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What are the screening tests for prostate cancer?
26.09.2010 12:42
What are the screening tests for prostate cancer?
Screening tests are those that are done at regular intervals to detect a disease such as prostate cancer at an early stage. If the result of a screening test is normal, the disease is presumed not to be present. If a screening test is abnormal, the disease is then suspected to be present, and further tests usually are needed to confirm the suspicion (that is, to make the diagnosis definitively). Prostate cancer usually is suspected initially because of an abnormality of one or both of the two screening tests that are used to detect prostate cancer. These screening tests are a digital rectal examination and a blood test called the prostate specific antigen (PSA).
In the digital rectal examination, the doctor feels (palpates) the prostate gland with his gloved index finger in the rectum to detect abnormalities of the gland. Thus, a lump, irregularity, or hardness felt on the surface of the gland is a finding that is suspicious for prostate cancer. Accordingly, doctors usually recommend doing a digital rectal examination in men age 40 and over.
The PSA test is a simple, reproducible, and relatively accurate blood test. It is used to detect a protein (the prostate specific antigen) that is released from the prostate gland into the blood. The PSA level is usually higher than 4ng/mL in people with prostate cancer than in people without the cancer. Situations of large prostate size, infection and inflammation are other reasons why the PSA may be elevated. The PSA, therefore, is valuable as a screening test for prostate cancer. Accordingly, doctors usually recommend doing a PSA in men age 40 and over. Subsequent screening is recommended based on individual preference and assessment of risk for developing prostate cancer. For example, patients with a high risk of developing prostate cancer due to a family history or a high initial PSA should have more frequent evaluation (usually annually).
Although, still considered controversial, most urologists recommend regular screening for prostate cancer in men who are likely to live for more than 10 years (for example, life expectancy > 10 years). The American Urological Association (AUA) issued their latest guidelines for prostate cancer in 2009. According to these, men at the age of 40 should be offered a baseline PSA test and a prostate exam (digital rectal exam or DRE) to ascertain the risk of prostate cancer. Subsequent screening and tests may be performed according to the findings on this initial evaluation and an individual's risk of getting the disease on the basis of other factors such as race, ethnicity, and family history of prostate cancer. As mentioned, most urologists currently would advise some form of screening in men with a life expectancy greater than 10 years. Most frequently, it would be performed on an annual basis. Although, there is no definite cutoff age to stop prostate cancer screening, most physicians would rarely screen men more than 75 years of age for this disease.
Results of the PSA test under 4 nanograms per milliliter of blood are generally considered normal. (See the next two sections on false-positive elevations of the PSA and on refinements in the PSA test.) There is a recent trend, however, to perform prostate biopsy in all patients with a PSA more than 2.5 ng/ml in order to detect prostate cancer at an earlier (and hopefully, completely curable) stage. The American Urological Association guidelines (2009) do not define a definite cutoff point but advise that all the other risk factors for prostate cancer should be taken into account while making a decision on whether to proceed for a biopsy. One of the important factors that need to be considered is the rate at which the PSA value has increased over time on repeated measurements (PSA velocity). Results between 4 and 10 are considered borderline. These borderline values are interpreted in the context of the patient's age, symptoms, signs, family history, and changes in the PSA levels over time. Results higher than 10 are considered abnormal, suggesting the possibility of prostate cancer. It has been shown that the higher the PSA value, the more likely the diagnosis of prostate cancer. Moreover, the level of PSA tends to increase when the cancer has progressed from organ-confined prostate cancer to local spread to distant (metastatic) spread. Very high values, such as 30 or 40 and over, are usually caused by prostate cancer.
What are false-positive elevations in the PSA test?
False-positive elevations in the PSA are increases in the PSA that are caused by conditions other than prostate cancer. For example, benign prostatic hyperplasia (BPH) and infection or inflammation of the prostate (prostatitis) from whatever cause can elevate the PSA. Note also that even a rectal examination or an ejaculation within the prior 48 hours can sometimes elevate the PSA. False-positive elevations are usually in the 4 to10 range, but they can go as high as 25 or 30. At these higher levels, however, caution in the interpretation of the test is warranted because a prostate cancer may well be present. Non-prostatic diseases or infections, medications, foods, smoking, and alcohol do not cause false-positive elevations of the PSA.
The ability of the PSA test to detect prostate cancer (called the sensitivity of the test) is high. The reason for this is that most patients, although not all, with prostate cancer have a borderline or an abnormally elevated PSA. The ability of the test to exclude other diagnoses (called the specificity of the test), however, is lower because of the other conditions that can cause false-positive elevations of the PSA.
What refinements have been made in the PSA test?
Recently, several refinements have been made in the PSA blood test. The purpose of these refinements is to help doctors to better assess a borderline or an elevated PSA. The goal is to determine more accurately who has prostate cancer and who has a false-positive elevation of the PSA from another condition. In other words, the purpose of the improvements is to improve the sensitivity and the specificity of the test.
One refinement is called the PSA ratio. This ratio is determined by dividing the amount of PSA that circulates freely in the bloodstream by the amount of PSA that is bound to proteins in the bloodstream. Research has shown the PSA that circulates freely in the blood tends to be associated with benign prostatic hyperplasia (BPH) whereas the PSA that is bound to protein tends to be linked with prostate cancer. Thus, a high PSA ratio suggests a false-positive elevation of the PSA and weighs against the diagnosis of prostate cancer. In contrast, a high PSA with a low PSA ratio favors the diagnosis of prostate cancer.
Another recent modification of the PSA test is based on the observation that as men age the amount of PSA in the blood can normally rise without the presence of a prostate cancer. Thus, doctors can use what is referred to as an age-specific PSA, especially to evaluate borderline values. In the age-specific PSA, the normal values are adjusted for the age of the patient. Accordingly, the age-specific normal ranges are 0 to 2.5 for men in their 40s, 0 to 3.5 in their 50s, 0 to 4.5 in their 60s, and 0 to 6.5 for men 70 years of age and over. Therefore, as an example, a PSA of 4 would be considered borderline for men in their 30s and 40s but could be normal for men in their 50s, 60s, and 70s.
Furthermore, another improvement of the PSA test is called the PSA velocity or slope. The velocity is calculated as the rate at which the PSA changes with repeated testing over time. The more rapid the rise in the PSA, the more likely is the presence of a prostate cancer. The less rapid the rise in the PSA, the less likelihood there is that a prostate cancer is present.
Prostate cancer gene 3 (PCA3) is a new gene-based test carried out on a urine sample. PCA3 is highly specific for the diagnosis of prostate cancer. Therefore, in contrast to PSA, the PCA3 is not increased by conditions such as benign enlargement or inflammation of the prostate. The PCA3 urine test can provide additional information over a PSA test that may help in deciding whether a prostate biopsy is really needed.
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