Alpha2-macroglobulin is a very large size (725 kDa) protease inhibitor, primarily synthesised by hepatic parenchyma cells. It is involved in the inhibition of enzymes acting in the kinin-kallikrein, complement, coagulation and fibrinolytic systems. In addition to these inhibitor roles, alpha2-macroglobulin is used as a transporter for many small peptides (such as cytokines or growth factors), and also for divalent cations (particularly zinc).
Alpha2-macroglobulin is normally absent in urine because of its very large size, even in advanced glomerular disease. The detection of alpha2-macroglobulin on urinalysis can reveal post-renal blood contamination of the sample, which could distort the other measurements made on the sample, for example caused by haemorrhage or inflammation of the urinary tract, or by female menstruation.