Free Prostate-Specific Antigen (PSA) Test Helps in Prostate Cancer Diagnosis
A relatively new and helpful addition in the diagnosis of prostate cancer is the free PSA Test.
This blood test is done in addition to the PSA Test and the finger (digital) examination.
The PSA Test measures a protein in the blood that is produced only by the prostate gland. Some of the PSA in blood is bound to other proteins and the rest circulates freely. This free PSA is a damaged variant of PSA that no longer is able to bind to other proteins and thus circulates in the blood in the unbound (free) form.
For reasons that are not yet known, patients with cancer have less free PSA than patients with benign prostate conditions which can also raise the total PSA level.
PSA Test Level | Interpreting PSA Scores
The standard PSA test measures both components of PSA: free and protein bound. The free PSA blood test measures only the unbound (or damaged) type of PSA, called the free PSA.
Until recently, the usual PSA threshold used to recommend a biopsy has been 4.0 nanograms per milliliter.
This standard approach not only misses 20% of prostate cancers, but also can raise false alarms.
In fact, only about 25 to 35% of elevated PSA levels are actually caused by cancer; benign enlargement or inflammation within the prostate cause the rest.
Dr. Catalona and other investigators have shown that lowering the PSA threshold for recommending a biopsy to 2.5 identifies about 20% more cancers, but also increases the number of biopsies that must be performed.
Looking at the ratio of free PSA to total PSA level in the blood helps the patient to decide about having a biopsy when the total PSA level is mildly elevated (2.6 to 10).
The percentage of free PSA can be used to estimate how likely it is that a biopsy will show cancer:
If the percentage of free PSA is higher than 25%, the likelihood of prostate cancer is about 8%.
If the percentage of free PSA is less than 10%, then the likelihood of prostate cancer rises to 56%.
The use of free PSA measurements can cut back on the number of unnecessary biopsies by about 20 percent, while missing only about 5% of the cancers. Also, some evidence suggests that the missed cancers are usually the least aggressive ones.
The protocol for the PSA study, with which investigators must comply, states that all men whose PSA level is 2.6 or higher should undergo a biopsy; however, the patient may decide to waive the biopsy.
That protocol also states that the public must be informed of the results that the likelihood of the biopsy showing prostate cancer is very low (only 8%) if the per cent of free PSA is higher than 25%.
Men may choose to use this information from the free PSA to decide whether or not to have a biopsy.
If they choose not to have the biopsy, they are taking a small risk that they will be missing or delaying the detection of prostate cancer. If they are willing to accept this risk, they can avoid the biopsy.
Also, and perhaps more important, the free PSA test can clarify the results when a biopsy fails to find cancer even though the total PSA level is elevated.
In that case, it is possible that the biopsy simply missed the cancer.
A low percentage of free PSA may warrant repeating the biopsy, while a higher percentage of free PSA would tend to strengthen the negative result.