Heparin-dependent Platelet Antibody (Serotonin Release Assay)

CPT: 82542
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Test Details

Synonyms

  • Functional HIT Assay

Use

Confirmatory assay used in the evaluation of heparin-induced thrombocytopenia (HIT)

Limitations

The diagnosis of heparin-induced thrombocytopenia should not be based entirely on the results of the Heparin-dependent Platelet Antibody assay and should take into account the results of the immunologic HIT assay, presence and timing of thrombocytopenia in relation to heparin administration, the presence of thrombosis or other HIT-related sequelae, as well as determination of other causes of thrombosis.

This test was developed, and its performance characteristics determined, by LabCorp. It has not been cleared or approved by the US Food and Drug Administration (FDA).

Methodology

The SRA employs washed donor platelets and detects their activation by measuring the release of endogenous serotonin induced by addition of patient serum. The serotonin levels are measured by a highly sensitive liquid chromatography/tandem mass spectroscopy method. Therapeutic levels of heparin included in the assay mix allow for the formation of PF4/heparin complexes on the donor platelet surface to provide an antigen target for the HIT antibodies. The specificity of the assay is increased by assessing the inhibition of platelet activation by adding heparin in excess, thus proving the heparin dependency of the serotonin release.

Additional Information

Heparin-induced thrombocytopenia (HIT) is a potentially catastrophic, antibody-mediated complication of heparin therapy caused by immunization against platelet factor 4 complexed with heparin or other polyanions.1-6 HIT antibodies bind to PF4/heparin complexes on the platelet surface, resulting in platelet activation that leads to a platelet count decrease that can be accompanied by life-threatening thrombosis. This prothrombotic disorder can produce devastating thromboembolic complications, including ischemic limb necrosis, pulmonary embolism, myocardial infarction, and stroke.

Moderate thrombocytopenia is common in the clinical settings where heparin is administered and most cases are not caused by HIT.1-6 Differentiation of HIT from other potential causes of thrombocytopenia is the most difficult diagnostic component in the evaluation of heparinized, thrombocytopenic patients and relies on a combination of a clinical assessment and laboratory investigation.7 Prompt diagnosis and management is critical to minimizing the risk of life-threatening thrombosis. Patients diagnosed with or suspected of suffering HIT must be taken off heparin and transitioned to an alternative nonheparin anticoagulant as quickly as possible.

The laboratory investigation of HIT is challenging and requires correlation between clinical symptoms and laboratory assays. The most common assay performed is a serologic assay that detects the presence of HIT antibodies without regard to their functional ability. Several serologic assays that are relatively easy to perform are available commercially, and these are highly sensitive. The results of these assays have excellent negative predictive values and a negative result can be used to exclude HIT in all but the most compelling clinical circumstances.6 However, these assays suffer from low specificity and frequently yield positive results in the absence of clinical HIT. A positive result, especially of low titer, does not differentiate between pathogenic antibodies and clinically irrelevant antibodies.6 Functional assays that measure platelet activation by HIT antibodies in the presence of heparin are considered "gold standard" diagnostic laboratory tests due to their ability to detect the patient's underlying procoagulable state in those with true HIT.7 Due to complexity of performance, functional assays that use washed-platelets are not widely available. The serotonin release assay (SRA) using washed-platelets is a well-established functional HIT assay that is both highly sensitive and specific for HIT.1-6

Currently recommended HIT diagnostic algorithms published by the American Society for Hematology, British Committee for Standards in Haematology (BCSH) and American College of Chest Physicians Evidence-based Clinical Practice Guidelines incorporate an estimate of clinical probability and use of a sensitive immunoassay to guide initial management with subsequent confirmatory testing by a more specific functional assay.1-8

Rarely, a spontaneous prothrombotic thrombocytopenia syndrome can occur without proximate heparin exposure.9 In this syndrome, the results of serologic tests and the serotonin release assay are indistinguishable from HIT, despite the fact that patients had not been treated with heparin. Warkentin and coworkers have proposed that a rigorous definition of spontaneous HIT syndrome should include otherwise unexplained thrombocytopenia/thrombosis without proximate heparin exposure and with anti-PF4/heparin antibodies that cause strong in vitro platelet activation—even in the absence of heparin.

The British Committee for Standards in Haematology (BCSH) 2012 published guideline states that "ideally, the diagnosis of HIT should be confirmed by a washed-platelet assay" such as the SRA.8 The 2012 CHEST Supplement on the diagnosis and treatment of HIT states "The washed platelet SRA and HIPA are generally accepted as the reference standard assays for HIT."2

Specimen Requirements

Specimen

Serum, frozen

Volume

1 mL

Minimum Volume

0.5 mL

Container

Gel-barrier or red top tube

Collection

Transfer serum to a plastic transport tube. To avoid delays in turnaround time when requesting multiple tests on frozen samples, please submit separate frozen specimens for each test requested.

Storage Instructions

Freeze.

Causes for Rejection

Room temperature specimen received; grossly hemolyzed, icteric, and lipemic samples

Clinical Information

Footnotes

1. Greinacher A. Heparin-induced thrombocytopenia. J Throm Haemost. 2009 Jul; 7(Suppl 1):9-12. 19630757
2. Linkins LA, Dans AL, Moores LK, et al. Treatment and prevention of heparin-induced thrombocytopenia: Antithrombotic Therapy and Prevention of Thrombosis, 9th ed: American College of Chest Physicians Evidence-Based Clinical Practice Guidelines. Chest. 2012 Feb; 141(2 Suppl):e495S-530S. 22315270
3. Warkentin TE. Think of HIT. Hematology Am Soc Hematol Educ Program. 2006:408-14. 17124091
4. Warkentin TE., Sheppard JA. Testing for heparin-induced thrombocytopenia antibodies. Transfus Med Rev. 2006 Oct; 20(4):259-272. 17008164
5. Francis JL. A critical evaluation of assays for detecting antibodies to the heparin-PF4 complex. Semin Thromb Hemost. 2004 Jun; 30(3):359-368. 15282659
6. Cuker A. Heparin-induced thrombocytopenia: Present and future. J Thromb Thrombolysis. 2011 Apr; 31(3):353-366. 21327506
7. Sadik ZG, Jennings DL, Nemerovski CW, Kuriakose P, Kalus JS. Impact of platelet function assays on the cost of treating suspected heparin-induced thrombocytopenia. J Pharm Pract. 2014 Feb 14; DOI:10. 1177/0897190014522065. 24532821
8. Watson H, Davidson S, Keeling D; Haemostasis and Thrombosis Task Force of the British Committee for Standards in Haematology. Guidelines on the diagnosis and management of heparin-induced thrombocytopenia: Second edition. Br J Haematol. 2012 Dec; 159(5):528-540. 16643427
9. Warkentin TE, Basciano PA, Knopman J, Bernstein RA. Spontaneous heparin-induced thrombocytopenia syndrome: 2 new cases and a proposal for defining this disorder. Blood. 2014 Jun 5; 123(23):3651-3654. 24677540

LOINC® Map

Order Code Order Code Name Order Loinc Result Code Result Code Name UofM Result LOINC
150018 Serotonin Release Assay 150014 SRA, Low Dose Heparin % 77728-4
150018 Serotonin Release Assay 150016 SRA, High Dose Heparin % 50727-7
150018 Serotonin Release Assay 150017 SRA, Interpretation 66488-8

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