August 2019: Laboratory Diagnosis of Heparin-Induced Thrombocytopenia (HIT)


Theodore (Ted) E. Warkentin, MD, BSc(Med), FRCP(C), FACP, FRCP(Edin.)
Professor, Dept of Pathology and Molecular Medicine, and Dept of Medicine, McMaster University, Hamilton, Ontario, Canada

Brief overview
HIT is a clinical‐pathological disorder; thus, laboratory detection of pathogenic heparin-dependent, platelet‐activating antiplatelet factor 4 (PF4)/heparin antibodies (“HIT antibodies”) is crucial for diagnosis. The platelet serotonin‐release assay (SRA) can detect heparin‐dependent and heparin‐independent platelet‐activating antibodies; this latter subgroup of antibodies explains “autoimmune HIT” disorders (delayed‐onset, persisting, spontaneous, heparin “flush”). Immunologic features of HIT include early antibody detectability (at onset of platelet count fall) and antibody transience (“seroreversion”). PF4-dependent enzyme-immunoassays (EIAs) are widely-used, have high sensitivity (~99%) but only moderate specificity for HIT, and results are usually not available in real time. In contrast, automated rapid immunoassays (e.g., chemiluminescence immunoassay [CLIA], latex immunoturbidimetric assay [LIA]) facilitate real‐time diagnosis that can incorporate pretest probability into posttest probability through Bayesian analysis.

Learning objectives

  • Describe what is meant by a “clinical-pathological” disorder.
  • Understand the clinical picture and laboratory diagnosis of “autoimmune” HIT.
  • Perform real-time HIT diagnosis (Bayesian analysis) using automated rapid assays for HIT.

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Answers to Questions from Live Webinar

Below is a list of questions received from the live webinar attendees. Click on any question to reveal the speaker's response.

It depends on the situation. If the test result and the clinical picture are highly concordant, no further testing may be required. If for any reason the clinician wishes further test confirmation/refutation (e.g., for research/publication, unclear clinical-laboratory picture, etc.) then testing with other complementary assays may be useful.

Therapeutic plasma exchange (TPE) has been used for patients with severe aHIT syndromes. Given the challenges of arranging TPE, and potential difficulties in maintaining adequate anticoagulation during TPE, my preference is to give high-dose intravenous immunoglobulin (IVIG) as adjunctive therapy to anticoagulation in patients with severe aHIT syndromes. [See Warkentin TE. Exp Rev Hematol 2019, an open access article reviewing the use of high-dose IVIG to treat HIT]

Very high concentrations of heparin with bind PF4 in such a way as to prevent PF4-PF4 interactions from occurring (closely-opposed PF4 tetramers appear to be required to form HIT antigens).

Controversial-- It is important to avoid bolus heparin administration to a patient who has circulating HIT antibodies (risk of fatal anaphylactoid reaction), arguing strongly for a heparin allergy flag. However, after several weeks or months, heparin can usually be given safely again, arguing such a flag may not be necessary, at least down the road. However, given that deliberate heparin rechallenges should only be done in special circumstances in patients with a previous history of HIT (e.g., for cardiac or vascular surgery), on balance, it makes sense to place a heparin allergy flag in a patient’s chart.

The CLIA kit (available from Instrumentation Laboratory) can be used if the lab has access to the ACL AcuStar instrument (manufactured by Instrumentation Laboratory) needed to run that assay.

Certain polyanions can trigger HIT, such as hypersulfated chondroitin sulfate (this may no longer be marketed), PI-88 (anti-cancer agent), and certain other polyanionic molecules.

I don’t believe so, but I haven’t inquired recently on this point.

Yes--presumably antibody seroreversion (loss of antibody reactivity) would be shown by the CLIA and LIA; however, for decisions such as heparin rechallenge, the functional test (SRA) is considered the gold standard—if the SRA is negative, then it is believed safe to perform a heparin rechallenge, if otherwise clinically warranted (e.g., cardiac surgery).

In general, the commercial PF4-dependent EIAs (including the Asserachrom IgG-specific assay) are believed to be essentially equivalent, and of high sensitivity (at least 97%, if not higher); there may be occasional HIT samples that (for unknown reasons) test positive in one commercial EIA but not in another.

Usually just one; the most important thing is that the “pedigree” platelet donor is known to react well to HIT sera.

I don’t know the answer to this question. Our lab invariably receives frozen samples for HIT testing, and we usually test a sample within a reasonably short time after thawing the serum or (for the LIA) plasma. Regarding the specifics of your question, in some jurisdictions, the manufacturer indicates that the samples for testing should be thawed rapidly with testing performed within 2 hours. In other jurisdictions (USA and Canada), it is indicated that samples may be frozen and maintained at -70C for 2 freeze/thaw cycles.

In general, HIT is becoming less common, with increasing use of low-molecular-weight heparin (10-fold less likely to cause HIT than unfractionated heparin), fondaparinux (perhaps ~100-1000-fold less likely to cause HIT vs unfractionated heparin), and non-heparin anticoagulants, such as the direct oral anticoagulants (DOACs). Note that a patient can still develop HIT during DOAC thromboprophylaxis, when it is given following knee arthroplasty (there are ~12 reported cases of spontaneous HIT syndrome following knee arthroplasty in which various non-heparin anticoagulants, including DOACs, were being given at the time that HIT began).

I don’t currently recommend that the PIFA assay be used.

Rapid, on-demand tests for which favorable data exist for diagnosis of HIT include: PaGIA (particle gel immunoassay), LFI (lateral flow immunoassay), LIA, and CLIA. No assay is 100% sensitive and specific, however (this caveat applies to the EIAs as well as to the functional tests such as the SRA).