the reference range or could have deviations even in healthy individuals. Secondly, an important fact is not taken into consideration, namely, ferritin is an indicator of iron availability, but G. Goranov, V. Goranova 40 also an acute phase protein, i.e. it can undergo signifi - cant quantitative fl uctuations, unrelated to iron metabolism. Unlike ferritin, TSAT changes to a lesser degree in response to infl ammation and correlates well with Fe availability [15]. This fact has long been discussed in specialized literature and has been the subject of reassessment in recent years [18]. Novel indicators of iron metabolism and defi - ciency. Serum soluble ferritin receptor (sTfR) and sTfR–ferritin index [19-21]. sTfR is part of the erythroid membrane protease, a marker of erythropoietic activity and of the size of erythroid precursors. Its concentration depends on age, altitude, ethnicity, erythropoietin administration, but is not directly aff ected by infl ammatory processes. Sierpinski R et al. found that sTfR ≥ 1.25 mg/L occurs in 47% of patients with HF, in 56% and 46% of anemic and non-anemic patients, respectively (p < 0.05). sTfR is an indicator of ID with a sensitivity of 84% and specifi city of 100%. The authors found that serum sTfR ≥ 1.41 mg/L was a signifi cantly more accurate prognostic indicator of 3-year mortality compared to plasma N-terminal pro-B-type natriuretic peptide and other variables. In a multivariate prognostic model of 3-year all-cause mortality, the addition of sTfR abolished the prognostic value of serum ferritin and TSAT [20]. sTfR levels are increased in the presence of iron defi - ciency, decreased in patients with iron overload, and are normal or low in infl ammation [39]. The ratio of sTfR (in mg/L) and log ferritin (in μg/L) or the so-called sTfR–ferritin index distinguishes FID in infl ammatory conditions and anemia in chronic disease [21]. In AID, the sTfR– ferritin index is > 1.5 (Dade-Behring) or > 3.2 (Roche), in FID (CRP > 5 mg/L) and ACD (reduced Hb and CRP > 5 mg/L) > 0.8 (Dade-Behring) or > 2.0 (Roche) [15]. Leszek et al. reported that serum sTfR signifi cantly correlated with myocardial and mitochondrial Fe status, unlike ferritin, serum Fe and TSAT [22]. Hepcidin is not only a major regulator of systemic iron metabolism, but also correlates with ID, although not strictly related. Elevated levels of IL-6, IL-1, and TNF-α (characteristic of comorbidities in HF) are associated with low serum Fe and high hepcidin [18]. The above relationship is more characteristic of FID than of AID [6]. Elevated circulating hepcidin levels are characteristic of early stages of HF and are not accompanied by anaemia or infl ammation. In patients with stable HF, an inverse relationship between hepcidin levels and HF severity has been established, with hepcidin levels in patients with NYHA class IV being signifi cantly lower even compared to healthy controls. HF progression is associated with a decrease in circulating hepcidin and the development of ID [6]. The sTfR:hepcidin ratio has been proposed as an informative indicator of ID. According to Jankowska E.A et al., patients with low hepcidin and high sTfR had peripheral oedema, high NT-proBNP, uric acid, low haemoglobin (p < 0.05), and 5% higher in-hospital and 12-month mortality [6]. CRP is not used for the diagnosis of ID but indicates the severity of infl ammation and the cause of ferFig. 3 41 Absolute and functional iron defi ciency in heart failure... ritin increase the latter being an acute phase protein. A signifi cant negative correlation was found between CRP and ferritin and CRP and the percentage of Fe defi ciency [23]. T2* MRI is of great benefi t for non-invasive assessment of myocardial Fe in the management of iron overload in transfusion-dependent homozygous β-thalassemia. In patients with HF, this modality yields a relatively accurate assessment of myocardial Fe levels [11]. In a systematic review, Dhaliwal S and Kalogeropoulos AP identifi ed several biomarkers for iron metabolism that showed signifi cant correlation with HF: serum ferritin < 15-20 μg/L indicated AID, TSAT was a good predictor of overall mortality, as well as the long-term risk of hospitalizations for HF; increased sTfR levels were associated with worse exercise tolerance and QoL; low serum Fe was associated with an increased risk of cardiovascular events; hepcidin was of prognostic value for overall mortality in HF.39 On the other hand, ID in HF correlated with a signifi cant number of biochemical parameters and clinical symptoms. For example, a study by Van der Wal H.H et al., conducted in 2357 HF patients analyses 92 cardiovascular parameters and their correlation with HF and their signifi cance for survival and fi rst rehospitalization. ID was found in 61.6% of patients and correlated with lower protein intake, tachycardia, peripheral