Prostate Specific Antigen (PSA)
Prostate cancer is the most common cancer diagnosis in men. Screening for prostate cancer became much more common after the development of the serum prostate specific antigen (PSA) blood test in the late 1980s. In fact, there was a rapid increase in the number of cases reported from 1988-1992, which was a result of more cases being detected by the new test. There was a sharp decline in cases from 1992-1995, which was likely because doctors had "caught up" with all the undiagnosed cases in the previous years. The number of cases per year stabilized in the 10 years following this decline. Screening for prostate cancer currently consists of two tests: a digital rectal exam (DRE) and a PSA blood test.
A PSA (prostate specific antigen) test is a blood test that measures a protein called PSA, which is only made by the prostate gland. PSA is a part of ejaculate material and is important to help increase sperm motility. Normal prostate tissue makes a small amount of PSA, but prostate cancer usually makes much more. By checking to see if your PSA level is elevated, a doctor can screen you for prostate cancer. However, the PSA test isn't perfect. Some tumors won’t elevate the PSA, while some other processes (like benign prostatic hyperplasia and prostatitis) can cause it to be falsely elevated. Remember, the "specific" in prostate specific antigen stands for specific to the prostate gland, not specific to prostate cancer. However, a higher PSA level is more likely to be caused by a prostate cancer.
What do PSA results mean?
The level considered "normal" is less than 4.0 ng/ml. There is agreement that men with a total PSA level greater than 10.0 ng/ml are at an increased risk for prostate cancer (more than a 67% chance, according to the ACS). More important than a single PSA value is the trend of the PSA over time. If the level increases over time, regardless if it is above 4, further testing may be done to rule out cancer. Studies have shown that men who have a steady increase in their PSA are more likely to have prostate cancer and if the rise is rapid, it is more likely to be an aggressive cancer.
There are a number of specialized PSA tests which are used to help differentiate between elevated PSA due to benign conditions and those due to prostate cancer, or to determine the likelihood of cancer and the need for a biopsy.
- The free PSA test evaluates the ratio between the PSA that is free in the blood and the PSA that is bound to proteins in the blood. In most men, the majority of PSA is bound to proteins in the blood and the percentage of free PSA is typically lower in men with prostate cancer.
- A test called complex PSA measures the bound form of PSA. The advantage is that while the free PSA test requires two tests in the laboratory, the complex test requires only one.
- The traditional PSA test is a combination of free PSA and complex PSA.
- The PSA velocity is used to describe the speed at which the PSA value increases over a series of blood tests (and time). The result is reported as an amount (ng/ml) per year. A series of three tests over a period of a minimum of 18 months is needed to calculate a PSA velocity. In general, a rate of 0.75 ng/ml/year is considered high and may be a sign of a more aggressive cancer. Very high levels are more likely a sign of prostatitis than prostate cancer and can be treated with antibiotics.
- PSA doubling time is the period of time it takes for the PSA to double.
- The PSA density is used to evaluate the level of PSA in relation to the overall size of the prostate gland. A transrectal ultrasound is done to determine the size of the prostate, and that number is divided into the PSA value. The theory is that a rise in PSA can be directly proportional to an increase in prostate size. This test is not used very often; as it has not been proven to be reliable, while other tests are more reliable and less costly. One of the major concerns is the poor reliability of transrectal ultrasound in measuring prostate volume correctly.
PSA testing after prostate cancer
The PSA test is often used to monitor men after prostate cancer treatment for recurrence. This "biochemical relapse" often occurs many months or even years before physical signs of a recurrence appear. A single elevated PSA after treatment does not always mean there is a relapse and often the healthcare provider will repeat the test or perform other tests to further evaluate.
Causes of elevated PSA
There are several conditions that can increase the PSA that are not cancer, including benign prostatic hypertrophy (BPH) and prostatitis. Any irritation of the prostate can also cause an elevation. For this reason, men are asked to refrain from ejaculation for 48 hours before the test, and the PSA level should be drawn before the digital rectal exam is performed. A group of medications called 5-alpha reductase inhibitors (finasteride, dutasteride) that are used to treat BPH can cause a false reduction of up to 60% in the PSA level. Saw Palmetto (an herbal supplement) may also affect PSA levels, though research is conflicting.
Several studies have found that prostate cancer screening may not reduce the number of deaths due to the disease and may expose men to unnecessary testing and treatment. However, the decision to have prostate cancer screening is something each man should discuss with his healthcare provider. Family history, race and health status play a big role in determining when and if screening should be performed. Talk with your healthcare provider to see if prostate cancer screening is right for you.
American Cancer Society – Prostate Cancer Screening
Prostate Cancer Foundation – Should I Be Screened?
Abeloff M, Niederhuber JE, Armitage JO, Doroshow, JH, Kastan MB, Tepper, JE. Abeloff’s Clinical Oncology. 5th edition. Philadelphia: Churchill Livingstone; 2014.
Prostate-Specific Antigen (PSA) Test [Internet]. Bethesda, MD: National Cancer Institute; 24 Jul 2012; cited 1 Dec 2014. Available from: /www.cancer.gov/cancertopics/factsheet/detection/PSA