What is the serotonin release assay (SRA)?

By William Aird

The SRA is a functional assay used to diagnose HIT. It identifies pathogenic IgG antibodies capable of binding and cross-linking platelet Fc gamma RIIA and triggering platelet activation. Test results are expressed as percentage of serotonin release compared to maximum release after detergent-induced platelet lysis (100%). Positive test is > 20% release at therapeutic heparin levels and < 20% release at supratherapeutic heparin levels. Specificity about 95%, but lower sensitivity reported compared with immunoassays.

Serotonin release assay begins with normal donor platelets that are incubated with radioactive 12C-serotonin. The platelets, which now contain 12C-serotonin, are then incubated with control buffer or patient serum or plasma and various concentrations of heparin. HIT serum (or plasma) causes platelet activation at therapeutic (0.1–0.3 IU/ml) heparin concentrations, but not in the presence of high (100 IU/ml) heparin concentrations (not shown).