Prostate-specific antigen (PSA) test
A prostate-specific antigen (PSA) test measures the amount of PSA in the blood. PSA is a protein made by prostate cells. It is mostly found in semen, but small amounts of PSA can also be found in the blood of healthy men.
Why a PSA test is done
A PSA test may be done to:
- help find prostate cancer early in men who don’t have any signs or symptoms of the disease
- check for cancer in men who have signs or symptoms of prostate cancer
- confirm a diagnosis when other tests suggest prostate cancer
- predict a prognosis (outcome) for prostate cancer
- predict if cancer has spread outside the prostate
- plan treatment for prostate cancer
- monitor men with prostate cancer who are being treated with active surveillance
- find out if cancer treatments are working
- find out if cancer has come back (recurred) after treatment
A PSA test is often used together with a digital rectal exam (DRE)digital rectal exam (DRE)A physical examination used to check for abnormalities of the rectum or prostate. to increase the chance of finding prostate cancer early when it is easier to treat. Using these tests together is better than using either test alone.
Who should have a PSA test
Men older than 50 years should talk to their doctor about their personal risk of developing prostate cancer and the benefits and risks of having a PSA test. The Canadian Cancer Society recommends that you should also talk to your doctor about PSA testing if you:
- will soon be 50 years old
- have a family history of prostate cancer or are of African ancestry
- have symptoms of prostate cancer
Benefits and risks
The PSA test can help detect prostate cancer early, but it can also cause false alarms or can miss prostate cancer when it’s really there. It’s important that men talk to their doctor about their personal risk of developing prostate cancer as well as the benefits and risks of PSA testing.
A PSA test can find prostate cancer early, before it grows large or spreads outside of the prostate. Finding cancer early can mean that treatments will be more successful.
PSA testing carries the following risks.
A false-positive result on a PSA test suggests that a man might have prostate cancer when he actually doesn’t. This happens quite often with PSA testing and only about 1 in 4 abnormal results is due to cancer. A false-positive result can lead to unnecessary follow-up testing that is more invasive, such as repeated biopsies. It can also cause men and their families unnecessary anxiety and distress.
A false-negative result means that the test shows that the PSA level is normal even though prostate cancer is present. PSA testing misses about 15% of prostate cancers. Getting a false-negative result may mean that a man and his doctor ignore symptoms of prostate cancer.
Overdiagnosis means diagnosing prostate cancer that would never pose a serious threat to a man’s health. Overdiagnosis can lead to giving treatments that aren’t absolutely necessary (overtreatment). Research shows that 23%–42% of prostate cancers that are found with PSA testing may never need to be treated. But most men diagnosed with prostate cancer still choose to have treatment. Unnecessary follow-up testing and treatment put a man at risk for problems, including erectile dysfunction and loss of bladder control (called urinary incontinence).
How a PSA test is done
A PSA test is a blood test that is done in a lab or hospital. Ejaculation can affect PSA levels in the body, so you may be told not to ejaculate for a few days before the test.
Tell your doctor if you are taking drugs to treat benign prostatic hyperplasia, prostatitis, urinary problems or baldness. These drugs can lower PSA levels.
Find out more about blood tests.
What the results mean
PSA levels depend on a man’s age. As men get older, their PSA levels naturally go up, but PSA can go up and down for many reasons. Doctors have a hard time agreeing on what is a normal PSA level. But some researchers use a cut-off PSA level of around 3 ng/mL to help them decide which men have the greatest risk of developing prostate cancer. It’s important for you to discuss your PSA level result and what it means for your risk of prostate cancer with your doctor. Your doctor will help you determine your risk of prostate cancer in relation to your age, family history and other personal information.
A higher than normal PSA level doesn’t always mean that a man has prostate cancer. High PSA levels can also be caused by:
- an enlarged prostate due to benign prostatic hyperplasia
- an inflamed or infected prostate (called prostatitis)
- a urinary tract infection
- a recent medical test or procedure on the prostate, such as a transrectal ultrasound (TRUS) or biopsy
- a urinary catheter
- a bladder exam
- sexual activity that includes ejaculation
- bike riding often or a recent long bike ride
- warmer climates
Sometimes the PSA level goes up temporarily after a couple of years of receiving radiation therapy to treat prostate cancer. This is called a PSA bounce. In most cases, the PSA level will fall the next time it is checked. Treatment isn’t needed unless the PSA level continues to rise.
