Lack of in vitro cross-reactivity predicts safety of low-molecular weight heparins in heparin-induced thrombocytopenia

Alternative anticoagulation in patients with heparin-induced thrombocytopenia (HIT) is often problematic. The relatively high cross-reactivity rate reported for the low-molecular-weight heparins (LMWH) has discouraged their use in this setting. This study has investigated the safety of using the LMWH Fragmin, based on a negative heparin-dependent platelet aggregation test using the latter, in patients with proven HIT. Fifty-three evaluable patients with clinical and laboratory evidence of HIT were evaluated for cross-reactivity with Fragmin using a Fragmin-dependent platelet aggregation test. In 20 of 38 patients who showed no in vitro cross-reactivity, Fragmin was substituted for unfractionated heparin. The outcome of these 20 patients was evaluated and compared to that of the remaining 33 patients, in whom anticoagulates were ceased or warfarin or Orgaran was used. Eighteen of 20 patients treated with Fragmin increased their platelet count by greater than or equal to 50 x 10(9)/l from a mean nadir of 57.9 +/- 4.7 x 10(9)/l within 2.8 +/- 0.29 days following substitution of Fragmin for unfractionated heparin. Twenty-eight of the 33 remaining patients who did not receive Fragmin increased their platelet count by greater than or equal to 50 x 10(9)/l from a mean nadir of 53.0 +/- 4.8 x 10(9)/l within 3.0 +/- 0.29 days. In seven patients (two treated with Fragmin), response could not be evaluated due to death within 36 h of cessation of heparin or discharge from hospital. The results indicate that in vitro cross-reactivity testing employing a heparin-dependent platelet aggregation assay can be safely used to select patients with HIT for further anticoagulation with LMWH.