Prostate Specific Antigen

PSA is an enzyme normally produced by the prostate and found in the blood.

 

Prostate specific antigen (PSA) is a protein which is produced by the prostate gland of all men - that is, it is normally present. PSA is made the epithelial cells of the prostatic acini (the individual glandular structures within the prostate - there are thousands and thousands of acini).  PSA is an enzyme (specifically a 'serine protease), the normal function of which is to liquefy the semen after it has been ejaculated.

 

The PSA measured in the serum can arise from non-cancerous conditions of the prostate as well as prostate cancer.

 

PSA normally leaks from the prostate gland into the bloodstream where it can be measured with a blood test. Because PSA is made by normal prostate tissue, PSA is always detectable and the blood even in normal men. There are many causes of why the PSA may be elevated.

  1. Bigger prostate = more PSA production = more leak in to the blood stream. This is the most common cause of elevations in PSA. Benign prostatic hyperplasia is the medical term for non-cancerous prostate enlargement
  2. Prostate cancer can result in increased leak of PSA into the blood stream
  3. Anything which damages the barrier between the prostate gland and blood stream. Examples:
    1. Any sort of prostate surgery (e.g. TURP)
    2. Any sort of urethral instrumentation (e.g. scope or catheter)
    3. Biopsy of the prostate
    4. Vigorous prostate exam or massage
    5. Prostate infection

It can be seen that there are many potential causes for increased levels of PSA - PSA IS NOT SPECIFIC FOR PROSTATE CANCER.

As a result, an elevation in PSA cannot diagnose the prosate cancer (with rare exceptions).

 

Different methods of measuring PSA can provide different results but the differences are not enough to affect clinical assessment. The various methods of measuring PSA may result in a 25% difference in the result. This sounds like a lot, but in the vast majority of men, the differences in the reported level between the different assays is a clinically insignificant 1 mcg/L or less for PSA levels under 10 mcg/L. Changes of this magnitude generally result in a 1% or less change in the absolute level of calculated risk of having prostate cancer (using the PCPT risk calculator). From a practical perspective, having a 6% risk of having prostate cancer vs. a 7% risk of prostate cancer, for example, is clinically irrelevant.

 

It is important to note that there is already a substantial amount of 'background random noise' in PSA measurements that results from day to day changes in the body. The consequence is that we are always looking at a rough estimate of what the level is - and to repeat the level if there is any uncertainty, especially if the PSA is high and there is nothing else to suggest an issue. Often a repeat measurement will show the PSA is lower.

 

The issue with differences in the PSA measurement were only an issue when the outdated approach of using 'PSA cutpoints' was used to determine if a biopsy should be done. In this outdated approach, men were treated differently based on 'PSA thresholds' - for example, a man was called 'normal' if the PSA was less than 4 mcg/L but 'abnormal' if the PSA level was over 4 mcg/L. Using 'PSA cutpoints' is a poor way to assess risk because it ignores the fact that there are other things besides PSA that help determine risk (age, race, how the prostate feels and family history of prostate cancer) and that every many has risk no matter what the PSA.

 

PSA exists in several different forms. These are measured by PSA assays in different ways.

 

There are different forms of PSA which may be measured including a free PSA.

  1. Total PSA = free PSA (fPSA) + complexed PSA (cPSA)
    1. cPSA = 70-90% of the total PSA; this PSA is bound to alpha-1-antichymotrypsin and other proteins in order to prevent potentially damaging protease activity
    2. fPSA = 10-30% of the total PSA; this is found in 3 forms
      1. benign PSA (BPSA): mainly results from PSA made in the transition zone where BPH occurs
      2. intact PSA (iPSA) similar to native active PSA but is enzymatically inactive
      3. proPSA (pPSA): mainly from the peripheral zone of the prostate; most commonly assicated with prostate cancer

 

Which of these is being reported on your 'PSA measurment'? If you look on your PSA report, you may see a note about the type of assay that was used (e.g. Hybritech Tandem-R or other). The variation between assays is generally less than 3-5% - this is a level far below one which would affect clinical decision making. The assays are all standardized to the WHO 90:10 PSA standard which uses 90% cPSA and 10% fPSA. 

 

There have been many attempts to refine the abililty of PSA measurements to be better at detecting prostate cancer. These have included free vs. total measurements, BPSA and proPSA. Unfortunately, these have not provided any meaningful clinical utility to using a fairly straight forward risk assessment tool such as the PCPT calculator.

 

Clinical use of PSA

 

PSA can be used and a number of different ways and it is important to define how this particular test is being applied since his interpretation can vary significantly.

  1. Screening for prostate cancer
  2. Surveillance for patients with known prostate cancer
  3. Post-treatment monitoring: note that interpretation depends on the type of treatment
    1. Surgery
    2. Radiation
    3. Hormone ablation
    4. Other
  4. Assessment of prostate volume and response to treatment for prostate disease

On the Web


General Prostate Cancer Web-Resources

Memorial Sloan-Kettering Cancer Center in New York is an excellent resource for information on prostate cancer. Balanced, unbiased discussions of the disease, including discussion regarding some of the controversies in prostate cancer.

General Information on Cancer

UNDERSTANDING CANCER - Metrovan Urology info on the principles of diagnosis, staging, prognosis and more.

American Cancer Society

BC Cancer Agency: Good general website from the British Columbia Cancer Agency. Has contact information on locations.

National Cancer Institute: Excellent source of understandable and mainly unbiased information. Several very good brochures on every stage of prostate cancer.

National Comprehensive Cancer Network: peer-reviewed expert content/prostate cancer guidance on evidence-based cancer diagnosis and management. Best for Prostate and Kidney Cancer. The most in-depth information is located in the physician section and requires registration.