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PSA is prostate specific antigen, a protein made by prostate cells that shows up in your blood with a PSA blood test. PSA levels increase with prostate cancer, so PSA has been used as a screening test for prostate cancer for many years. A cancer screening test is a test your doctor orders to find cancer early, before you have any symptoms. [1-4]
Studies show that PSA screening saves lives for people with prostate cancer, because it finds cancer early when it is most treatable.  Prostate cancer is the second leading cause of cancer deaths in men after lung cancer.  So, you would think all men should get PSA testing throughout their lives to screen for prostate cancer. Unfortunately, it is not that simple. [1-4]
What’s a Normal PSA?
The main problem with PSA is that it is not a very reliable test. There is no settled normal range that can tell if you have prostate cancer.  You can have a normal PSA and still have prostate cancer, and you can have an abnormal PSA and not have prostate cancer. In fact, most men with a high PSA don’t have prostate cancer. 
A PSA level goes up with prostate cancer but it also goes up with age. PSA can go up with two very common prostate conditions, benign prostatic hyperplasia (BPH) and inflammation of the prostate (prostatitis). [1-3] PSA can even go up after having sex or riding a bicycle. 
PSA is measured in nanograms per millimeter (ng/mL). Although most doctors would consider a PSA of less than 4 ng/mL normal, some doctors say anything over 2.5 is abnormal. This is what the studies say:
- About 15 percent of men with a PSA under 4 have prostate cancer.
- About 25 percent of men with a PSA between 4 and 10 have prostate cancer.
- About 50 percent of men with a PSA over 10 have prostate cancer. 
The Problem With Prostate Cancer Screening
The only way to diagnose prostate cancer is with a prostate biopsy. You would think it would be worthwhile to do PSA screening for all men and a prostate biopsy on every man with an abnormal PSA, but that is not the case. [1-4] Prostate biopsy has risks. It can cause bleeding, pain, urinary retention, and a urinary tract or prostate infection.  Since most men with a high PSA do not have prostate cancer, these risks are important to consider. [1-4]
There is also a problem with prostate cancer itself. Even if a biopsy shows prostate cancer, many men with prostate cancer will never have any serious problems from their cancer. They will die from something else. For these men, the treatment can be worse than the disease. Treatment can cause serious urinary and sexual problems. [1-4]
Because most men with a high PSA do not have prostate cancer and many men with prostate cancer do not need treatment, guidelines for PSA screening rely heavily on a discussion of the risks and benefits with your doctor. [1-5]
PSA Screening Recommendations
Different medical organizations have slightly different guidelines. These are the guidelines for PSA screening from the American Urological Association, the specialists who diagnose and treat prostate cancer:
- PSA is not needed under age 40.
- PSA is not needed for men ages 40 to 54 unless they are at higher risk for prostate cancer. Being African American or having a family history of prostate cancer puts you at higher risk.
- PSA should be considered for men ages 55 to 69 after discussing the risks and benefits of the test and of prostate cancer treatment.
- PSA is not recommended for men age 70 or older. 
What Happens After a PSA Test?
If you and your doctor decide to do a screening test and the test is normal, you may not need to think about another test for a few years. If the test is abnormal, you and your doctor will discuss the risks and benefits of further testing:
- You may have another screening test before or after your PSA called a digital rectal exam (DRE). During DRE, your doctor places a gloved finger into your anus to feel your prostate. If your prostate feels enlarged or abnormal, your doctor may recommend further testing or a biopsy.
- Further testing could include another PSA test or a more specialized PSA blood test that may tell your doctor more about your prostate cancer risk. There are many options including tests called prostate health index, PSA velocity, PSA density, free PSA and others.
- Further testing may include an imaging study (MRI or trans rectal ultrasound)) of your prostate. 
- Your doctor will consider any further testing, your PSA test, and your DRE, along with your age, overall health and your risk factors. You and your doctor should use this information to weigh the risks and benefits of a prostate biopsy. [1-4]
The bottom line on PSA as a screening test for prostate cancer is that it is still a valuable test when used along with your DRE, other tests, and your personal risk factors to help you and your doctor decide if you would benefit from a prostate biopsy. [1-5]
Other Uses for PSA Testing
Outside of prostate cancer screening, there are other uses for PSA testing. If you have had a biopsy that shows a slow growing prostate cancer, your doctor may advise watchful waiting. In this case, PSA testing may be done periodically to see if your cancer becomes more active. Any increase in PSA could be considered abnormal. If you have been treated for prostate cancer, periodic PSA testing may be done to see if there are any signs of cancer coming back. Any increase would be abnormal. 
If you have symptoms of prostate cancer, PSA testing may be done to find out if you have prostate cancer. When you already have symptoms, PSA is no longer considered a screening test. It is a diagnostic test.  Let your doctor know if you have any of these prostate cancer symptoms:
- Problems passing urine, like slow flow, increased frequency, or loss of control
- Blood in your semen or urine
- Trouble getting or keeping an erection
- Weakness or numbness in your legs or feet
- Bone pain in your hips, back, or ribs 
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- Memorial Sloan Kettering Cancer Center, Prostate Cancer Screening
- American Cancer Society, Screening Tests for Prostate Cancer
- American Society for Clinical Chemistry, Lab Tests Online, Prostate Specific Antigen (PSA)
- CDC, Prostate Cancer, What Are the Benefits and Harms of Screening?
- American Urological Association, Early Detection of Prostate Cancer (2018)
- American Cancer Society, Signs and Symptoms of Prostate Cancer