The body/venous hematocrit ratio: its constancy over a wide hematocrit range.

Most observers have found that when circulating red cell volume is deduced from measurement of plasma volume and venous hematocrit a higher value is obtained than when the red cell volume is measured directly. It is widely believed that this is because the proportion of red cells in the blood in large vessels is higher than that in the body as a whole and various correction factors have been proposed. Some confusion arises because the venous hematocrit must be corrected on two accounts: firstly because, under standard conditions of centrifugation, some plasma is trapped in the red cell column of the hematocrit and thus the uncorrected reading over-estimates the proportion of red cells in the sample; when this correction has been applied one may speak of the "true" venous hematocrit. Secondly, this corrected venous hematocrit value still over-estimates the proportion of red cells in the circulating blood as a whole and must be multiplied by a factor of approximately 0.9 to give an estimate of "body hematocrit." Although these concepts are accepted by the majority of investigators, the precise