Heparin-induced
thrombocytopenia (HIT) is a common adverse effect of
unfractionated heparin (UFH)
therapy. In contrast, only a few patients have been reported with HIT following
low-molecular-weight heparin (
LMWH)
therapy (LMW-HIT). To define the clinical and biological characteristics of LMW-HIT, 180 patients treated for suspected HIT at 15 French centres were investigated. Clinical history was recorded and HIT was confirmed in 59 patients with positive
serotonin release assay results: 57 of them had high levels of
antibodies (Abs) to
heparin-
platelet factor 4 complexes (H/PF4) and two had Abs to
interleukin 8. Eleven patients were treated exclusively with
LMWH (LMW-HIT) and 48 with UFH either alone (UF-HIT, n = 34) or combined with
LMWH (UF/LMW-HIT, n = 14). The LMW-HIT and UF-HIT groups were similar with respect to sex, age, platelet count before
heparin therapy, frequency of
bleeding and occurrence of
disseminated intravascular coagulation. The interval to onset of HIT was longer in LMW-HIT patients compared with UF-HIT patients (P = 0.03). Severe
thrombocytopenia (platelets < 15 x 10(9)/l) was more frequent in the LMW-HIT group (P = 0.04).
Thrombosis occurred in three of 11 LMW-HIT patients, i.e. as frequently as in UF-HIT patients. LMW-HIT is potentially severe and may be observed after longer
heparin treatment compared with UF-HIT. It is highly recommended, therefore, that platelet counts be monitored carefully whenever
LMWH is administered.