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Total blood volume (TBV) and red cell volume (RCV) determinations have become an established diagnostic procedure in a variety of conditions; they are being used increasingly to estimate the requirement of blood transfusion. A common laboratory practice is to determine indirectly the RCV and/or the TBV by measuring the plasma volume (PV) and the venous hematocrit (VH). This procedure is technically simple and has been made popular by a number of semiautomatic electronic devices. In the healthy population we find results of the direct and the indirect measure of the RCV to be not statistically different, although they may differ by as much as ± 10% in individual cases. Larger discrepancies between the results of the two methods have been reported in patients by numerous investigators. In the present study the RCV based on the PV was found to be significantly larger in many patients than the RCV measured directly by tagging of red cells. Emphasis is here placed on the severity of this discrepancy in the general bed‐patient population. The differences in the RCV cannot be explained alone by the variability of the hematocrit in the large and small vessels, but must involve an early loss of the albumin and/or the tag from the vascular bed. These findings cast doubt on the validity of the measure of the PV, and consequently of the estimated RCV, in a considerable number of patients. It will be shown that the discrepancies noted can be reduced significantly by basing the measure of the plasma volume on the highest T‐1824 density or on the highest 131I (RISA) activity obtained three to ten minutes postinfusion of the tag.

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