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Heparin-Induced Thrombocytopenia

Heparin-Induced Thrombocytopenia

Test Highlight

Heparin-Induced Thrombocytopenia

  

Clinical Use

  • Diagnose heparin-induced thrombocytopenia (HIT) type II

Clinical Background

HIT, the most common form of drug-induced thrombocytopenia, may develop in up to 5% of patients treated with heparin. The clinical course of HIT is varied, but can include catastrophic outcomes such as amputation and death secondary to sudden arterial or venous thrombosis. HIT usually results from the formation of antibodies that react with a trimolecular complex between platelet factor 4 (PF4), heparin, or other glycosaminoglycans and the platelet membrane receptor Fc(gamma)RIIa, leading to reduced platelet count and paradoxically increased thrombosis. Thrombocytopenia typically develops after 5 days in naive patients and can develop within minutes to hours post-exposure in those who have received heparin therapy within the past 6 months.

A presumptive diagnosis of HIT can be based on a >50% reduction in platelet count more than 5 days after initiation of heparin therapy, a platelet count <150,000/µL, or the development of thromboembolic complication(s). Laboratory confirmation is important, but withdrawal of all heparin sources should not be delayed while awaiting confirmatory results.

The 14C-serotonin release assay (SRA) is considered the gold standard for laboratory diagnosis because of its high sensitivity and specificity. An enzyme-linked immunosorbent assay (ELISA) detecting antibodies to the platelet factor 4 (PF4)/heparin complex offers rapid and sensitive detection, but lower specificity than the SRA. Physicians often order the 2 assays together. Quest Diagnostics offers the SRA and ELISA separately and as a panel.

Method

SRA: After incubation of donor platelets with 14C serotonin, porcine heparin (0.1, 0.5, and 100 U) is added along with patient serum. False-positive results are excluded by running a parallel study with anti-Fc(gamma)RIIa. Results are reported as percent serotonin released; values <20% are considered negative. Specimens with borderline results are re-tested the next day with fresh platelets from a different donor. Positive results are called in to the ordering physician and consultation is available.

This test was developed and its performance characteristics have been determined by Quest Diagnostics Nichols Institute. It has not been cleared or approved by the U.S. Food and Drug Administration. The FDA has determined that such clearance or approval is not necessary. Performance characteristics refer to the analytical performance of the test.

ELISA: This assay utilizes the PF4/heparin complex as the capture antigen and an alkaline phosphatase-labeled anti-human globulin as the detection antibody. The optical density (OD) is reported: OD values >0.40 are considered positive and indicate the presence of heparin-induced antibodies.

Interpretive Information

SRA: A positive result (≥20%) with 0.1 and 0.5 U of heparin and a suppression of serotonin release with 100 U of heparin strongly suggests HIT; consider substituting heparin with an appropriate direct thrombin inhibitor. A negative result suggests the absence of HIT.

ELISA: A positive result indicates the presence of heparin-induced antibodies but is not diagnostic of HIT; other clinical and laboratory findings should be considered prior to diagnosis. The presence of immune complexes or other immunoglobulin aggregates may cause false-positive results. The higher the titer of heparin-induced platelet antibodies, the more likely that HIT is present. A negative result suggests the absence of heparin-induced antibodies, although this assay may not detect low-titer, low-avidity antibodies.

Panel: SRA is more specific for HIT than the ELISA. Thus, a patient who has a positive SRA and a negative ELISA is more likely to have HIT than one with a positive ELISA and a negative SRA.

 

Content reviewed 02/2013

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