Hematocrit Correlates Well with Red Cell Volume in Preterm Infants 784

Objective: Hematocrit (HCT, the packed cell volume) is often used in clinical practice to draw inferences about circulating red cell volume (RCV, routinely expressed per kg body weight) because the RCV is considerably more difficult to measure. In some, but not all, previous studies of low birth weight (LBW) infants, HCT correlated poorly with RCV. The present study reexamined correlation between HCT and RCV in preterm infants using a nonradioactive method for RCV that is less susceptible to HCT artifact. Study design: Using flow cytometric detection of the in vivo dilution of biotinylated red blood cells, RCV (mL/kg) was determined on 43 occasions in 28 stable infants with birth weights < 2.2 kg at unselected ages between 7 and 79 days; mean age at time of study = 36 days (SD = 16 days). HCT was determined by automated cell counting. Results: Hematocrit correlated highly with RCV (P < 0.0001). Excluding the single outlier shown, correlation = 0.94, intercept = -4.8, and slope = 0.88 with 95% confidence limits for the population depicted by (---) lines. However, the prediction of RCV from HCT remained imprecise; for example, an infant with HCT = 30 could have an RCV from 15 to 28 mL/kg (95% confidence level). Conclusion: In the population of stable LBW infants older than 7 d, the degree of correlation between HCT and RCV is high enough to provide some guidance in transfusion decisions, but HCT may still substantially overestimate or underestimate RCV in individual infants. Figure