Accuracy of the PSA Test
Although the PSA test is not perfect, it is the best currently available test for early detection of prostate cancer. Since doctors started using the test, the number of prostate cancers found at an early, curable stage has increased. Because most men have normal test results, they can be reassured that they are unlikely to have prostate cancer, especially if the digital rectal exam (DRE) is also negative.
While the PSA test is considered a major advance in diagnosing early-stage prostate cancer, it has some drawbacks. For 100 men over 50 at average risk for prostate cancer, the following would be found if they all had a PSA test:
- Ten of the 100 men would have a PSA level higher than normal (over 4.0). The 10 men would need further testing to clarify their abnormal levels.
- Three of the 10 men would be found to have prostate cancer.
- Seven of the 10 men would be found not to have prostate cancer. They would have an elevated PSA for other reasons—most likely BPH.
- Ninety of the 100 men would have PSA levels in the normal range (<4.0).
- One or 2 of these 90 men would be found to have significant prostate cancer that becomes life threatening.
This shows that the PSA test is moderately sensitive. Of 100 men with prostate cancer, it will detect only about 70 of them, a sensitivity of about 70%. But the positive predictive value of the PSA test is low. Only 3 out of 10 positive results were cancer (PPV of about 30%). And 7 out of 10 positive PSA results (i.e., greater than 4.0) are false-positive results; this means that 2 out of 3 men who are told that they may have cancer after taking the PSA test actually do not have it. When the PSA is greater than 10.0, the test is more accurate. There is about a 50-50 chance of having cancer at this level of PSA.
Negative predictive value is high reflecting the low likelihood of having significant prostate cancer with a normal PSA level (NPV of about 98%).
Refinements of the PSA test have been developed to reduce the number of false positive results. These include measuring the proportion of free PSA, the density of PSA, and the velocity of PSA changes. Not all authorities agree on how to use these additional PSA tests. If your PSA is found to be high, ask your primary healthcare practitioner to discuss your cancer risk and the possible use of other evaluations of PSA before having a biopsy.
