After centrifugation, the height of the RBC column is measured and compared with the height of the column of original whole blood. The percentage of RBC mass to original blood volume is the Hct. Anticoagulated whole blood is centrifuged in a special tube. Since whole blood is made up essentially of RBC and plasma, the percentage of packed RBCs after centrifugation gives an indirect estimate of the number of RBCs/100 ml of whole blood (and thus, in turn, is an indirect estimate of the amount of Hb). Hct thus depends mostly on the number of RBCs, but there is some effect (to a much lesser extent) from the average size of the RBC. In most automated cell counting systems the Hct is not measured directly but is calculated from the RBC count value and the mean corpuscular volume (MCV) value obtained from electric pulse height sizing of the RBCs. Reference values are 40%-54% for men and 37%-47% for women. The average error in Hct procedures is about 1%-2%. Microhematocrits are generally as accurate as the older standard Wintrobe (macrohematocrit) technique. The Hct may be decreased when going from upright to recumbent position and increased (1.5%-5.8% units) in the same manner as the Hb by heavy smoking.

Useful relationships between Hb, Hct, and RBC count include:

Useful relationships between Hb, Hct, and RBC count

*at mean corpuscular hemoglobin concentration (MCHC) of 33; this factor varies from 2.7-3.2 depending on the MCHC value.