Correction for Plasma Trapped in the Red Cell Column of the Hematocrit

1. In the hematocrit, the percentage of the red cell column occupied by trapped plasma increases directly with the packed cell volume; this is due to a corresponding reduction of the effective radius of centrifugation.

2. When P.C.V. is determined in Wintrobe hematocrit tubes spun at 3000 r.p.m. in a centrifuge of radius 15 cm., the variation in plasma trapping between one sample and another is much smaller after 55 minutes centrifugation than after 30 minutes. If a 55 minute period is used, variation is so small, even when pathologic bloods are tested, that it is satisfactory to use a calibration curve. Such a curve is presented.