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. The prostate is part of the male reproductive and urinary systems. PSA is mostly found in semen, which is also made in the prostate, but small amounts of PSA can also be found in the blood. Higher levels of PSA in the blood can be caused by prostate cancer, among other things.

Why a PSA test is done

A PSA test may be done to:

  • help find prostate cancer early in those who don't have any signs or symptoms of the disease
  • check for cancer in those who have signs or symptoms of prostate cancer
  • plan treatment for prostate cancer
  • monitor those with prostate cancer who are being treated with active surveillance
  • find out if prostate cancer treatments are working
  • find out if prostate cancer has come back (recurred) after treatment

A PSA test is often used together with a digital rectal exam (DRE) 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

Make an informed decision about whether the PSA test is right for you by talking with your doctor and considering your risk.

  • If you are at average risk of developing prostate cancer, consider testing from age 50.
  • If you are at high risk of developing prostate cancer, consider testing from age 45. Black men and those with a family history of prostate cancer are considered to be at high risk.

Your PSA levels will determine how often you get tested. The decision to stop testing is based on your PSA levels, age and general health.

Benefits and limitations

Almost every test or procedure has benefits and limitations. Before having any test, it's important to be aware of them so that you're making an informed decision that's right for you. Testing for prostate cancer early – before you have any symptoms – is your choice.

Benefits 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.

Limitations

PSA testing has the following limitations.

A false-positive result on a PSA test suggests that you might have prostate cancer when you actually don'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 testing that is more invasive, such as repeated biopsies. It can also cause you and your family 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. Not all prostate cancers cause a high PSA level. PSA testing misses about 15% of prostate cancers.

Overdiagnosis means diagnosing prostate cancer that would never pose a serious threat to your health. Overdiagnosis can lead to having treatments that aren't necessary (called overtreatment). Research shows that 23% to 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 testing and treatment put you at risk for side effects, including erectile dysfunction and loss of bladder control (called urinary incontinence).

Although PSA testing may find prostate cancer early, research has shown that it still may not reduce the chance of dying from prostate cancer.

How a PSA test is done

A PSA test is a blood test that is done in a lab or hospital. Many things can affect PSA levels including ejaculation, riding a bike, certain medical procedures involving the prostate and certain medicines or herbs. Your doctor will give you instructions for how to prepare for your PSA test.

Certain drugs can lower PSA levels and affect your results. Tell your doctor if you are taking drugs for benign prostatic hyperplasia (BPH), prostatitis, urinary problems, baldness or transgender hormone therapy.

What the results mean

PSA levels depend on age. If you have a prostate, your PSA levels naturally go up as you get older. But PSA can go up and down for many reasons. Most labs consider a normal PSA level to be around 4 ng/mL. Some researchers use a lower cut-off PSA level to help them decide who has the greatest risk of prostate cancer. It's important for you to discuss your PSA test 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 mean that you have prostate cancer. High PSA levels can also be caused by:

  • an enlarged prostate due to BPH
  • 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 level 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 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 result of 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
  • nomograms
  • PSA density

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 TRUS
  • x-rays
  • a bladder exam (called a cystoscopy)
  • a prostate biopsy

Follow-up tests

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 your 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 if you don't have prostate cancer. But those with prostate cancer will have PSA levels that rise more quickly over time than those 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 to 4 PSA tests that are done over 18 to 24 months.

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 those with non-cancerous conditions of the prostate and lower in those with prostate cancer. If you have a total PSA level between 4 and 10, doctors may test your blood for free PSA. A percent-free PSA above 25% is considered normal. Some doctors recommend that if you have a percent-free PSA of 18% or less you should have a prostate biopsy. Other doctors recommend having a biopsy if the percent-free PSA is around 12% or less.

Nomograms

Nomograms are statistical models that predict probable outcome. Prostate cancer nomograms look at PSA test results and several other factors, such as age, ethnicity, family history, urinary symptoms, DRE results and prostate biopsy results. The nomograms are then used to:

  • calculate your 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 your risk of developing prostate cancer.

PSA density

PSA density (PSAD) is the level of PSA in the blood in relation to the size (volume) of the prostate measured during a TRUS. It is calculated by taking the PSA level and dividing by the prostate volume. PSA levels are usually higher in those with enlarged prostates. Some doctors may use PSAD to help decide whether to do a biopsy because a high PSAD (above 0.1) is more often related to prostate cancer.

PSAD of the transition zone (the part of the prostate that surrounds the urethra) 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 is another test that some doctors may use to help decide whether or not to do a biopsy.

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