Rapid Assay Particle Immunofiltration Assay

More recently, another rapid immunoassay for anti-PF4-H antibodies, the "HealthTEST® Heparin/Platelet factor 4 Antibody Assay" (Akers Laboratories, Inc., Thorofare, NJ) received approval by the U.S. Food and Drug Administration (Fig. 5). This assay utilizes a system known as "particle immunofiltration assay" (PIFA), wherein patient serum (fresh not frozen/thawed) is added to a reaction well containing dyed particles coated with PF4 (not PF4/heparin). The lack of requirement for heparin presumably reflects formation of the HIT antigens through close approximation of PF4 tetramers achieved under the conditions of PF4 binding to the particles (Greinacher et al., 2006). Subsequently, nonaggluti-nated—but not agglutinated particles—will migrate through the membrane filter. Thus, a negative test is shown by a blue color in the result well, whereas no color indicates a positive test. FDA approval was based upon the assay being judged by

PIFA® (Particle ImmunoFiltration Assay) PF4-coated polystyrene microspheres

Heparin Binding Platelet Factor

Blue color

FIGURE 5 Schematic drawing showing the PIFA. Abbreviations: HIT, heparin-induced thrombocytopenia; PIFA, particle immunofiltration assay; PF4, platelet factor 4. Source: From Warkentin and Sheppard, 2006a.

Blue color

FIGURE 5 Schematic drawing showing the PIFA. Abbreviations: HIT, heparin-induced thrombocytopenia; PIFA, particle immunofiltration assay; PF4, platelet factor 4. Source: From Warkentin and Sheppard, 2006a.

the FDA as substantially equivalent to the solid-phase EIA available from GTI. However, in a preliminary report (Francis et al., 2006), and in the laboratories of both authors, this assay showed unacceptable sensitivity and specificity for detecting anti-PF4/H antibodies.

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