Association between antibiotic resistance and community prescribing: a critical review of bias and confounding in published studies.

The reported association between antibiotic prescribing and resistance may be subject to bias or confounding. Bias describes any effect at any stage of investigation or inference tending to produce results that depart systematically from the true value. A confounding variable is one that is associated independently with both exposure and outcome. Confounding variables may create an apparent association or mask a real association. Pharmacoepidemiology is the study of the use and the effects of drugs in large numbers of people. We have used standard pharmacoepidemiological methods to investigate sources of bias and confounding in the association between prescribing and resistance. We conclude that the association is statistically valid and that the consistency of evidence supports a cause-effect relationship. Nonetheless, several important sources of bias and confounding must be taken into account in future studies that analyze the impact of prescribing policies on resistance.