Circulating microRNA-21 (miR-21) has been utilized as a diagnostic tool in the assessment of heart failure (HF). Blood constitution may be altered when HF occurs and miR-21 may affect hematopoiesis. Sample hemolysis may influence the determination of circulating miRNAs, challenging the diagnostic use of miRNAs. We examined the relationship between blood measurements and miR-21 levels in ambulant chronic HF patients with reduced ejection fraction (HFrEF;  = 19). Healthy volunteers ( = 11) served as controls. Serum miR-21 levels were measured through quantitative reverse transcription polymerase chain reaction (RT-qPCR) and we calculated the hemolysis score (H-score). Study was approved by an Institutional Review Board (EK FaF UK 02/2018). MiR-21 serum levels were reduced in HFrEF patients compared with the controls ( < 0.05), without relationship to New York Heart Association class, left ventricular ejection fraction or N-terminal prohormone of brain natriuretic peptide levels. MiR-21 levels decreased markedly in anemic patients, compared with those with normal hematocrits ( < 0.05). We found a significant relationship between miR-21 to hematocrit ( < 0.05) and hemoglobin concentration ( < 0.05). Importantly, we found a correlation between hematocrit and sample H-score ( < 0.05) in the cohort of HFrEF patients; however, there was no correlation between hemolysis and miR-21. Circulating miR-21 levels were decreased in HFrEF patients and hematocrit was identified as a factor associated with this abnormality. This suggests that miR-21 mirrors other characteristics of HFrEF patients rather than the standard identifiers of HF severity and progression.