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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.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.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 R2 of 0.99 and the following relations applied: Ht (%) = 2.953 × Hb (g/dL) or Hb (g/dL) = 0.334 × Ht (%). Lactate dehydrogenase was not related with Ht and Hb, indicating hemolysis was not relevant.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.