Since most adverse drug reactions are the so-called "type A'' reactions, which are caused by the pharmacological effects of the product, and commonly occur within a short period after exposure, comparing the rates of events occurring soon after exposure with subsequent periods provides a useful means to generate possible drug safety signals.
PEM provides a numerator (the number of reports) and a denominator (the number of patient-months or patient-weeks of exposure), both collected within a known time frame (the difference, for each patient, between the start and stop dates of the drug being monitored).
The incidence density (ID) for a given time period, t, for each of the event terms in the DSRU dictionary is calculated as follows:
IDt number of events during treatment for period t number of patient-months (weeks) of treatment for period t
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