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It’s well known that the prostate-specific antigen (PSA) test used to screen for prostate cancer is imprecise. Elevations in PSA 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 assess a man’s need for a prostate biopsy, which is necessary to diagnose prostate cancer.
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What Affects the PSA Lab Test?
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). Again, as men get older, the risk of BPH rises.
3. Prostatitis. This prostate disorder also can cause PSA elevations. (See our post Prostatitis Causes More Than Pain.)
4. Urinary tract infections.
5. Prostate procedures. Examples include prostate surgery 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.
7. Prostate stimulation. Prostatic massage may cause minor PSA elevations.
8. Physical activity. Bicycle riding in particular can 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. Among the controllable factors that can result in lower PSA levels and potentially affect the results of the PSA lab test is your weight. PSA concentrations tend to be lower in obese men, possibly because they have greater blood volumes, in which PSA can become more diluted.
10. Medications. A number of medications can trigger lower PSA results. One of the most notable drug classes causing this effect are BPH medications known as 5-alpha reductase inhibitors—dutasteride (Avodart) and finasteride (Proscar)—which can reduce PSA levels by about 50 percent in many men who take them. However, finasteride and dutasteride actually increase the sensitivity of the PSA test and improve its utility.
Studies suggest that aspirin and other nonsteroidal anti-inflammatory drugs (NSAIDs), such as ibuprofen (Advil, Motrin) and naproxen (Aleve, Naprosyn), as well as blood pressure medications known as thiazide diuretics—which include hydrochlorothiazide (Microzide, Dyazide, Maxzide), chlorothiazide (Diuril), and chlorthalidone—may result in lower PSA levels. Similarly, statin medications, such as atorvastatin (Lipitor), rosuvastatin (Crestor), and simvastatin (Zocor), may lower your PSA levels in addition to reducing your cholesterol.
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 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.
To better assess your likelihood of having prostate cancer found on a biopsy, your doctor may augment the PSA lab test with tests for several derivatives of PSA. He or she may recommend a PSA velocity measurement, which calculates the rate at which your PSA rises over time.
Another option is the free-PSA test, which measures whether the PSA in the blood is floating free on its own or is attached to a protein molecule. (Benign conditions tend to produce more free PSA, while cancer tends to produce the attached form. Men with prostate cancer have lower percentages of free PSA compared with their cancer-free counterparts.)
In studies, two newer blood tests, the Prostate Health Index (PHI) and the 4Kscore, have outperformed the PSA test at detecting prostate cancer and predicting clinically significant disease. The PHI measures [-2]pro-PSA (an isoform of free PSA) along with other forms of free PSA, and PSA. The 4Kscore measures blood levels of four prostate-derived proteins: total PSA, free PSA, intact PSA, and human kallikrein 2. These biomarkers are combined with a patient’s age, digital rectal exam, and biopsy history to calculate the likelihood of finding a high-grade prostate cancer on biopsy.
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