When case–control data are available, it is usual to make inferences on the IV-exposure association using only the controls.37 (link) This makes the assumption that the association of the IV with the exposure in the controls is similar to that in the general population, which is true for a rare disease.38 (link) This is necessary for two reasons. The first is reverse causation, as measurements of the exposure in a retrospective setting may be distorted by a disease event. Second, over-recruitment of cases into the study means that the distribution of confounders in the case–control sample is different to that in the general population. An association may be induced between the IV and the confounders, leading to possible bias in the IV estimate.9 This affects not only the ratio method but all IV methods. The IV–outcome association estimate should not be affected provided that logistic regression is used, as logistic regression estimates are invariant to outcome-dependent sampling. It is also necessary to assume that the probability of recruitment into the study is not dependent on the genetic variants used as IVs. For example, a genetic variant associated with increased mortality after an outcome event would appear to be protective rather than deleterious if the individuals who died were excluded from the study.
If the outcome is common and its prevalence in the population from which the case–control sample was taken is known, such as in a nested case–control study, then inferences on the IV–exposure association can be obtained using both cases and controls, provided that measurements of the exposure in cases were taken prior to the outcome event. This analysis can be performed by weighting the sample so that the proportions of cases and controls in the reweighted sample match those in the underlying population.38 (link)
If the outcome is common and its prevalence in the population from which the case–control sample was taken is known, such as in a nested case–control study, then inferences on the IV–exposure association can be obtained using both cases and controls, provided that measurements of the exposure in cases were taken prior to the outcome event. This analysis can be performed by weighting the sample so that the proportions of cases and controls in the reweighted sample match those in the underlying population.38 (link)