Can obscure the true risk of certain adverse drug reactions

BMJ 1995; 310 doi: https://doi.org/10.1136/bmj.310.6992.1470 (Published 03 June 1995) Cite this as: BMJ 1995;310:1470
  1. K R Paterson
  1. Consultant physician Clinical Pharmacology, Royal Infirmary, Glasgow G4 0SF

    EDITOR,—The concept of events per person year is not only of limited utility in describing events in osteoporosis.1 In the study of adverse drug reactions the use of event rates can also be inappropriate and obscure the true risk of certain reactions.

    When an event is (or is believed to be) likely to occur at any stage during continuous treatment with a drug then an event rate with a time component (rate per person year, etc) has a true meaning. For example, 1000 patient years' exposure to a drug might be achieved by studying 100 patients for 10 years or 2000 patients for six months, but in both cases the event rate per person year will be the same if the event in question occurs uniformly through the period of exposure to the drug.

    If, however, an event is likely to occur either early in a patient's exposure to a drug or not at all (a common situation with idiosyncratic adverse drug reactions) then the use of an event rate with a time component is inappropriate. Extending the example above and assuming that one patient in 50 experiences a specific adverse drug reaction in the first three months of treatment, then the rate per patient year will be 0.002 in the group of 100 patients (=two events in 1000 patient years) and 0.04 in the group of 2000 patients (=40 events in 1000 patient years).

    Less commonly, a specific adverse drug reaction may not occur until late in the exposure to a drug, in which case the rate per patient year will be much lower in short term studies than in long term studies.

    For adverse drug reactions of this type it is the number of events per patient treated (or per 100 or 1000 patients treated) that is the true rate, reflecting the actual risk to an individual recipient of the drug. Those reporting rates of adverse drug reactions should take careful note of the temporal pattern of reactions and, when risk is not uniform through time, report numbers of events as well as, or even instead of, rates with a time component.


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