Positive predictive value

Assistant Editor(s)-In-Chief: Kristin Feeney, B.S.

Overview
The positive predictive value (PPV), or precision rate, is the proportion of individuals with a positive test result who actually have preclinical disease. It is considered the physician's gold standard, as it reflects the probability that a positive test reflects the underlying condition of interest.

Definition
The Positive Predictive Value can be defined as

$$ PPV = \frac{\rm number\ of\ True\ Positives}{{\rm number\ of\ True\ Positives}+{\rm number\ of\ False\ Positives}}$$

or, alternatively,

$$ PPV = \frac{({\rm sensitivity}) ({\rm prevalence})}{({\rm sensitivity}) ({\rm prevalence}) + (1 - {\rm specificity}) (1-{\rm prevalence})} $$

Problems with Positive Predictive Value

 * Predictive values are often used in medical research to evaluate the usefulness of a diagnostic test. Hence the PPV is used to indicate the probability that in case of a positive test, that the patient really has the specified disease. However there may be more than one cause for a disease and any single potential cause may not always result in the overt disease seen in a patient.
 * An example is the microbiological throat swab used in patients with a sore throat. Usually publications stating PPV of a throat swab are reporting on the probability that this bacteria is present in the throat, rather than that the patient is ill from the bacteria found. If presence of this bacteria always resulted in a sore throat, then the PPV would be very useful. However the bacteria may colonise individuals in a harmless way and never result in infection or disease. Sore throats occurring in these individuals is caused by other agents such as a virus. In this situation the gold standard used in the evaluation study represents only the presence of a bacteria (that might be harmless) but not a causal bacterial sore throat illness. It can be proven that this problem will affect positive predictive value far more than negative predictive value. To evaluate diagnostic tests where the gold standard looks only at potential causes of disease, one may use an extension of the predictive value termed the Etiologic Predictive Value.
 * PPV is a function of the population sampled whereby a higher yield occurs in a higher risk group.
 * PPV is influenced by sensitivity, specificity, prevalence, and diagnostic pre-clinical phase of disease.

Related Chapters

 * Negative predictive value
 * Relevance (information retrieval)
 * Receiver-operator characteristic