Summary
The platelet aggregation test is widely used for the diagnosis of heparin-induced
thrombocytopenia (HIT), a potentially serious complication of heparin therapy. We
have evaluated its sensitivity and specificity in comparison with those of the 14C-serotonin release test. The sensitivity of the platelet aggregation test was found
to vary with the heparin concentration and the donor of the platelets used in the
test. The optimal heparin concentrations were between 0.1 and 1.0 U/ml. Using these
heparin concentrations, the mean sensitivity varied from 39% (with the least reactive
platelets) to 81% (with the most reactive platelets). In comparison, the sensitivity
of the release test ranged from 65% to 94%. The specificities of the platelet aggregation
test were 82%, 90% and 100% for the following control groups: (1) non-thrombocytopenic
patients given heparin, (2) patients with thrombocytopenia due to other causes, and
(3) normal controls not given heparin, respectively. The corresponding specificities
for the release test was 94%, 90% and 100%. The specificities can be further increased
to 100% for all controls with the adoption of a two-point system which defines a positive
result as one in which platelet aggregation occurs with a low heparin concentration
(0.5 U/ml) but not with 100 U heparin/ml. For optimal results, a two-point platelet
aggregation test should be performed with heparin concentrations of 0.5 and 100 U/ml
and using platelets of more reactive donors.
