The Accuracy of a Test at Detecting a Disease
No test is perfectly accurate. This is certainly true with the PSA test. The accuracy of a test is evaluated by a number of factors. One is its ability to detect a high percentage of people who have the disease in question. This is called sensitivity. If a test is very sensitive, it is good at detecting the disease when it is present. A sensitivity of 95% means that it would be positive for 95 out of 100 people who had the disease. Another measure of a test is the ability to rule out those who do not have the disease. This is called specificity. It is the percentage of those without the disease who test negative. A specificity of 95% means that 95 out of 100 who do not have the disease get negative results.
A more important aspect of accuracy is called predictive value. This has to do with the confidence one can have in a specific test result. Positive predictive value (PPV) is the chance that a positive test result is really due to cancer. Negative predictive value (NPV) is the probability that a negative result means no cancer is present.
