Specific ADR

All other reactions

Specific drug

a

b

All other drugs

c

d

than they would be otherwise. This conservatism applies when the numbers are small and does no harm when using the PRR or POR for prioritisa-tion.

A more general approach is to ask, "What is the expected number of reports for this ADR and this drug?'' and then to compare the observed number with the expected number. A first attempt to obtain the expected number is to assume that the proportion of reports for this ADR with this drug will be the same as the proportion for this ADR in the database as a whole, PADR. The expected number can then be obtained using the total reports for this drug, Ndrug:

Table 20.2. Example calculation of PRR.

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The deviation of the observed number from the expected number can be expressed as a ratio, that is, the PRR

This approach can more easily be seen to be generalisable to allow the expected number to be calculated in a less crude way. It can be modified to allow for age and sex to be taken into account. This is equivalent to having a set of 2 x 2 tables stratified by age and sex, where a POR can be derived using a general Mantel-Haenszel estimator from several 2 x 2 tables (Rothman and Greenland, 1998). It is also possible to use logistic regression to obtain such an estimate.

These measures have allowed for the magnitude of the effect to be assessed; they have not made any allowance for chance variation. The simplest way to make such allowances is to calculate statistical significance tests of the hypothesis that the PRR or POR is one. It is also possible to use the equivalent confidence intervals Tubert-Bitter et al. (1996). The usual chi-square test (corrected using Yates' method to be conservative), or for stratified tables using the Mantel-Haenszel method, can be calculated. This chi-square value indicates the contribution of chance to the magnitude of the PRR. Table 20.2 gives an example of an extreme PRR. The proportion of reports of uveitis with the drug rifabutin is

Table 20.2. Example calculation of PRR.

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