According to Marin
Urology
Several forms of PSA are found in the serum. About 60 to 65% are bound to alpha-1-antichymotrypsine and alpha 2 macroglobuline and 35% to 40% of are considered as free (while PSA is almost totally free in seminal fluid).
The free PSA / total PSA fraction is significantly diminished in men with prostate cancer (almost all of the PSA is bound).
Taking this fraction into account would improve the specificity of a PSA elevation between 4 to 10 ng/ml. However, no urological examination should have been performed during the days prior to the test since prostate manipulation releases free PSA.
When the fraction is under 10%, the risk of prostate cancer is high. When it is above 25%, the elevation of PSA is probably related to benign hyperplasia.
The following table enables us to judge the risk of prostate cancer in patients with a normal rectal examination and a PSA value between 4 and 10ng/ml.
| % free PSA | 50-64 years | 65-75 years |
| 0% -10% | 56% | 55% |
| 10% -15% | 24% | 35% |
| 15% -20% | 17% | 23% |
| 20% -25% | 10% | 20% |
| > 25% | 5% | 9% |
The following diagram, inspired by the Dianon system website shows that most patients with a prostate cancer have a fraction inferior to 15%.
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According to Marin
Urology |