Haptoglobin is a transport and acute phase protein which is synthesized in hepatocytes. It is a glycoprotein which consists of two light α‐chains and two heavy β‐chains. The genetic polymorphism of the α‐chains leads to three phenotypes Hp 1‐1, Hp 2‐1 and Hp 2‐2 differing in molecular weight. Haptoglobin binds hemoglobin in a strong haptoglobin‐hemoglobin complex (Hp‐Hb), the hemoglobin resulting from pathologically elevated hemolysis. These complexes are deposited in the hepatocytes, the deposition process having a half‐life of less than 10 minutes. Hemoglobin is enzymatically metabolized and haptoglobin is liberated after approximately 3 days. Complex formation and the extremely rapid elimination from circulating blood prevent the occurrence of hemoglobinuria with excess renal loss of iron. A reduction in the level of free haptoglobin is indicative of intravascular hemolysis.
As a strong positive acute phase reactant, a hemolysis‐mediated reduction or, to a certain extent, an elevation with accompanying acute inflammation can be compensated for. Indications for haptoglobin assays have been published and include the assessment of the severity and stage of intravascular hemolysis, evaluation of acute inflammatory processes and phenotype differentiation in paternity diagnostics.