The positive predictive value, or precision rate, is the probability that a positive test represents a true positive. The positive predictive value decreases as the prevalence of the organism in a given population decreases. This is true even for tests that have high specificity. In the case of B. pertussis NAAT, where specificity is not high due to the similarity (high homology) of genetic sequences between the Bordetella species, this is a particular concern. Consequently, the CDC recommends that an initial positive NAAT be considered presumptive evidence of infection. Lack of confirmatory testing can lead to a false-positive result and incorrect treatment of the patient.
