In pharmacoepidemiology studies, covariates are often used for selecting and matching study subjects, comparing characteristics of the cohorts, developing propensity scores, creating stratification variables, evaluating effect modifiers and adjusting for confounders. Reliable assessment of covariates is therefore essential for the validity of results. Patient characteristics and other key covariates that could be confounding variables need to be evaluated using all available data. A given database may or may not be suitable for studying a research question depending on the availability of these covariates.
Some patient characteristics and covariates vary with time and accurate assessment is time dependent. The timing of assessment of the covariates is an important factor for the correct classification of the subjects and should be clearly specified in the protocol. Assessment of covariates can be done using different periods of time (look-back periods or run-in periods).
Fixed look-back periods (for example 6 months or 1 year) are sometimes used when there are changes in coding methods or in practices or when is not feasible to use the entire medical history of a patient. Estimation using all available covariates information versus a fixed look-back window for dichotomous covariates (Pharmacoepidemiol Drug Saf. 2013; 22(5):542-50) establishes that defining covariates based on all available historical data, rather than on data observed over a commonly shared fixed historical window will result in estimates with less bias. However, this approach may not be applicable when data from paediatric and adult periods are combined because covariates may significantly differ between paediatric and adult populations (e.g., height and weight).
|10. Specific topics|
|Annex 1.||Guidance on conducting systematic revies and meta-analyses of completed comparative pharmacoepidemiological studies of safety outcomes|