Myth or reality: hematocrit and hemoglobin differ in trauma.

Background: Estimating blood loss in trauma patients usually involves the determination of hematocrit (Ht) or hemoglobin (Hb). However, in trauma patients, a poorly substantiated habit exists to determine both Ht and Hb in assessing acute blood loss. This suggests that Ht and Hb provide different information. Moreover, a survey of the literature showed a significant association of the subject trauma with the use of Ht. We investigated whether Ht and Hb differ in trauma patients.

Methods: Trauma patients with an Injury Severity Score>15 admitted from 1996 to 2004 to the University Medical Center Groningen were analyzed. All blood samples obtained during the first 7 days postinjury in which both Ht and Hb were determined were studied. Ht and Hb were measured with a Coulter Counter. The relation between Ht and Hb was analyzed with linear regression. The potential effect of hemolysis was studied by analyzing lactate dehydrogenase levels.

Results: In 671 patients 2,461 paired Ht levels and Hb levels were obtained. The mean Ht was 30.9%+/-6.9% (interquartile range 25.8%-35.8%). The mean concentration of Hb was 10.4+/-2.3 g/dL (interquartile range 8.7-12.1 g/dL). Ht and Hb had a Pearson's R of 0.99 and the following relations applied: Ht (%)=2.953xHb (g/dL) or Hb (g/dL)=0.334xHt (%). Lactate dehydrogenase was not related with Ht and Hb, indicating hemolysis was not relevant.

Conclusions: In a large series of trauma patients, Ht and Hb behaved as identical parameters. The idea that Ht is different from or even superior to Hb is a misconception. There is no reason for determining both Ht and Hb in trauma patients.