Hematocrit correlates well with circulating red blood cell volume in very low birth weight infants

Although circulating red blood cell (RBC) volume is a better measure of tot al body oxygen delivering capacity than hematocrit (HCT), circulating RBC v olume is more difficult to measure. Thus, the HCT is often used in RBC tran sfusion decisions. However, several previous studies of low birth weight in fants have reported that the correlation between HCT and circulating RBC vo lume is poor. Using a robust nonradioactive method based on in vivo dilutio n of biotinylated RBC enumerated by flow cytometry, the present study reexa mined the correlation between HCT and circulating RBC volume in very low bi rth weight infants. Venous and capillary HCT levels were compared with circ ulating RBC volume measured using the biotin method. Twenty-six stable very low birth weight infants with birth weights less than 1300 g were studied on 43 occasions between 7 and 79 d of life. Venous HCT values correlated hi ghly with circulating RBC volume (r=0.907; p<0.0001). However, the mean 95% confidence limits for prediction of circulating RBC volume from venous HCT (the average error of prediction) was <plus/minus>13.4 mL/kg. The correlat ion between HCT and circulating RBC volume is strong in older stable very l ow birth weight infants. However, clinically important uncertainty exists i n estimating circulating RBC volume and the associated RBC transfusion need s of an individual infant based on venous HCT. Because direct measurement o f circulating RBC volume is not yet practical, the HCT (or the blood Hb con centration) remains the best available indirect indicator.