RLO: Positive and Negative Predictive Value of Diagnostic Tests: Introduction

Diagnosis is an essential step in medical care. Usually this involves ‘diagnostic tests’ of some description. A health check is also a group of diagnostic tests. Decisions about medical treatment are made on the basis of test results.

An ideal, or truly accurate, test will always give a positive result with disease, and a negative result without disease. This is not the case for all tests. In practice this means that not all positive test results will represent disease. This is described by the Positive Predictive Value (PPV). Equally, not all negative results will represent no disease. This is described by the Negative Predictive Value (NPV).

Positive predictive value is defined as the proportion of those with a positive test result who actually have disease.

Negative predictive value is defined as the the proportion of those with a negative test result who do not have disease.

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