We review methods of calculating the positive predictive value of a test (the probability of having a condition given a positive test for it) in situations where there is no ’gold standard’ way to determine the true classification. We show that Bayesian methods lead to illogical results and instead show that a new approach using imprecise probabilities is logically consistent.

Keywords: Diagnostics, Bayes’ rule, False positives, Prevalence, Sensitivity, Specificity, Uncertainty, Gold standard.