Free PSA and How It Is Used
PSA in the blood is found either alone or bound with other proteins. When PSA is alone, it is called free PSA. The percent-free PSA is the proportion that is alone, or free. When prostate cancer is present, less PSA is free. Why this is so is not known. But, determining the proportion of PSA that is free, when the total PSA level is less than 10.0, appears to be the best test currently available to differentiate an elevated PSA due to an enlarged prostate from one resulting from cancer. The test is rapidly becoming routine use with borderline-high PSA levels. The lower the percent-free PSA is, the more likely cancer is. The percent-free PSA increases with an age-related enlargement of the prostate. The purpose of the test is to reduce the number of biopsies in men without cancer, without missing cancer cases.
The challenge researchers have faced is determining the best threshold level to rule out BPH while detecting cancer. The lower the threshold point is, the higher the specificity (more men with BPH ruled out, not needing biopsy) is, but the lower the sensitivity (more men with cancer who are overlooked) is. Several thresholds, ranging from 14% to 28% have been evaluated. It appears that 25% may be the optimal level with PSA levels between 4.0 and 10.0. This has been shown to detect 95% of cancers and to eliminate 20% of unnecessary prostate biopsies when used routinely in men with PSA levels in the borderline 4.0–10.0 ng/ml range. In the 2.5 to 4.0 range, a threshold of 27% was shown to detect about 90% of cancers while eliminating 18% of unnecessary biopsies.
