Clinical usefulness of testing for a heparin-dependent platelet-aggregating factor in patients with suspected heparin-associated thrombocytopenia.

It is important to confirm the diagnosis of heparin-associated thrombocytopenia, because discontinuation of heparin requires the institution of other antithrombotic therapy. Reports describing a heparin-dependent platelet-aggregating factor in the serum of patients with heparin-associated thrombocytopenia suggest the potential for a specific diagnostic test. No study has evaluated the diagnostic usefulness of testing for this factor, and we tested for a heparin-dependent platelet-aggregating factor in sera from (1) 14 patients with heparin-associated thrombocytopenia, (2) 14 nonthrombocytopenic controls given heparin, (3) 16 patients with consumptive thrombocytopenic disorders not associated with heparin, and (4) 13 healthy laboratory personnel. Coded serum samples plus varying concentrations of heparin (final concentration 0.01, 0.1, 1.0, and 5 U/ml) plus platelets from healthy individuals were incubated together, and aggregation was measured. Positive results (greater than 20% aggregation) occurred in (1) five of 14 patients with heparin-associated thrombocytopenia, (2) 0 of 16 consumptive thrombocytopenic controls not given heparin therapy, (3) one of 14 nonthrombocytopenic controls given heparin, and (4) one of 13 healthy controls, giving a sensitivity of 36% and specificities of 100%, 93%, and 92%, respectively. No unique clinical or laboratory feature distinguished those patients with heparin-associated thrombocytopenia with positive aggregation results from those patients with heparin-associated thrombocytopenia with negative results. Fourteen of the 16 patients (86%) with heparin-associated thrombocytopenia had elevated levels of platelet-associated IgG; however, the test was also positive in 67% of the 16 patients with other consumptive (and probably immune) thrombocytopenic disorders not associated with heparin administration.(ABSTRACT TRUNCATED AT 250 WORDS)