Serine proteases are bound mostly to various serum proteins. A small percentage of serum PSA exists as free PSA (fPSA), but the majority exists as complexed PSA (cPSA) and is bound to either alpha2 -macroglobulin (AMG) or alpha1 -antichymotrypsin (ACT). These are the 2 major serine protease inhibitors in the blood, constituting 10% of total serum protein. The ejaculate primarily contains fPSA, in a concentration of 1 million ng/mL.
The Goteborg screening study evaluated 604 men with a total PSA (tPSA) higher than 3. These men underwent digital rectal examination (DRE), transrectal ultrasonography (TRUS), and sextant prostate biopsies. Cancer was identified in 144 men (23.8%). Significantly higher levels of hK-2 and tPSA were found in those with cancer, whereas the ratio of free PSA (fPSA) to tPSA was lower. The optimum equation predicting the presence of cancer was as follows:
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The specificity of PSA at levels higher than 4.0 ng/mL is 60-70%. Specificity can be improved by using age-adjusted values, PSA velocity (PSA-V), and the ratio of free PSA (fPSA) to total PSA (tPSA). Another method is to adjust the PSA according to the size of the prostate or volume determinations of the transitional zone, which produces most of the PSA, and the peripheral zone, which produces less PSA but a majority of prostate cancers. 2ff7e9595c
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