Medical Staristics: Sensitivity and specificity

Interpreting the sensitivity and specificity of a test depends on what you are using it for.

The poor specificity of this test means that it would be inappropriate to use it as reason for telling people they have HIV infection; in the at-risk population over 50% diagnosed positive will not have the disease. By contrast, the balance of risk is different in screening blood for HIV, where the risk of missing a positive case far outweighs the risk of discarding some blood units unnecessarily, and similarly in looking for bowel cancer.

The test would also potentially be appropriate in screening for head lice where the disease is not serious but you do not want to miss cases and the treatment is simple and safe.

Predictive value depends as much on prevalence of a condition as the sensitivity. The positive predictive value in young patients means most army recruits with a positive test would be false-positive, with more being false positive than true positive, but since very few are likely to have heart disease a preliminary screening test with a positive predictive value of 48% would be a reasonable test to use.