In pharmacoepidemiological studies, covariates use includes 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. A given database may or may not be suitable for studying a research question depending on the availability of information on these covariates.
Some patient characteristics and covariates vary with time and accurate assessment is therefore time dependent. The timing of assessment of the covariates is an important factor for the correct classification of the subjects and should be clearly reported. Capturing covariates can be done at one or multiple points during the study period. In the latter scenario, the variable will be modelled as time-dependent variable (See Chapter 4.3.3).
Assessment of covariates can be performed using different periods of time (look-back periods or run-in periods). Fixed look-back periods (for example 6 months or 1 year) can be appropriate when there are changes in coding methods or in practices or when using the entire medical history of a patient is not feasible. 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 always be applicable, for example when data from paediatric and adult periods are combined because covariates may significantly differ between paediatric and adult populations (e.g., height and weight).