Use

Measure the percentage of free (uncomplexed) PSA relative to the total amount of PSA in men with serum PSA concentrations between 4.0 and 10.0 ng/mL.

In general, serum PSA levels increase due to physical changes to prostate architecture caused by trauma, infection, inflammation, prostate manipulation, benign prostatic hypertrophy (BPH), or malignancy. The sensitivity of PSA levels to these changes serves as the basis for the clinical use of the test. The PSA concentration in the serum of healthy men is a millionfold lower than that in seminal fluid. PSA in seminal fluid is predominantly free or uncomplexed. In serum, the majority of PSA is bound to inhibitors, including α₁-antichymotrypsin (ACT) and α₂-macroglobulin (A2M). Measured total PSA consists of free and ACT-bound, since PSA complexed to A2M is not immunologically detectable.

Catalona and coworkers found that one in four patients with normal DRE and PSA levels between 4.0 and 10.0 ng/mL have prostate cancer. They recommend using a cutoff of 25% free PSA for this group of men (see Limitations) to identify individuals with an increased risk of prostate cancer. They found that 95% of men with cancer (as determined by biopsy) with normal DRE and total PSA between 4.0 and 10.0 ng/mL had percent free PSA of ≤25%. Their study further indicated that 20% of men with benign disease (as determined by biopsy) with normal DRE and a total PSA between 4.0 and 10.0 ng/mL had percent free PSA greater than the 25% cutoff.

Alternatively, percent free PSA may be used to determine the relative risk of prostate cancer in individual men.

Values obtained with different assay methods should not be used interchangeably in serial testing. It is recommended that only one assay method be used consistently to monitor each patient’s course of therapy.

Patients taking finasteride, an alpha-reductase inhibitor, will have diminished levels of PSA. PSA complexes are more stable than free PSA. The serum levels of both total and free PSA increase with prostate manipulation but the free returns to premanipulation concentrations quicker. This can result in a transient elevation in percent free PSA.

Preparation

Specimens should not be drawn immediately after digital rectal examination (DRE), prostatic massage, or transrectal ultrasound (TRUS). PSA sampling should not be performed for at least six weeks after prostatic biopsy.