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Quick Question: Reporting the HIT Immunoassay

Our current “quick question” was provided by Dr. Michael Blechner:

“When performing ELISA testing for heparin-induced thrombocytopenia (HIT), how do you report the results?

Answers from twenty reponses are below…

a. Positive, negative, equivocal, confirmed by saturating heparin dose: 8 (40%)
b. Positive, negative, equivocal, unconfirmed by heparin: 5 (25%)
c. Positive, negative, equivocal with optical density: 5 (25%)
d. Optical density alone: 2 (10%)

This is a remarkable diversity, and it reflects comments from the 2007 report of the Platelet Immunology subcommittee of the SCIENTIFIC SUBCOMMITTEE SESSION 6 July 2007 Palexpo, Geneva, Switzerland:

Zehnder J, Price EA, Hayward C, Warkentin T, Moffat K, Moore J. NASCOLA Survey of laboratory practices regarding testing for heparin-induced thrombocytopenia .

In this survey of 44 specialized labs that perform testing for HIT antibodies, two objectives were stated: to determine current practice in laboratory diagnosis of HIT, and to identify areas in need of standardization and improvement. A wide variety of specimen types/pre-analytic variables were accepted, with two “concerns” being heparin contamination within samples (78%) and that the timing of the blood draw could be too early for antibody detection (75%). Antigen assays (e.g. EIA [ELISA]) are most commonly performed, especially the assay from GTI (71%) versus the one from Stago (27%), with a wide variety of reporting strategies and test cutoffs utilized. A minority of labs had tried the “rapid” assays (particle gel immunoassay and particle immunofiltration assay). Platelet activation assays were performed in only about 1/3 of labs, with similar numbers using “washed platelets” as platelet-rich plasma. Most labs used prescreened donors known to react well in the activation assays. Again, a wide variety of test conditions (e.g., heparin concentrations) were used. The most common pattern of practice (55% of labs) involved screening with a commercial EIA, and referring out of samples to a reference lab for platelet activation assays. A number of items that might benefit from standardization were presented.

You may wish to go to the subcommittee discussion to review some of the controversies associated with HIT assays

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