Causes of a falsely raised PSA
A falsely raised PSA (prostate-specific antigen, a protein made by prostate cells) is a high result without cancer, and irritating or injuring the prostate can cause it.
Remember
Let the prostate settle before you measure PSA: take the blood before a rectal examination or at least 48 hours after one, and wait 6 to 8 weeks after a biopsy.
PSA is a protein made by prostate cells and measured in a blood test. It is used as a tumour marker, a substance in the blood that can signal a cancer. It is specific to the prostate but not to prostate cancer, so anything that disturbs the prostate can push the level up.
A prostate biopsy puts a small needle into the prostate to take tissue samples, so it injures the tissue it samples. Needle biopsy usually causes a marked rise in PSA that can persist for weeks. A PSA should not be measured within 6 to 8 weeks of one, because the result could be a false positive (a raised test without cancer).
A digital rectal examination (DRE: a clinician gently presses on the prostate with a gloved finger through the back passage) can squeeze a little extra PSA into the blood. The effect is usually small, but it can lift a borderline result over the age-specific PSA level at which referral is considered. Teaching advises avoiding a PSA within 48 hours of a DRE as a precaution, so take the blood first or leave a gap. The same 48 hours applies to ejaculation and vigorous exercise such as cycling.
A urine infection needs a longer gap: NHS advice is to wait 4 to 6 weeks after it has cleared. Prostate inflammation, urinary retention (being unable to empty the bladder) and recent instrumentation of the urinary tract (passing a tube or camera, for example) can also raise PSA.
Some changes in PSA are steady, not short-lived. Benign prostatic hyperplasia (non-cancerous overgrowth of the prostate) can leave PSA somewhat higher all the time. 5-alpha reductase inhibitors (drugs that shrink the prostate, such as finasteride) pull PSA down, by about half on average during treatment, so they can hide a rise.
Key facts
- After a biopsy
- wait 6 to 8 weeks before relying on a PSA result.
- Rectal examination
- can cause a small, temporary rise in PSA, so the sample is taken first or at least 48 hours afterwards.
- Ejaculation and exercise
- avoid PSA testing within 48 hours of ejaculation or vigorous exercise such as cycling.
- After a urine infection
- wait 4 to 6 weeks once it has cleared.
- Steady changes
- benign prostatic hyperplasia can raise PSA persistently, and 5-alpha reductase inhibitors lower it by about half on average during treatment.
- Food and drink
- you can eat and drink as usual before a PSA blood test.
- Raised result
- a result that may have been affected is often repeated after the wait, before biopsy or MRI is discussed.
Exam traps
- Repeating a PSA 4 weeks after a prostate biopsy is earlier than the recommended interval, because UK guidance advises waiting at least 6 weeks, and other teaching allows 6 to 8 weeks.
- Repeating a PSA within 24 hours of a prostate biopsy does not help, because the marked rise can persist for weeks and the repeat would probably show the same raised value.
- A 5-alpha reductase inhibitor does not cause a short-lived false rise, because it pulls PSA down, by about half on average during treatment, and so can hide a rise rather than cause a false positive.
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