A hematocrit test, or HCT, measures the percentage of your blood volume that is made up of red blood cells. It is typically part of a complete blood count (CBC). 

Purpose: Because testosterone can cause the body to produce too many red blood cells, HCT is monitored during TRT to screen for a condition called erythrocytosis or polycythemia. An elevated HCT can increase the risk of heart attack and stroke.