The Test Sample
What is being tested?
Prostate specific antigen (PSA) is a protein produced primarily by cells in the prostate, a small gland that encircles the urethra in males and produces a fluid that makes up part of semen. Most of the PSA that the prostate produces is released into this fluid, but small amounts of it are also released into the bloodstream. This test measures the amount of PSA in the blood.
The PSA test is used as a tumor marker to screen for and to monitor prostate cancer. It is a good tool but not a perfect one, and most experts agree that screening should be done on asymptomatic men only after thorough discussions with their healthcare providers on the benefits and risks and after informed decisions are made to undergo screening. Elevated levels of PSA are associated with prostate cancer, but they may also be seen with prostatitis and benign prostatic hyperplasia (BPH). PSA levels tend to increase in all men as they age, and men of African American heritage may have levels that are higher than other men, even at earlier ages.
PSA is not diagnostic of cancer. The gold standard for identifying prostate cancer is the prostate biopsy, collecting small samples of prostate tissue and identifying abnormal cells under the microscope. (Read the article on Anatomic Pathology for more information on biopsies.) The total PSA test and digital rectal exam (DRE) are used together to help determine the need for a prostate biopsy.
The goal of screening is to detect prostate cancer while it is still confined to the prostate. Once the presence of prostate cancer is confirmed by biopsy, another decision must be made with regard to treatment. Prostate cancer is relatively common in men as they age and many, if not most, of the tumors are very slow-growing. While prostate cancer is the number two cause of death in men, the slow-growing type is an uncommon cause of death. A pathologist may be able to help differentiate between slow-growing cases and cancers that are likely to grow aggressively and spread to other parts of the body (metastasize).
Over-diagnosing and over-treatment are issues with which health practitioners are currently grappling. In some cases, the treatment can be worse than the cancer, with the potential for causing significant side effects such as incontinence and erectile dysfunction. The PSA test and DRE cannot, in general, predict the course of a person's disease.
PSA exists in two main forms in the blood: complexed (cPSA, bound to other proteins) and free (not bound). The most frequently used PSA test is the total PSA, which measures the sum of complexed and the free PSA in the blood.
The free PSA test is sometimes used to help to determine whether a biopsy should be done when the total PSA is only slightly elevated. PSA is an enzyme (protein that helps different chemical reactions to occur) and when it is released into the blood, some circulating proteins inactivate PSA by binding to it. Benign prostate cells in BPH tend to release PSA that is not active (and, therefore, less likely to be bound by circulating proteins) and cancerous prostate cells tend to release PSA that is already protein-bound.
Therefore, men with BPH tend to have higher levels of free PSA and men with prostate cancer tend to have lower amounts of free PSA. A relatively low level of free PSA increases the chances that a cancer is present, even if the total PSA is not significantly elevated.
How is the sample collected for testing?
A blood sample is taken by needle from a vein in the arm.
NOTE: If undergoing medical tests makes you or someone you care for anxious, embarrassed, or even difficult to manage, you might consider reading one or more of the following articles: Coping with Test Pain, Discomfort, and Anxiety, Tips on Blood Testing, Tips to Help Children through Their Medical Tests, and Tips to Help the Elderly through Their Medical Tests.
Another article, Follow That Sample, provides a glimpse at the collection and processing of a blood sample and throat culture.
Is any test preparation needed to ensure the quality of the sample?
Ejaculation should be avoided for 24 hours before sample collection, as it has been associated with elevated PSA levels. The sample should also be collected prior to the health practitioner performing a digital rectal exam (DRE) and prior to (or several weeks after) a prostate biopsy, as both of these can also elevate PSA levels.