Relative-risk ratio was a useful measure of differential association in cohort and case-series studies

ObjectiveThe framework consists of cohort or case-series studies with intermittent exposure and two types of events. The aim is to define and estimate an association measure between the exposure and the occurrence of one type of event rather than the other.Study Design and SettingThe model and the estimation method are obtained by extending Farrington's approach for one type of recurrent event. The proposed association measure “RRc” is the ratio of the relative risks pertaining to each type of event. The estimated RRc and its confidence interval are derived under the independence assumption between the counts of the two types of events. The data that are analyzed are part of the data of a study on antimicrobial resistance in children.ResultsAn interpretation of the RRc is proposed in terms of an odds ratio, which parallels a similar association measure defined in cross-sectional studies (“ORc”). The estimated value of the RRc agrees with the ORc reported in previous studies.ConclusionThe RRc appears as a useful tool for evaluating the risk of colonization (or infection) with resistant rather than susceptible bacteria following a previous intake of a given antibiotic conditional on colonization (or infection) with any bacteria.