Nevertheless, respiratory diagnostics are particularly limited by the inability to determine whether the detection of a particular pathogen in a symptomatic patient indicates that it is causative of the illness or results from contamination, colonization, or prolonged shedding from a prior unrelated infection, particularly when testing specimens from the upper respiratory tract. This issue is not typically addressed in diagnostic evaluation studies, but it has become more relevant as molecular diagnostics have expanded the lower limits of pathogen detection by several orders of magnitude. Attempts to answer this question have suggested an additional category of test performance, the ''epidemiological'' specificity of a test, to describe the ability of an assay to assign true etiologic status to a pathogen for a specific illness. Ultimately, determination of the epidemiologic specificity of a respiratory diagnostic would require the interpretation of microbiologic results in conjunction with all other clinical and laboratory data, perhaps in the form of a predictive model. Such analyses are uncommon but will be a main focus of the PERCH study.