Venous Hematocrit: Caveats to Informing Blood Volume and Relative Risk in Chronic Heart Failure

Venous hematocrit (vHct) is commonly used to inform blood volume (BV) status and stratify cardiovascular risk. However, vHct defines a percentage of red blood cells (RBCs) to total BV; therefore, similar vHcts can reflect significantly different absolute BVs and RBC mass/plasma volume profiles. This analysis explores how quantitative measures of BV enhance vHct reliability as a biomarker in heart failure.

A retrospective analysis was undertaken in 395 patients stratified by vHcts reflecting clinically heart failure-relevant cut points (35% and 40%). BV was quantitated by the indicator-dilution methodology using iodinated-131 labeled albumin. Kaplan-Meier analyses compared outcomes.

Quantitative absolute BVs on average did not differ between vHct subgroups above or below cut points; however, the distribution of RBC mass and plasma volume profiles varied significantly. A normal BV profile was measured in approximately one third of cohort patients and BV hypervolemia in 58%, driven largely by plasma volume expansions (72% of cohort), all with similar distributions above and below cut points. In contrast, profiles of deficits in RBC mass (anemia) and excess (erythrocythemia) were more frequent below and above cut points, respectively, but not exclusively, with 14% of patients demonstrating contrasting RBC mass profiles. Kaplan-Meier analyses provided aggregate support for vHct to stratify heart failure-related outcomes but do not account for misclassified patients.

VHcts lack capacity to discriminate absolute BVs or the variability in RBC mass and plasma volume profiles above or below cut points. Therefore, quantitative BV measurements are key to adding specificity to individual vHct values, which permits reliable identification of BV status, assignment of appropriate risk, and guidance in management.