Thrombocytopenia Is More Common and Nadir Occurs Earlier with Unfractionated Heparin Than with Low Molecular Weight Heparin - Results from the Prospective Catch Registry.

Background: Thrombocytopenia in the patient on heparin can have various etiologies, including benign reversible causes and serious, potentially life-threatening ones, such as heparin induced thrombocytopenia (HIT). Knowledge about the prevalence of and timecourse of thrombocytopenia in heparin-treated patients may be helpful to develop systems of alerting clinicians to possible HIT and determining how frequently to check blood counts to detect thrombocytopenia that may be heralding HIT.

Methods: The CATCH registry is a prospective registry of inpatients enrolled between March 2003 and April 2004 at over 50 US hospitals in 3 strata: [1] receiving > 96 hours of unfractionated heparin (UFH) or low molecular weight heparin (LMWH), [2] developing thrombocytopenia (platelets >50% reduction from baseline or <150,000/mm³) in the cardiac care unit and [3] patients on whom HIT tests were ordered. Here we present the data of the prolonged heparin stratum.

Results: 1,121 and 861 patients received UFH and LMWH, respectively, for > 96 hours. A platelet count decrease of > 50 % from baseline was seen more frequently in patients on UFH than in patients on LMWH (10.7% and 7.9 %, respectively; p = 0.03). The parameters that predicted development of thrombocytopenia in the UHF group were length of heparin therapy, body mass index, and admission to a cardiac service; the only parameter predicting thrombocytopenia in the LMWH group was length of LMWH treatment. Of the patients with decreased platelet count by > 50 % from baseline (for UFH n = 120; for LMWH n = 68), this drop occurred in the UFH group at a median of 3.0 days after initiation of heparin and at a median of 4.0 days in the LMWH group. The difference in timing between the 2 groups was not statistically significant (p = 0.205). The platelet nadir was reached after a median /mean of 4.0 / 7.4 days in the UFH group, and 8.0 / 11.4 days in the LMWH group. This was statistically significantly different between the 2 groups (p-value = 0.0025).

Conclusions: A platelet count decrease of > 50 % from baseline occurs frequently in inpatients treated with prolonged heparin. It occurs slightly more frequently on UFH than on LMWH. The median time to onset of thrombocytopenia (> 50 % decrease from baseline) occurs early, at 3–4 days. Daily platelet count checks in the first few days of heparin therapy may be helpful to rapidly discover thrombocytopenia that may then prompt HIT testing.