FALSE-POSITIVES IN SPONTANEOUS REPORTING - SHOULD WE WORRY ABOUT THEM

1 Spontaneous reporting remains the most used and, undoubtedly, the mo st cost-effective approach for the identification of adverse drug reac tions (ADRs). Most of the limitations of this method are well recognis ed but the possibility of receiving false-positive reports of coincide ntal drug-event associations has received little attention. 2 In this paper we propose a method based on the Poisson distribution for comput ing the maximum number of reports of an ADR that could be expected to be reported coincidentally. Three parameters are required: (i) the bac kground risk of the event in the reference population, (ii) the total number of patients treated with the drug considered and, (iii) the pro portion of cases that have been reported to the pharmacovigilance syst em. 3 For most empirical situations occurring in the post-marketing su rveillance setting, the expected number remains low and only a maximum of one to three cases could be accepted as possibly coincidental. 4 F or rare adverse events such as agranulocytosis or toxic epidermal necr olysis, coincidental associations are so unlikely that a number of rep orts greater than three constitutes a strong warning and requires furt her investigation. 5 These findings suggest that for rare events, repo rts of coincidental drug-event associations are too unlikely to be con sidered as an important limitation of spontaneous reporting.