Free prostate-specific antigen (PSA) tests can help detect signs of prostate cancer. They measure the level of a biomarker that may indicate the condition.
PSA is a substance that the prostate gland produces. Levels vary according to a person’s age and other factors.
This article will examine when a doctor may use a free PSA test, how these differ from total PSA tests, and what the results might mean.
PSA is a protein that the body produces in the prostate gland and parts of the urinary system. There are different forms of PSA, including total and free PSA. They can either bind to another protein or float freely.
A total PSA test measures all the PSA, including both the bound and the free-floating antigens.
A free PSA test, on the other hand, only measures PSA that is floating freely in the bloodstream and not bound to a different protein.
A doctor can test free and total PSA levels by taking a sample of blood and sending it to a laboratory for analysis. Comparing the two results can help them understand the risk of prostate cancer being present.
Comparing total PSA levels with free PSA levels can give an idea of the risk of having prostate cancer.
According to the American Cancer Society, total PSA results may indicate the following risk:
|Total PSA levels in nanograms per milliliter (ng/ml)||Risk that prostate cancer is present|
|10 or above||50% or above|
Comparing free and total PSA
Comparing the results of total and free PSA tests can give doctors an idea of how likely it is that a person has prostate cancer.
Free PSA ranges can vary, but generally, a higher ratio of free PSA to total PSA indicates a lower risk of prostate cancer. On the other hand, a low ratio of free PSA to total PSA indicates a higher risk of having prostate cancer.
However, PSA tests alone will not confirm a diagnosis, as other factors can affect PSA levels. A digital rectal exam (DRE) and prostate biopsy may also be necessary.
One reason the result may not be definitive is that other factors can affect PSA levels. These factors include:
- an enlarged prostate gland
- anal sex
- prostate stimulation, such as during a rectal examination
- medications to treat an enlarged prostate gland
- vigorous exercise
- urinary tract infections
- prostate surgery
- a prostate infection, or prostatitis
A doctor will consider the timing of a PSA test to ensure minimum interaction with other procedures.
For example, a person may need to wait 6 weeks to have a test after:
- bladder or prostate surgery
- the insertion of a catheter
- a prostate biopsy
- a urine infection
Also, a person should not engage in any vigorous exercise for 48 hours before the test.
PSA levels also increase with age.
The following table shows the
|Age in years||PSA levels in ng/ml|
In people with prostate cancer, a factor known as
PSA kinetics reflect how levels change over time, as can happen when a person has or is undergoing treatment for prostate cancer. The factors involved are PSA velocity and doubling time.
PSA velocity measures how rapidly levels are rising, and it can indicate that prostate cancer is recurring or progressing.
Doubling time is a calculation of how long it takes for PSA levels to double. It can give an idea about how quickly the cancer is progressing. For example, a faster doubling time can suggest a more aggressive cancer.
Free PSA tests are quicker, cheaper, and less invasive than biopsies, but they cannot confirm or rule out cancer.
A doctor may use a free PSA test to help make a diagnosis when total PSA levels are high but a biopsy shows no sign of cancer.
Also, if PSA levels initially indicate a possible risk of prostate cancer, the doctor may recommend a biopsy as the next step.
Testing for other biomarkers
Free and total PSA are not the only biomarkers that can help diagnose prostate cancer.
Tests for other markers may include:
- human kallikrein-related peptidase 2, a biomarker related to PSA
- prostate cancer antigen 3, a protein, tested by taking a urine sample after a DRE
- [-2] proPSA, a component of PSA
- TMPRSS2-ERG gene fusion, tested by taking a urine sample
Free and total PSA tests can help diagnose prostate problems, including cancer.
However, the results are only meaningful when compared with those of a total PSA test. A doctor may order a free PSA test if total PSA levels are high. Even then, it cannot confirm a diagnosis of cancer.
To confirm or rule out a cancer diagnosis, a person will likely need a DRE and a biopsy.