oedema, orthopnoea, hypoalbuminemia, CKD, low haemoglobin, high CRP levels, and use of P2Y12 inhibitors (p < 0.05). None of these indicators were gender-specifi c, although ID was more common in women [24]. According to the German prospective observational PReP registry (Prävalenz des Eisenmangels bei Patienten mit Herzinsuffi zienz) in 1198 patients, ID was an independent indicator of reduced exercise tolerance in HF, adjusted for age, sex, anaemia, serum creatinine, C-reactive protein, LVEF, and NP level [3]. R  ? The literature on the importance of ferritin and TSAT in the assessment of ID in patients with HF, according to cardiology guidelines, is too contradictory to interpret. In fact, the indicators of ID used are inclusion criteria, empirically derived from the numerous studies of ID and its impact on HF. In practice, the adopted defi nition includes conditions with both AID and FID. It is unacceptable and misleading for patients with ferritin < 100 ng/mL (normal level, also found in healthy people) to be automatically diagnosed with ID. Conversely, the common clinical scenario of FID with high serum ferritin levels (≥ 300 μg/L), low serum Fe concentrations (≤ 13 μmol/L) and low TSAT (< 20%) is not addressed by the HF guidelines [18]. The reference ranges provided in the guidelines are clearly not accurate. At ferritin < 100 ng/mL, neither ID nor suffi cient iron availability at values > 400 ng/mL (hyperferritinemic syndromes) can be excluded, nor can AID be distinguished from FID. The generally accepted defi nition has a sensitivity of 82.4% and a specifi city of 72% for detecting ID in patients with HF compared to the gold standard [25] Defi nitions based solely on TSAT ≤ 19.8% or serum iron ≤ 13 μmol/L, however, have a sensitivity of 94% and a specifi city of 88%, (p < 0.05) [21, 25]. On the other hand, if data from clinical trials are analysed, it is evident that ferritin has no prognostic signifi cance and does not correlate with a number of indicators of HF and cardiac mortality. In a study conducted among 4422 outpatients with HF, Masini G et al found that only TSAT < 20% and serum iron ≤ 13 μmol/L were associated with higher 5-year mortality (HR: 1.27; 95% CI: 1.14-1.43; P < 0.001; and HR: 1.37; 95% CI: 1.22-1.54; P < 0.001). Serum ferritin < 100 ng/mL showed a trend for lower mortality without statistical signifi cance (HR: 0.91; 95% CI: 0.81-1.01; P = 0.09) [18]. According to Rita Del Pinto et al, low TSAT (< 20%) and iron levels (≤ 72.6 μg/dL), but not ferritin (< 100 ng/mL), refl ect bone marrow Fe depletion and are associated with increased overall mortality in HF [21]. Similar results were obtained by Palau P et al in a retrospective study of 1701 patients with decompensated HF: lower TSAT (< 20%) but not ferritin was associated with the risk of 30-day readmission or death [26]. R. I. S. Alnuwaysir et al. found that serum Fe (≤ 13 μmol/L) and TSAT (≤ 19.8%) were signifi cantly better at predicting ID than the FAIR-HF defi nition, with areas under the curves (AUC) of 0.922 and 0.932, respectively [12]. The addition of ferritin to either defi nition did not lead to a signifi cant increase in AUC, i.e. ferritin does not contribute to a more accurate identifi cation of patients with ID and HF and has no prognostic signifi cance. Cleland et al. demonstrated that high ferritin levels are associated with a higher risk of all-cause and cardiovascular mortality [27]. Such results could be explained by the multifactorial pathogenesis of ID in the context of various comorbidities, most of which are characterised by infl ammation, resulting in higher ferritin, increased hepcidin levels and decreased Fe absorption. Even relatively common conditions such as obesity, hyperand dyslipidemia are pro-infl ammatory. For example, in FID dynamic changes in ferritin levels could be observed without reciprocal alterations in iron metabolism. On the other hand, ferritin fl uctuations in FID are proportional to pro-infl ammatory cytokine levels and to the severity of HF. Along with other acute phase proteins such as C-reactive protein (CRP) and alpha-1-acid glycoprotein (AGP), ferritin increases rapidly with increased expression of pro-infl ammatory cytokines (IL6) [28]. The predictive value of ferritin in terms of therapeutic response in clinical trials is also debatable. With an inclusion criterion of ferritin < 100 (20-99) μg/L in G. Goranov, V. Goranova 42 the HEART-FID and IRONMAN studies, not all patients showed a reliable reduction in adverse HF outcomes with intravenous Fe administration [29, 30]. Literature suggests that TSAT is a more informative indicator of total Fe depletion and demonstrates smaller variations in response to infl ammatory processes and hepcidin. TSAT correlates with increased overall mortality and rehospitalization rate in patients with HF, regardless of ejection fraction [14]. According to Beverborg NG