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Rapid HIT Assays

Informal discussions with Ginger Weeden and Lucinda Lojka from Bio-Rad, Incmade me curious about rapid assays for the antibody that causes heparin-induced thrombocytopenia with thrombosis (HIT), anti-heparin/PF4.

In North America we typically screen with one of three enzyme immunoassay (EIA) methods, from GTI Diagnostics Inc., Waukesha, WI, USA, PF4 Enhanced®: GTI PF4 Enhanced Package InsertHyphen Bio-med, Neuville-Sur-Oise, France, Zymutest HIA® Assay; or Diagnostica Stago, Asnières, sur Seine, France: Asserachrom® HPIA. These employ platelet factor 4 (PF4)-linked heparin or a heparin analogue as their solid-phase target and require incubation periods of approximately 2 hours. EIAs detectanti-PF4/heparin antibodies in patients exposed to heparin, and a substantial sub-population of these antibodies is responsible for HIT, though some anti-PF4/heparin antibodies appear to have no platelet-activating properties. The EIA methods are 100% sensitive, but only modestly specific, so we confirm positive results with platelet aggregometry, for instance the heparin-induced platelet activation assay (HIPA) or the serotonin release assay (SRA), performed at a reference laboratory. Laboratory diagnosis of HIT is time-consuming, requires batching, and is thus not available for rapid turn-around in emergent situations.

Akers Biosciences, Inc. Thorofare, NJ, USA, released their PIFA Heparin/PF4® Rapid Assay to the US market in 2004, and further modified the assay for enhanced readability in 2009. This is a moderately complex manual lateral flow immunoassay (LIA, particle immunofiltration assay, PIFA) that requires a serum specimen and produces a colored endpoint on a cellulose strip in 10 minutes. If you use or have used the second-generation Akers kit, please send a comment about its current readability. PIFA package insert: 1-2011_PIFA_HPF4_pn_1322003_r5.

In communications with Norbert Benattar, Directeur général, Cryopep, Montpellier, France, and Hans Zwerger, Produktspezialist, CoaChrom Diagnostica, Vienna, Austria, I’ve learned of three additional rapid assays, available in Northern Europe. Unlike the EIAs that are semiquantitative, the rapid assays produce binary results. They are…

  • Instrumentation Laboratory, Bedford MA, USA HemosIL® AcuStar HIT Assay, a 30-minute automated chemiluminescence method designed for the IL AcuStar and described in the attached package insert: IL Chemiluminscence HIT Assay.
  • Diagnostica Stago‘s Stic Expert HIT®, a manual LIA that uses serum or plasma, described in a 2012 American Society for Hematology (ASH) poster:Poster ASH 2012 Stic Expert Abstract.
  • PaGIA® Heparin/PF4 Antibody Test, a particle gel immunoassay fromDiaMed, Ltd., a division of Bio-Rad Laboratories Inc, Cressier sur Morat, Switzerland, that employs PF4/heparin complexes affixed to red, high-density polystyrene beads suspended in a gel similar to those used in transfusion medicine and is read like a blood group test. PaGIA is compared to the three standard EIAs in Warkentin TE, Linkins LA. Immunoassays are not created equal. J Thromb Haemost 2009; 7: 1256–9 and in a companion article, Bakchoul T, Giptner A, Najaoui A, et al. Prospective evaluation of PF4/heparin immunoassays for the diagnosis of heparin-induced thrombocytopenia. J Thromb Haemost 2009; 7: 1260–5.

All three assays provide for rapid turn-around, but are, like the EIA methods, 100% sensitive and relatively non-specific, requiring confirmation by a platelet activation method such as HIPA or SRA. All HIT assays must be preceded by a pre-test probability determination using the 4T assessment, as described in Fritsma GA, Thrombosis risk testing, Chapter 42 in Rodak BF, Fritsma GA, Keohane EM. Hematology: Clinical Principles and Applications, Fourth Edition, 2012, Elsevier, St. Louis, Page 688.

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