PSA Test Accuracy

Several statistical terms are important to understand when trying to make sense of who should have their PSA tested or their prostate biopsied.

• The sensitivity of a test refers to the proportion of individuals in a population that will be correctly identified when tested for a particular disease.
• The specificity of a test refers to the probability that a person who does not have the particular disease and is tested for the disease will be correctly identified as not having the disease.

These statistical analyses provide percentages that relate to identifying false negatives (those with a negative test who actually have the disease), false positives (those with a positive test who do not have the disease), true positives (those who test positive and have the disease) and true negatives (those who test negative and do not have the disease).

Any strategy to assess PSA and whether or not testing should occur needs to take into consideration these important concepts. By utilizing age-related PSA, the sensitivity would increase in younger men and increase the specificity in older men. This strategy would reduce unnecessary prostate biopsies. Similarly, this would reduce sensitivity in the older population. This approach may lead to a suitable balance between the specificity and sensitivity of this test. Total PSA is an extremely sensitive test (if the PSA is higher than 4), however more sensitive tests result in more false positives. Lowering the PSA cut-off to less than 4 will increase the sensitivity but decrease the specificity. A sensitivity of 86%, specificity of 33%, and positive predictive value of 41% have been identified for PSA levels higher than 4.0. Free PSA testing is associated with higher specificity.

Despite all the parameters that we are using today to interpret PSA, false negatives (missed cancers) and false positives (unnecessary biopsies) will always occur.