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prostate specific antigen

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Prostate-specific antigen (PSA) is a serine protease which is usually specifically expressed by both normal and malignant prostate cells (1).

  • it is useful to detect cancer at an early stage or before symptoms develop resulting in early treatment of the cancer (2)
  • repeat PSA may help in diagnosis of prostate cancer 
  • however PSA values are likely to be increased with age and in conditions such as benign enlargement of the prostate, prostatitis and lower urinary tract infections, (2)

Elevation of serum PSA is more a sensitive and specific indicator of prostatic carcinoma than prostatic acid phosphatase (PAP)

  • PSA is raised in over 90% of cases when carcinoma is first detected by comparison to 50% for PAP.
  • however, PSA lacks the required specificity to be a test for prostatic cancer as it is also increased in most patients with benign prostatic hypertrophy.

The most common PSA test measures the total amount of PSA (both free and protein bound) in the blood. Recently an alternative test has been used which calculates the ratio of free: total PSA. Free PSA is associated with benign conditions while bound PSA is associated with malignancy. Hence a low ratio (<25%) may be indicative of cancer (2).

PSA test is not diagnostic and require further investigations to confirm prostate cancer (2).

Note:

  • before undertaking a PSA test, patients should be informed carefully as to why the test is done and its implications (3)

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