To the Editor.
—The work of Valeri and Cooper (Arch Intern Med 132: 534-538, 1973) shows well the problem of measuring plasma volume accurately with sodium chromate Cr 51 serum albumin. But the authors come to the unwarranted conclusion that the best method of determining whole blood volume is to measure only red blood cell (RBC) mass. Their experimental design did not include an examination of the problems of measuring RBC volume with tagged RBC, and they apparently ignored the wide range of "f" values—the ration of total body hematocrit to "peripheral" venous hematocrit values. An average f factor may be used successfully for computing the average blood volume of a group of patients, but for a specific patient it may cause a major error. A more rational conclusion suggested by their data is that a reliable total whole blood volume demands independent measurement of both plasma volume and RBC