Spontaneous heparin-induced thrombocytopenia syndrome: 2 new cases and a proposal for defining this disorder.

[heparin-induced thrombocytopenia]

The existence of spontaneous heparin-induced thrombocytopenia (HIT ) syndrome (or autoimmune HIT ), defined as a transient prothrombotic thrombocytopenic disorder without proximate heparin exposure serologically indistinguishable from HIT , is controversial . We describe 2 new cases presenting with thrombotic stroke /thrombocytopenia : one following shoulder hemi-arthroplasty (performed without heparin ) and the other presenting to the emergency room without prior hospitalization , heparin exposure , or preceding infection . Both patients tested strongly positive for anti-platelet factor 4 (PF 4 )/heparin immunoglobulin (Ig )G in 2 different immunoassays and in the platelet serotonin-release assay . Crucially , both patients ' sera also caused strong (>80 %) serotonin release in the absence of heparin , a serologic feature characteristic of delayed -onset HIT (ie , where heparin use precedes HIT but is not required for subsequent development or worsening of thrombocytopenia ). We propose that a rigorous definition of spontaneous HIT syndrome should include otherwise unexplained thrombocytopenia /thrombosis without proximate heparin exposure and with anti-PF 4 /heparin IgG antibodies that cause strong in vitro platelet activation even in the absence of heparin .