et al. TSAT is a predictor of the therapeutic response to IV iron carboxymaltase as well [25]. Analyses show that in patients included in clinical trials based on traditional recommendations, the risk of cardiovascular mortality and hospitalization is reduced by an average of 13%. (0.75-1.01) [31]. A meta-analysis of 4 double-blind, randomized trials in 839 patients, as well as the IRONCRT trial, found that intravenous ferric carboxymaltase signifi cantly reduced rehospitalizations, cardiovascular mortality, and improved left ventricular ejection fraction in patients with TSAT < 20.1% compared with patients with TSAT > 20.1% [32]. Another meta-analysis of 10 trials in patients with inclusion criteria of TSAT < 20% and ferritin < 400 μg/L reported a 33% reduction in the risk of cardiovascular death [33]. The IRONMAN trial, demonstrated that patients with TSAT < 20% had a 20% risk reduction with intravenous Fe – hazard ratio 0.80 (0.63-1.02), compared with a 4% risk reduction in subjects with ferritin < 100 μg/L-TSAT > 20% – risk ratio 0.96 (0.69-1.34). Analytical variability (technical?) and diff erent reference ranges (54% diff erence in a comparative study by B. A. Ford et al.) for ferritin should also be considered [34]. Unlike systemic Fe metabolism, the assessment of myocardial iron metabolism is neither easy nor theoretically clear. Literature indicates that about 50% of patients with HF and reduced ejection fraction have ID by generally accepted criteria, but low myocardial Fe is detected by MRI in only 20-25% [35]. On the other hand, serum Fe and serum ferritin levels do not clearly distinguish patients with and without myocardial iron defi ciency. The dynamics of myocardial and bone marrow Fe do not show a strict correlation – Fe stores assessed by endomyocardial biopsy do not correlate with blood biomarkers, and total myocardial Fe does not accurately refl ect cytosolic Fe2+ [36]. This justifi es the recommendation for a redefi nition of ID in HF using more precise criteria based on TSAT and serum Fe [37-40]. According to Packer M et al. ID can be defi ned by TSAT < 20% and serum ferritin < 400 μg/L; serum ferritin < 100 μg/L should be discarded as a stand-alone diagnostic criterion [40]. Such a redefinition identifi es patients at high risk of ID who would benefi t from intravenous treatment. ID   HF The treatment of HF with reduced ejection fraction aims to prevent rehospitalization and improve exercise tolerance, clinical status and QoL and to reduce mortality. None of these goals are achievable without correction of ID, regardless of erythropoiesis [41]. The 2021 ESC HF Guideline recommendations for iron replacement therapy include [42]: • Symptomatic patients who have a left ventricular ejection fraction (LVEF) < 45% to relieve symptoms, improve exercise tolerance and QoL (Class IIa recommendation, Level of Evidence A). • Preand post-discharge follow-up of patients hospitalized for acute HF to improve symptoms and reduce rehospitalization (Class IIa recommendation, Level of Evidence B). • Symptomatic patients recently admitted for HF with an LVEF < 50% to reduce the risk of hospitalization for HF (Class IIa recommendation, Level of Evidence B). The main therapeutic problem is overcoming/ bypassing hepcidin blockage, redistribution, sequestration and impaired iron transport in HF and/or secondary anaemia in patients with HF and multiple comorbidities. Ferric carboxymaltose is the most widely studied IV Fe and the only Fe compound specifi cally recommended for the treatment of HF in the 2021 ESC guidelines [42]. Reticulocytes increase after only 24-48 hours, and Hb levels no earlier than 1–2 weeks. A rapid increase in ferritin > 400 μg/L may signal a short-term overload of cytosolic Fe, which provokes lipid peroxidation (malonaldehyde ↑) and cardiomyocyte injury (troponin ↑) [13]. In the IRONMAN [29] study, ferritin levels increased 10-fold within 4 weeks and remained > 500 μg/L for the next 4 months. In the AFFIRM-AHF [43] and HEART-FID [30] studies, serum ferritin levels increased to ≈ 350 μg/L within 6 weeks, with similar values maintained for up to 6 months. The therapeutic protocol recommends no more than 1000-2000 mg of iron carboxymaltose during the fi rst 6 weeks, regular assessment of iron levels, and 500 mg every 3-4 months as needed. The results of treatment, according to meta-analyses of 10 RCTs, are unidirectional: reduction of hospitalizations for decompensated HF, improvement of NYHA class and QoL, 6-minute walk test, left ventricular ejection fraction, NT-proBNP and CRP, reduction of the risk of death by any cause. However, no signifi cant difference in survival between treated and untreated patients has been demonstrated [21]. The benefi t of long-term treatment with IV Fe remains unclear, given the diffi cult-to-correct etiology of iron defi ciency in HF. It seems logical to maintain