In a retrospective analysis of 395 Class II–III chronic heart failure patients, only 30% of those labeled as having 'normal blood volume' actually had both normal red blood cell (RBC) mass and normal plasma volume simultaneously. Among the 123 patients with ostensibly normal total blood volume, 60% harbored a true RBC mass deficit—genuine anemia—yet hemoglobin levels below 12 g/dL flagged only 29% of those cases, meaning standard hemoglobin screening missed roughly half the truly anemic patients in this subgroup.
This finding exposes a critical diagnostic blind spot in heart failure management. Hemoglobin concentration is the near-universal clinical proxy for anemia, but plasma volume expansion—common in heart failure—dilutes hemoglobin, creating a falsely reassuring reading even when total circulating red cells are depleted. The indicator-dilution methodology used here separately quantifies RBC mass and plasma volume, offering a more granular picture that conventional labs cannot replicate at the bedside. Unrecognized anemia in heart failure carries real consequences: reduced oxygen delivery worsens myocardial stress, accelerates functional decline, and is independently associated with hospitalization and mortality. The erythrocytosis cases (4%) represent an equally overlooked risk, as excess RBC mass raises viscosity and thrombotic burden. Limitations include the retrospective single-cohort design, lack of clinical outcome data, and absence of cause-specific analysis. As a medRxiv preprint not yet peer-reviewed, these results require independent validation before reshaping clinical protocols—but the signal is compelling enough to challenge the assumption that a 'normal' total blood volume is reassuringly normal.