Adjustment of prognostic effects in prevalent case-control studies on genotype.

Since genotypes are unchangeable, adjustment of prognostic effects in prevalent case-control studies may produce an unbiased estimate of odds ratio (OR) for disease occurrence. In this paper, the prognostic effects on OR is demonstrated, then three approaches to examine and/or adjust the OR are presented. The demonstration shows that the prognostic effects are larger in diseases with poor prognosis than in those with better prognosis. Genotypes increasing disease risk and fatality rate are underestimated, while those increasing the risk and improving prognosis are overestimated. The simplest approach to examine the OR derived from prevalent case-control studies is to conduct stratified analysis according to the interval between diagnosis and study enrollment. When the stratified analysis finds no substantial difference in the estimate, the OR reflects mainly the relative risk for disease occurrence. The proportion of genotype among putative cases at diagnosis can be estimated from prevalent cases by a logistic model, producing the OR adjusted for the interval from diagnosis. An incomplete-data case-control design is also applicable to adjust the prognostic effects. An actual prevalent case-control study on breast cancer is used to demonstrate the three approaches. They are useful to compensate the disadvantage of prevalent case-control studies.