It’s well known that the prostate-specific antigen (PSA) test used to screen for prostate cancer is imprecise. Elevations in PSA levels may signal prostate cancer, but they also may be due to nonmalignant prostate conditions.
Further complicating the screening process is that several medications and a number of other modifiable factors may alter the results of the PSA lab test, leading to inaccurate readings that may overestimate or underestimate your risk of having prostate cancer found on a biopsy.
PSA is a liquid protein produced by the prostate that helps liquefy semen and is crucial to successful natural conception. The prostate normally secretes a small amount of PSA into the blood. The PSA lab test simply analyzes your blood to see how much PSA it contains. PSA is measured in nanograms (one-billionth of a gram) per milliliter (one-thousandth of a liter). The PSA lab test results are used to help assess a man’s need for a prostate biopsy, which is necessary to diagnose prostate cancer.
What Affects the PSA Lab Test and PSA Levels?
A number of factors are known to affect, to varying degrees, your PSA levels:
1. Age. PSA levels, on average, rise as men get older, possibly because the prostate leaks more PSA into the bloodstream.
2. Benign prostate enlargement (BPH). BPH is a non-cancerous enlargement of the prostate that occurs with age. As men get older, the risk of BPH rises.
3. Prostatitis. This prostate disorder is an infection or inflammation of the prostate. It also can cause PSA elevations. (See our post Prostatitis Causes More Than Pain.)
4. Urinary tract infections.
5. Prostate procedures. Examples include prostate surgery, cystoscopy, or a prostate biopsy. Your doctor will inform you about how long you should wait after one of these procedures before undergoing a PSA test.
6. Sex. Ejaculation can cause the prostate to transiently leak more PSA into the blood for about one to two days.
7. Prostate stimulation. Prostatic massage or a digital rectal exam may cause minor PSA elevations.
8. Riding a bicycle. Bicycle riding for a long distance may cause temporary spikes in PSA, possibly because the seat applies pressure on the prostate. You may have to abstain from bicycle riding at least 24 hours before having your PSA measured.
9. Obesity. Being very overweight can result in lower PSA levels.
10. Medications. A number of medications can trigger lower PSA results. These include:
- 5-alpha reductase inhibitors—dutasteride (Avodart) and finasteride (Proscar)—which can reduce PSA levels by about 50 percent in many men who take them.
- Some herbal and dietary supplements
- Thiazide diuretics
So, if you choose to undergo PSA screening, it’s important to understand these factors that can influence your PSA lab test results and to inform your physician if any of them apply to you.
Don’t Rely on PSA Levels Alone
Given the inexact nature of the PSA lab test and the various factors that can influence its results, experts recommend that the test be used in conjunction with a digital rectal exam (DRE). During that procedure, a doctor inserts a gloved finger into the rectum to feel the prostate through the rectal wall and find any lumps or other abnormalities that might indicate cancer. If you decide to be screened, ask for both tests, because that combination is least likely to miss a cancer. DRE is usually done after your blood test to prevent falsely raising PSA from the exam.
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