Heparin-induced thrombocytopenia: ELISA optical density value and 4T score in correlation with panel donor platelets activation in functional flow cytometric assay

Abstract

Background - Serological assays for the diagnosis of heparin-induced thrombocytopenia (HIT) detect both platelet-activating and platelet non-activating anti-heparin/platelet factor 4 (PF4) antibodies and have therefore a limited positive predictive value. Functional assays confirm the presence of platelet-activating antibodies but require platelets from healthy donors, whose response to patient serum can differ. Our aim was to investigate the correlation between the level of anti-heparin/PF4 antibodies, 4T score, and the extent of panel donor platelet activation in the functional assay.
Materials and methods - In total, 38 sera from enzyme immunoassays (ELISA) positive patients were tested against panel platelets obtained from 10 healthy, randomly selected donors, using our routine flow cytometry functional test for CD62P expression. Levels of anti-heparin/PF4 antibodies from medical and surgical patients and 4T pretest probability scores (where available) were correlated with the number of activated panel platelets.
Results - Sera with low ELISA optical density (OD) values (0.4-1) activated on average 5.6, sera with intermediate ELISA OD values (>1-2.5) activated on average 7.3, and sera with high ELISA OD values (>2.5) activated on average 8.6 out of 10 panel platelets. One serum with low 4T score did not activate donor platelets, 12 sera with intermediate 4T score activated on average 6.3 donors, 8 sera with high 4T score activated on average 8.5 panel platelets.
Discussion - Sera with higher ELISA OD values activated platelets from a higher number of platelet donors, independently of patient type (medical or surgical). The average number of activated panel platelets increased with rising 4T score. Results indicate that both donor platelet reactivity and quantity of anti-heparin/PF4 antibodies affect the result of the functional assay, meaning special attention is needed in platelet donor selection when testing sera with low levels of antibodies.