Test Background PSA is a single chain chymotrypsin-like serine protease of approximately 30 kDa. It is produced by the epithelial cells lining the acini and ducts of the prostate gland, and plays a role in the liquefaction of seminal fluid. Low levels of PSA are found in the blood as a result of leakage from the prostate gland. Increasing levels of serum PSA are associated with prostatic pathology, including prostatitis, benign prostatic hyperplasia (BPH), and cancer of the prostate. Urinary retention, transurethral resection of the prostate (TURP), prostate biopsy, prostate massage and ejaculation may also give rise to increases in serum PSA levels. In the sera PSA is mostly bound to either α-2 macroglobulin (AMG) or α-1 antichymotrypsin (ACT). The total PSA test measures free PSA and that bound to ACT with approximate equimolarity. PSA bound to AMG is not detected by the total PSA test. The proportion of protein-bound PSA increases in malignancy compared with that observed in BPH or prostatitis. Hence, separate measurement of free PSA and calculation of the ratio (the %fPSA test) is helpful in distinguishing men with prostate cancer from men with BPH, particularly when direct rectal examination is negative yet clinical suspicion remains high. A percentage free PSA ≤10 % in a healthy male with total PSA 4-10 µg/L increases the probability of biopsy-positive prostate cancer approximately 3 fold.
Clinical IndicationsDiagnosis: the use of %fPSA is recommended as an aid in distinguishing men with prostate cancer from those with BPH when total PSA level is between 4-10 µg/L and DRE is negative. It can also be helpful in identifying prostate cancer in men despite initial negative biopsy findings.
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