Previous studies investigated the role of each RBC variable to determine the prognosis of heart failure patients separately [3, 4, 11,12,13,14], but a comparative study of RBC indices in terms of sensitivity, specificity, and accuracy to determine the best prognostic predictor has not been done yet. In the present study, whilst proving the predictive role of Hb,  HCT, MCHC, and RDW in heart failure patients, the most sensitive variable was MCHC and the most specific one was RDW.

Contrary to the other measurements, RBC count is the variable that is not influenced by the plasma alteration effect in the setting of heart failure [15]; thus it may not change significantly. Following previous studies, RBC count was not found as an independent predictor.

There are different physiologic compensatory mechanisms for low Hb levels. One of them is increasing the cardiac output to maintain proper oxygen delivery to different organs, so this cardiac overloading will lead to heart failure in the future. Thus, it is known that a low level of Hb in heart failure patients can play a part in decompensation and a worse prognosis [16]. Okuno et al. [3] found that the Hb level at the time of AHF diagnosis in patients with preserved EF was an independent factor in predicting mortality for both men and women. On the other hand, Abebe et al. [11] divided severe heart failure patients into two anemic and non-anemic groups based on their Hb level, and Kaplan-Meier diagrams did not show a significant discrepancy in survival rate between the mentioned groups. Therefore, the role of Hb as a prognosis predictor in heart failure patients is contradictory. Predicting the role of Hb in the mortality of patients with AHF was illustrated by multivariate analysis. The current investigation showed appropriate sensitivity and slightly acceptable specificity with a remarkable size effect for predicting one-year mortality. The Kaplan-Meier analysis showed that the groups of patients which were divided based on Hb normal adult values had significantly different survival rates.

Blood oxygen content decreases by declining  HCT.  HCT is a determinant of blood viscosity. Hemodilution reduces the oxygen content and the viscosity of the blood, thereby increasing regional blood flow and cardiac output [17]. Hemodilution affects patients with heart failure as it results in impaired peripheral oxygen delivery. Compensatory mechanisms to evade tissue hypoxia include an increase in cardiac output by sympathetic stimulation, redistribution of blood flow, an increase in whole-body oxygen extraction ratio, and activation of aortic chemoreceptors with an increase in venomotor tone [18]. Achievement of hemoconcentration in hospitalized AHF patients showed to have better survival, compared to hemodilution [19]. Ling et al. [20] showed that plasma volume, which depends on weight and  HCT, was a predictor for prognosis in heart failure patients so  HCT was also associated with prognosis in heart failure patients. Guglin et al. [4] stated that a low level of Hb reduced  HCT and decreased blood concentration and viscosity, so stroke volume would increase, but this does not affect the prognosis of heart failure. The paper presented by Oczan Cetin et al. [21] reported a direct relationship between blood viscosity and the prognosis of patients with heart failure. In this study,  HCT has been illustrated to own a prognostic role in determining the mortality of patients with heart failure. The sensitivity of this indicator was acceptable, although its specificity was barely noticeable.

Plasma volume may increase in patients with decompensated heart failure, which exacerbates the prognosis; besides,  HCT and Hb, contrary to RBC count, are indirectly affected by plasma volume. Thus, this fact provides a base to justify different predicted results of these variables. Opposed to the RBC count,  HCT and Hb are adjusted to the plasma volume, indicating the prognosis predicting utility [15, 22, 23].

Mean corpuscular volume is the measure of the average size of the circulatory erythrocyte, and it is principally used as an index for the differential diagnosis of anemia. Recently, MCV has been associated with mortality in many clinical settings [13]. Wolowiec et al. [5] found that there was no statistically significant relationship between MCV and the prognosis of patients with heart failure with 1-year follow-up. In our study, no mortality-predicting role can be assumed for MCV in heart failure patients.

Mean corpuscular hemoglobin represents the average amount of Hb in RBCs, and Hb is essential for the distribution and delivery of oxygen to the tissues [24]. Following a study by Wolowiec et al. [5] that showed MCH was not a prognostic factor in heart failure patients, the current project failed to determine the MCH as a 1-year mortality predictor. Although it possesses an acceptable significant sensitivity in ROC analysis, in the multivariate model, with the effect of other co-factors, the prognostic utility was alleviated, and it cannot be supposed as a predictor.

MCHC is a measure of the concentration of hemoglobin per volume of packed RBCs. If the reduced hemoglobin synthesis rate is faster than the reduced synthetic RBC volume, then the MCHC level is decreased. Low MCHC, therefore, represents a gross estimate of the presence of relative hypochromia. MCHC provides information on the hemoglobin concentration of each RBC. If it decreases for a long period, the organs' oxygenation will reduce [6, 25]. Different mechanisms play a part in hypochromia. First of all, there is the probability of the existence of an issue with availability or adhesion of iron into Hb. Other mechanisms might be related to renal insufficiency, where the underlying renal disease causes erythropoietin insufficiency or resistance. Also, there is a possibility of a dilution effect, because changes in osmotic pressures in the setting of congestion may theoretically affect the relative concentration of hemoglobin within the erythrocyte [6]. Simbaqueba et al. [6] reported that hypochromia, which reflects the low level of MCHC, was associated with a worse prognosis in heart failure patients. Hammadah et al. [26] mentioned MCHC as an independent predictor of poor prognosis in patients with heart failure. On the other hand, in the study by Wolowiec [5], this determining role was rejected. In the current project, MCHC was identified as an independent factor in predicting the prognosis of patients with AHF. Despite the low level of specificity, the highest sensitivity makes this index more profitable among all RBC indices.

Several physiological and pathological conditions may impair erythropoiesis and, hence, promote a higher degree of heterogeneity of RBC volumes. This process is characterized by the variability in the size of circulating erythrocytes, which is conventionally known as anisocytosis. In patients with heart failure, the presence of anisocytosis may be interpreted as a homeostatic response to the disease, thus reflecting the existence of a potential link between ineffective erythropoiesis and chronic inflammation [27, 28]. Nutritional deficiencies are the other reason for anisocytosis as they are involved in the onset and progression of heart failure [29]. Progressive renal dysfunction is another major cause of anemia and anisocytosis, but it is also an important indicator of poor outcomes in heart failure patients. Anisocytosis also increases with aging as the result of numerous metabolic dysfunction. On the other hand, advanced age is also an effective factor for cardiac dysfunction [29]. Therefore, these facts show that heart failure and anisocytosis are common in many pathogenic processes. Nonetheless, anisocytosis can directly result in the onset and progression of heart failure. Anisocytosis leads to reduced oxygen delivery to the peripheral tissues; also, abnormal RBCs may play a part in the pathogenesis of cardiac fibrosis by amplifying inflammation, stress of cardiomyocytes, and apoptosis [29]. Different studies have proven the RDW role in prediction of heart failure prognosis [5, 7, 14, 30,31,32]. In current perusal, the highest effect size (hazard ratio), specificity, and accuracy for determining the long-term mortality risk in heart failure patients indicate RDW advantageous rather than other RBC indices.

This study reached its goal of evaluating the prognostic role of RBC markers in heart failure patients; however, there were also a few limitations. Conducting the project in one center might have influenced the external validity. Determining the type of anemia based on (Hb or MCV) and its relationship with groups of patients was not carried out. The iron profile of AHF patients was not included as an influential factor during this study. Also, other prognostic factors such as electrocardiogram changes in abnormalities were not evaluated.