There is no limit to how high a PSA level may rise. But some men with prostate cancer will have a normal PSA level. For these reasons, researchers are still trying to find out the best way to use the PSA test to find prostate cancer. They are also looking for other ways to find prostate cancer early.
What happens if your PSA is high
If you have a high PSA level, your doctor will talk to you about your options. Together you will decide if you need other tests, procedures, follow-up care or treatment. The type of follow-up care you receive will depend on whether or not you have already been diagnosed with prostate cancer.
Usually the first thing doctors will do to follow up on a high PSA level is another PSA test and a DRE. The result of the follow-up PSA test is compared to the first test. If the PSA level is still high, your doctor may use one of these follow-up procedures:
- serial PSA testing, including PSA velocity and PSA doubling time
- testing for free PSA in the blood
- PSA density of the prostate and PSA density of the transition zone (the part of the prostate that surrounds the urethraurethraThe tube that carries urine from the bladder to the outside of the body.)
If your PSA level continues to rise or the doctor feels a lump on the prostate during a DRE, follow-up tests may also include:
- urine tests, such as a urinalysis
- a transrectal ultrasound (TRUS)
- a bladder exam (called a cystoscopy)
- a prostate biopsy
The following are newer PSA tests and different ways of using the PSA test. They can be used to follow up on a high PSA level. Researchers are also trying to find out if they can find prostate cancer better than with regular PSA testing.
Serial PSA testing
Serial PSA testing is mostly used to monitor a man’s response to cancer treatment. It is less useful at diagnosing prostate cancer.
Serial PSA tests look at changes in the PSA level over time. This is important because PSA levels tend to go up and down even in men who don’t have prostate cancer. But men with prostate cancer will have PSA levels that rise more quickly over time than men who don’t have prostate cancer.
Serial PSA testing measures the PSA velocity and PSA doubling time.
PSA velocity measures the change in the level of PSA over time. To find out the PSA velocity, doctors compare the results of 3–4 PSA tests that are done over 18–24 months. Some research shows that PSA velocity that rises quickly can help doctors diagnose prostate cancer.
PSA doubling time measures the time it takes the PSA level to double. The result is calculated in time. For example, a PSA doubling time of 3 years means that, on average, the PSA level doubles every 3 years. PSA doubling time can help doctors find out if a prostate cancer is aggressive, which means it is more likely to grow quickly and spread.
Free and bound PSA
In the blood, PSA is either bound or free (unbound). Bound PSA means that it is attached to other proteins. PSA that is not attached to other proteins is called free PSA because it circulates freely in the blood.
Percent-free PSA is a ratio that compares the amount of free PSA to the total PSA level. The total PSA level includes the amount of both free and bound PSA in the blood.
Free PSA levels are often higher in men with non-cancerous conditions of the prostate and lower in men with prostate cancer. If a man has a total PSA level between 4 and 10, doctors may test his blood for free PSA. A percent-free PSA above 25% is considered normal. Some doctors recommend that men with a percent-free PSA of 18% or less should have a prostate biopsy. Other doctors recommend having a biopsy if the percent-free PSA is around 12% or less.
PSA density and PSA density of the transition zone
PSA density (PSAD) is the level of PSA in the blood in relation to the size (volume) of the prostate measured during a transrectal ultrasound (TRUS). It is calculated by taking the PSA level and dividing by the prostate volume. PSA levels are usually higher in men with enlarged prostates. PSAD can help doctors tell which men have prostate cancer and which men have benign prostatic hyperplasia because a high PSAD (above 0.1) is more often related to prostate cancer.
PSAD of the transition zone is the level of PSA in the blood in relation to the volume of the transition zone of the prostate. PSAD of the transition zone seems to be more accurate than PSAD of the entire prostate at identifying which men have benign prostatic hyperplasia and which men have prostate cancer.
Nomograms are statistical models that predict probable outcome. Prostate cancer nomograms look at PSA results and a number of other factors, such as age, ethnicity, family history, urinary symptoms, DRE results and prostate biopsy results. The nomograms are then used to:
- calculate a man’s risk of developing prostate cancer
- determine the need for a biopsy
- determine how aggressive the cancer may be
Researchers are studying nomograms to see how they can help doctors determine a man’s risk of developing prostate cancer.