Dominating medical therapy for many centuries, bloodletting was primarily intended to correct humoral imbalance by ridding the body of noxious humors. When the humoral theory became obsolete, venesection found a more modest application in the reduction of blood volume or blood pressure in congestive heart failure and hypertensive crises. To these limited and now dwindling hemodynamic uses have later been added indications of a metabolic nature calling for removal of excessive metabolites and blood constituents. Some of these conditions, such as hepatic coma or erythroblastic crises which require exchange transfusions or cross-circulation procedures, are transient and acute; others, such as polycythemia vera and hemochromatosis, are chronic, requiring repeated bloodletting. To the latter category may also belong porphyria cutanea tarda.
An acquired metabolic disorder, which is characterized by excretion of large amounts of uroporphyrin in the urine and stool and by distinctive photocutaneous manifestations, porphyria cutaneatarda has consistently defied effective therapy.