Background: Eleven cases of thrombotic thrombocytopenic purpura (TTP) associated with clopidogrel therapy have been published.
Objectives: To perform a comprehensive causality assessment of the 11 published cases of TTP to assess quantitatively the extent to which clopidogrel was the causative factor.
Methods: The 11 reports of TTP were analyzed using the Bayesian Adverse Reaction Diagnostic Instrument to calculate the posterior probability (PsP) that clopidogrel caused the TTP based on epidemiological and clinical trials data (expressed as prior odds) and the clinical characteristics of each case (expressed as likelihood ratios).
Results: Clopidogrel was implicated as the causative factor of the TTP (PsP>0.75) in only five of the 11 cases. The PsP for clopidogrel was lowered by concomitant use of a statin, a history of cancer and the occurrence of relapsing TTP (in the absence of clopidogrel) in the remaining six cases.
Conclusions: This systematic analysis provides strong evidence, based on the case information reported, that clopidogrel was the causative factor in at least some cases of TTP.