Review of Related Disease (RRD) 5 Practice Exam

Hematocrit tells us what fraction of blood is made up of red blood cells. Polycythemia means there’s an actual increase in red cell mass, so the hematocrit climbs above the upper limit of normal. In adults, the upper normal hematocrit is about 52% for men (and lower for many women), so using a value greater than 52% as the threshold best captures when red cell mass is abnormally high. This cutoff is practical because once hematocrit passes that point, blood becomes thicker and the risk of clotting rises. There are different causes of polycythemia (relative, absolute primary like polycythemia vera, or secondary from hypoxia), but the criterion most often used to define it clinically is hematocrit above the upper normal limit around 52%.