Positive and negative predictive values

[1] The PPV and NPV describe the performance of a diagnostic test or other statistical measure.

Still, if the individual's pre-test probability of the target condition is the same as the prevalence in the control group used to establish the positive predictive value, the two are numerically equal.

The NPV can also be computed from sensitivity, specificity, and prevalence: The complement of the NPV is the false omission rate (FOR): Although sometimes used synonymously, a negative predictive value generally refers to what is established by control groups, while a negative post-test probability rather refers to a probability for an individual.

Still, if the individual's pre-test probability of the target condition is the same as the prevalence in the control group used to establish the negative predictive value, then the two are numerically equal.

Thus it will be necessary to follow up any positive result with a more reliable test to obtain a more accurate assessment as to whether cancer is present.

[11] PPV is directly proportional[dubious – discuss] to the prevalence of the disease or condition.

[citation needed] When an individual being tested has a different pre-test probability of having a condition than the control groups used to establish the PPV and NPV, the PPV and NPV are generally distinguished from the positive and negative post-test probabilities, with the PPV and NPV referring to the ones established by the control groups, and the post-test probabilities referring to the ones for the tested individual (as estimated, for example, by likelihood ratios).

[citation needed] Bayes' theorem confers inherent limitations on the accuracy of screening tests as a function of disease prevalence or pre-test probability.

needed is: where Of note, the denominator of the above equation is the natural logarithm of the positive likelihood ratio (LR+).

As described Balayla et al.,[12] repeating the same test may violate the this independence assumption and in fact "A more natural and reliable method to enhance the positive predictive value would be, when available, to use a different test with different parameters altogether after an initial positive result is obtained.".

[12] PPV is used to indicate the probability that in case of a positive test, that the patient really has the specified disease.

In this situation the gold standard used in the evaluation study represents only the presence of bacteria (that might be harmless) but not a causal bacterial sore throat illness.