iron stores, respectively the ferritin level, within the range of about 400 μg/L. 43 Absolute and functional iron defi ciency in heart failure... Indefi nite IV Fe therapy every 4-6 months was tested in the IRONMAN, HEART-FID and AFFIRM-AHF trials with cut-off values of ferritin > 400 μg/L or TSAT ≥ 25% and > 300 μg/L or TSAT ≥ 20%, respectively [29, 30, 43]. It is reasonable to recommend that such therapy be considered only if ferritin and TSAT levels remain low. With regard to oral therapy, the IRONOUT HF trial [44] did not report any clinical benefi t due to slow pharmacokinetics and the potential for Fe sequestration in macrophages and hepatocytes. Oral preparations are currently not recommended for the correction of iron defi ciency in HF, although iron defi ciency can be corrected orally with bivalent iron drugs. No eff ect has been demonstrated after the administration of erythropoietin (the risk of thromboembolic events is increased). N   The main goal of increasing iron absorption and mobilization of sequestered Fe can be achieved through the reduction of hepcidin synthesis and/or function and manipulation of the hepcidin-ferriportin link. Direct agents against hepcidin (LY2787106, Lexaptepid pegol or anticalins), blocking IL-6 with the anti-IL-6 ligand antibody ziltivekimab, suppression of bone morphogenic protein with LY3113593, as well as the use of direct ferroportin antibodies have been tested in phase 1 and 2 clinical trials [45]. Inhibition of the prolyl hydroxylase/hypoxia-inducible factor (PHD/HIF) domain is another therapeutic strategy being investigated. Phase 3 clinical trials are testing the HIF stabilizers Vadadustat, Daprodustat, and Roxadustat, and initial analyses indicate that PHD inhibitors increase haemoglobin levels, serum transferrin, increase intestinal Fe absorption, and reduce hepcidin levels in anaemic patients with CKD, regardless of infl ammation [46, 47]. C Absolute and functional iron defi ciency (ID), along with the classical iron defi ciency anemia and anemia of chronic disease, exhibit distinct clinical features and biochemical markers for diagnosis. Traditional cardiology guidelines have relied on serum ferritin levels for diagnosing ID; however, these levels can be controversial and often misleading, as they typically fall within the reference range and are based on criteria developed from clinical trials focused on treating ID in heart failure (HF). Recent literature suggests new diagnostic criteria for ID, primarily focusing on transferrin saturation (TSAT) levels below 20%. This approach demonstrates high specifi city and sensitivity, allowing for more accurate identifi cation of patients who could benefi t from treatment. Moreover, it holds signifi cant prognostic and predictive value, correlating with survival rates, cardiovascular events, and the likelihood and eff ectiveness of therapeutic responses. Nevertheless, ferritin testing remains important; it not only helps defi ne absolute iron defi ciency when levels are below 15-20 μg/L but also serves as an indicator of iron overload. Additionally, as an acute phase protein, ferritin correlates with infl ammatory markers, providing valuable insights for diff erential diagnosis. Table 2 Clinical Trial Objective Findings Key outcomes Notes IRONMAN [29] Evaluate the effi cacy of intravenous ferric carboxymaltose in heart failure patients with iron defi ciency. Signifi cant reduction in hospitalizations for decompensated heart failure. Improved NYHA class, QoL, reduced NT-proBNP and CRP levels. Ferritin levels increased 10-fold within 4 weeks. AFFIRM-AHF [43] Assess the eff ects of intravenous iron on patients with acute heart failure and iron defi ciency. Ferric carboxymaltose improved symptoms and reduced hospital readmissions. Serum ferritin levels increased to ~350 μg/L within 6 weeks. Similar results with HEART-FID. HEART-FID [30] Investigate the impact of iron replacement therapy on clinical outcomes in heart failure patients. Patients experienced improved exercise capacity and reduced symptoms of heart failure. Reduction in hospitalizations and improved quality of life. Ferritin levels maintained around 350 μg/L for up to 6 months. IRONOUT HF [44] Evaluate the eff ectiveness of oral iron repletion in heart failure patients with reduced ejection fraction. No signifi cant clinical benefi t observed from oral iron supplementation. Slow pharmacokinetics; potential for iron sequestration. Oral iron is not recommended for heart failure management. Phase 3 Trials (Vadadustat, Daprodustat, Roxadustat) [45-47] Test prolyl hydroxylase inhibitors for anemia in CKD patients. Increased hemoglobin levels, enhanced intestinal iron absorption, reduced hepcidin levels. Potential to improve iron metabolism and anemia management. Investigational drugs showing promise for future therapies. No confl ict of interest was declared