What is a PSA test?
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PROSTATE CANCER
What is a PSA Test?
PSA is a blood test that is performed to assess a man’s risk of having prostate cancer. PSA stands for prostatic specific antigen. It is a chemical that is produced by cells within the prostate gland and small amounts are normally found in the blood stream. PSA is also found within a man’s ejaculate fluid where its role is to liquify seminal fluid. PSA is used in conjunction with a DRE (rectal examination to assess a man’s prostate) to help determine the prostate cancer risk of a particular individual.
Our understanding and interpretation of the PSA test has evolved significantly over the last few years. Previously a value of over 4ng/ml was considered elevated and less than this was considered a normal value. This is no longer the case. In an attempt to improve the utility of the PSA test there are several aspects of the test that we now look at. These include:
- Absolute value
- Relationship to the median value
- Age specific value
- Free/total ratio
- Velocity
The reason we now look at these various aspects of PSA is with a view to improve the diagnostic yield for prostate cancer and to reduce the number of unnecessary prostate biopsies. It should be noted that part of the confusion with the test is because PSA can be elevated in several prostate conditions including:
Age Specific Values
As men get older there tends to be a gradual increase in prostate size due to an increase in prostate cell numbers. This is called benign prostatic hyperplasia or BPH. It is this benign enlargement which results in urinary symptoms in many men but is also responsible for a gradual increase in PSA. It is for this reason that we accept PSAs of different levels for men in various age brackets.
| AGE | PSA (NG/ML) | MEDIAN VALUE (NG/ML) |
|---|---|---|
| *Figures are median (average common figure) per age decade. | ||
| 40s | < 2.5 | 0.6 |
| 50s | < 3.5 | 0.9 |
| 60s | < 4.5 | 1.2 |
| 70s | < 5.5 | |
PSA Median Values
More recently we have started to compare values to the median (most common value) for a particular decade. The Urological Society of Australia and New Zealand now offer a PSA test at the age of 40 years following discussion regarding the pros and cons of the test. A PSA of 0.6ng/ml (median value for 40s) or less is indicative of a lower risk of prostate cancer whereas a value above 0.6ng/ml but lower than 2.5ng/ml should be watched more closely with more regular PSA testing. This also facilitates PSA velocity or rate of change to be calculated.
PSA Velocity
Serial measurements of PSA can provide more valuable information than a single sample and are used to help determine a man’s likelihood of having prostate cancer. In terms of kinetics or velocity our aim is to determine the rate of PSA change over time. Strictly speaking we need 3 values over 12 month period for this to be accurate. A value of 0.3 ng/ml/year is in keeping with BPH or a benign enlargement of the prostate where as a value of up to 0.6-0.7ng/ml/year is more suggestive of a cancerous process.
Free/Total Ratios
PSA that is found within the blood stream is partly bound to proteins which act as part of a transport system. PSA that is produced by prostate cancer sticks more readily to these proteins than does PSA that is produced by benign conditions. As a result there exists an inverse correlation between free/total ratio and prostate cancer risk. For those men with a free/total ration 24% the risk of prostate cancer is around 7%.
If your PSA is elevated or there are abnormalities of the other aspects of PSA this may lead to a prostate biopsy being recommended to further assess your risk of having prostate cancer.
Prostate Health Index (phi)
To date there are several aspects of the PSA test which are evaluated to improve the specificity of the test. The issue with the test has been that although a specific test for the prostate, different conditions which affect the gland, can result in a falsely elevated value. These conditions include inflammation and enlargement or BPH.
Recently an assay for a new marker for prostate cancer has become available. This is called truncated proPSA (p2PSA). This molecule circulates as part of the Free PSA fraction, and is present as a higher proportion of the Free PSA fraction in patients with prostate cancer. A recent study has shown that it has a greater specificity for cancer detection when the proPSA was combined with Free PSA and total PSA, as a calculation known as the Prostate Health Index (phi).
TRUS Biopsy
A biopsy is an investigation that involves placing a small needle into the prostate gland to obtain samples of tissue. These are then sent to the laboratory where they are examined under a microscope looking for cancer cells.