Digital Rectal Exam (DRE) and Prostate-Specific Antigen (PSA) Test
The diagnosis of prostate cancer typically begins with an abnormal prostate-specific antigen (PSA) test or perhaps a worrisome finding on a digital rectal exam (DRE). If either the PSA test or DRE suggests cancer, transrectal ultrasound or a prostate biopsy will be performed.
Digital Rectal Examination
Diagnosing prostate cancer at an early stage can be difficult with DRE alone because the cancer cells tend to spread throughout the prostate instead of forming a solid mass that can be easily felt. Used alone, digital rectal exams miss approximately 30 to 40 percent of prostate cancers, and most cancers found with the exam are in a later, less treatable stage. The most reliable way to detect prostate cancer in its early stages is to combine digital rectal exams with PSA testing.
Prostate-Specific Antigen Test
The prostate-specific antigen (PSA) test measures an enzyme produced almost exclusively by the glandular cells of the prostate. It is secreted during ejaculation into the prostatic ducts that empty into the urethra. PSA liquefies semen after ejaculation, promoting the release of sperm.
Normally, only very small amounts of PSA are present in the blood. But an abnormality of the prostate can disrupt the normal architecture of the gland and create an opening for PSA to pass into the bloodstream. Thus, high blood levels of PSA can indicate prostate problems, including cancer. PSA blood levels are expressed as nanograms per milliliter (ng/mL).
A blood test to measure levels of PSA was first approved by the U.S. Food and Drug Administration (FDA) in 1986 as a way to determine whether prostate cancer had been treated successfully and to monitor for its recurrence. Today, PSA tests are approved for prostate cancer detection and are widely used to screen men for the disease.
Clinical studies, including a randomized trial known as the European Study of Screening for Prostate Cancer (ESRPC), have shownthat PSA testing saves lives by detecting and treating prostate cancer earlier. In the most recent results from this trial, prostate cancer deaths were reduced by approximately 40 percent among men who were screened with the PSA test compared with those who were not screened.
This reduction in prostate cancer deaths is similar to the decline in prostate cancer deaths seen in the United States sincethe onset of widespread PSA testing in the late 1980s. Clinical studies to date have demonstrated the following benefitsof PSA testing:
- An elevated PSA is the single best predictor of the presence of prostate cancer.
- PSA testing detects prostate cancer about five to 10 years earlier than digital rectal exams.
- Most cancers detected with PSA testing are curable.
- Regularly scheduled PSA testing virtually eliminates the diagnosis of advanced prostate cancer.
- A baseline PSA prior to age 50 can help predict the risk of prostate cancer up to 25 years later.
Because some of the cancers detected by PSA screening are so small or slow growing that they might never have become life threatening, the tradeoff of routine screening is the overdiagnosis of nonlife-threatening cancers for which treatment is not necessary. This occurs more often in older men, who have less to gain from screening because of a shorter life expectancy.
Another drawback of PSA testing is that most men with an elevated PSA do not have prostate cancer. Instead, prostate enlargement (BPH) or inflammation is to blame. These men may undergo unnecessary diagnostic tests and treatments and may experience undue anxiety. Because of these uncertainties, men should discuss both the benefits and limitations of PSA testing with their physician before having their PSA levels measured.
The likelihood of having prostate cancer varies by PSA level and whether or not the DRE is normal. The risk increases incrementally as PSA rises and, within any given PSA range, is higher if the DRE is abnormal. The risks of finding prostate cancer on a biopsy in men with a normal DRE are as follows:
- a PSA of 0 to 2 ng/mL translates to a risk of 12 percent
- 2 to 4 ng/mL, a 15 to 25 percent risk
- 4 to 10 ng/mL, a 17 to 32 percent risk
- 10 to 20 ng/mL, a 50 to 75 percent risk and
- a PSA higher than 20 ng/mL, a 90 percent risk
.A study based on data from the National Institute on Aging found that a PSA level above the median value for age (0.6 ng/mLfor men age 40 to 49; 0.7 ng/mL for men age 50 to 59) was associated with a three- to fourfold greater risk of prostate cancer up to 25 years later when compared with a value below the median. Investigators from Sweden showed that men with a PSA of 0.51 to 1.0 ng/mL or 1.01 to 2 ng/mL before age 50 had a twofold and sevenfold greater risk, respectively, of advanced prostate cancer up to 25 years later than men whose PSA level was 0.5 ng/mL or below before age 50.
There is no PSA level below which physicians can reassure a man that he does not have prostate cancer. Therefore, it is not possible to define a "normal" level. In men in their 40s or men without prostate enlargement, PSA levels above 2.5 ng/mL can signal trouble. For older men, levels above 3 to 4 ng/mL usually indicate the need for a prostate biopsy. Most experts agree that PSA should be used in conjunction with other information (for example, family history, race, and age) to assess the overall likelihood that prostate cancer is present after a discussion with the patient about the benefits and risks.
The American Cancer Society and the American Urological Association recommend that PSA testing and DRE be offered beginning at age 50. Men at increased risk for prostate cancer—black men and men with a family history of prostate cancer—should be offered PSA and DRE beginning at age 40 or 45. Previously, both organizations recommended annual testing for men who chose to be screened. However the American Cancer Society now recommends screening every two years for men with a PSA level below 2.5 ng/mL.
The U.S. Preventive Services Task Force (USPSTF, an organization that makes recommendations about periodic health examinations) recommends that men age 75 and older not undergo screening for prostate cancer and contends that the current evidence is insufficient to assess the balance of benefits and harms of prostate cancer screening in men younger than age 75.
A study by the Medical Editor of this information found that testing all men at age 40, age 45, and then every other year after age 50 might be a better strategy, saving more lives and being less expensive than other screening schedules. Other studies have concluded that men age 50 and older who have PSA levels below 2 ng/mL do not need to be tested every year. However, the testing schedule should be more frequent if a man's PSA is rising but has not reached a threshold level where a biopsy would be recommended.
Several factors may affect the results of a PSA test. For example, some studies show that ejaculation one or two days before a PSA test may increase PSA levels in the blood. Consequently, men should abstain from sex for 72 hours prior to a PSA test. Digital rectal exams and biopsies of the prostate may also affect PSA levels, although the increase in PSA caused by a digital rectal exam is not believed to be significant enough to produce a false-positive test result in most men. A prostate biopsy, however, may elevate PSA levels for as long as four weeks.
Other prostate problems (such as BPH or prostatitis) also can inflate PSA levels, and 5-alpha-reductase inhibitors for BPH treatment (Proscar and Avodart) can lower PSA levels by about 50 percent. When men who are taking 5-alpha-reductase inhibitors have a PSA test, this drug-related reduction in PSA level must be taken into account to avoid misinterpretation of the test result.