Heparin-induced thrombocytopenia (HIT) is a complication of heparin therapy. HIT is characterized by a 30–50% drop in platelet count 5 to 10 days after heparin therapy is started and places a patient at increased risk of serious complications from blood clots.


Orthopedic and cardiac bypass surgical patients develop HIT syndrome more frequently than medical or obstetric patients.

How Does HIT Occur?

When a person is given heparin, the drug can combine with a substance called platelet factor 4 (PF4) found in platelets and form a complex. In some patients, the heparin-PF4 complex triggers an immune response, which results in an antibody being produced against the heparin-PF4 complex. When the antibodies bind to the heparin-PF4 complex on the surface of platelets, the IgG platelets activate, leading to a drop in platelet count and increased risk of thrombosis. Activated platelets release additional PF4, causing the cycle to continue.

Possible complications of HIT include:

  • Deep-vein thrombosis, pulmonary emboli
  • Myocardial infarction, stroke
  • Compromised blood flow to limbs
  • Skin necrosis, end-organ damage
  • Death

For additional information, please call ARUP Client Services at 800-522-2787 and mention keyword: HIT.

Test Information

For additional information regarding testing strategies, refer to our Laboratory Test Directory.

Serotonin Release Assay (SRA) Testing Features and Benefits

  • Gold-standard confirmatory test
  • Complex, highly sensitive and specific functional assay that uses high-performance liquid chromatography (HPLC) to measure serotonin released when platelets are activated by HIT antibodies
  • Measures heparin-dependent platelet activation
  • Medical director review and consultation available
  • No radioactive waste

Method Description

ARUP performs the SRA test by using washed donor platelets incubated with patient sera and low (therapeutic) and high concentrations of heparin. In the presence of low (therapeutic) heparin concentrations, HIT-positive sera result in platelet activation and platelet granule release. The platelet granules contain serotonin, making it a surrogate marker for platelet activation. Any serotonin released from the donor platelets is identified and quantified by high-performance liquid chromatography (HPLC). The results are expressed as percent release (amount of serotonin released/total amount of platelet serotonin).


Functional Assay

Serotonin release assay (SRA) is a highly specific, functional assay that detects whether antibodies in complex with heparin/PF4 are capable of activating platelets in the presence of heparin.


ELISA assays are sensitive for HIT antibody detection but have poor specificity for clinical HIT, as they are incapable of determining if the antibodies have platelet-activating properties.

Additional Resources

  • Spotlight on Testing Video
    Heparin-induced Thrombocytopenia and the Serotonin Release Assay. Presented by Kristi J. Smock, MD
  • ARUP Consult
    Information for Heparin-Induced Thrombocytopenia - HIT

For additional information, please call ARUP Client Services at 800-522-2787 and mention keyword: HIT.