Purpura rash on a patient's back. The rash is the result of bleeding into the skin from small blood capillaries. The tiny purple spots of the rash are called petechiae. Purpura may be due either to defects in the capillaries (nonthrombocytopenic purpura) or to a deficiency of blood platelets (thrombocytopenic purpura). Capillary wall damage may arise from various causes, including infection (such as scarlet fever), toxins (such as certain drugs), allergic reactions, scurvy, and malnutrition and wasting.