PSA (Prostate Specific Angtigen)
PSA (Prostate Specific Angtigen) - What it Means?
Prostate Specific Antigen, or PSA for short, is a protein that is produced by prostate cells. The PSA increases as men age, when the prostate enlarges or becomes inflamed and when prostate cancer is present. There are also transient bumps in PSA after digital rectal exam (DRE), ejaculation, and bike riding. PSA is prostate specific but not cancer specific. There are different ways in which to look at your PSA.
When PSA was introduced a cut off of 4.0 was used to achieve what scientists and doctors felt to be an acceptable level of sensitivity (how many it catches) and specificity (how accurate it is if positive). We soon realized that although this was helpful it missed too many cancers in young men and resulted in too many biopsies and over diagnosis in older men.
Age Based PSA
We know that PSA rises as you age. By having a lower cut off in younger men and a higher cut off in older men we are able to catch cancers sooner in the younger population while eliminating a lot of unnecessary biopsies in the older population. There are several accepted reference ranges; one of the most common is this:
46-55 < 2.5
56-65 < 3.5
66–75 < 4.5
>75 < 6.5
PSA Free and Total
While PSA is a protein that is in our blood stream it is not all existing in the same form. Some is free floating and some is bound to other proteins. It was found that benign disease (not cancerous) tends to have higher levels of free PSA (>25%) and patients with prostate cancer tend to have lower free PSA (<15%).
These numbers can help stratify the risk in some men. For example if a patient has a PSA of 8.0 and had a negative biopsy, knowing that he has a PSA free >25% gives us more assurance that he doesn’t have prostate cancer.
We know that PSA rises as the prostate gets larger. Men with large prostate glands tend to have higher PSA’s.
A normal size prostate is about 25 grams. We consider anything larger than 40 grams a enlarged. A normal PSA density is 0.1 PSA for every gram. This makes sense as a large, 40 gram prostate would have a corresponding upper limit PSA of 4.0. This helps us in men who have had a negative biopsy and have enlarged prostates. We are less likely to worry about someone with a PSA of 6.0 if there prostate is > 60 grams. Conversely, we are more worried if the PSA is elevated in a smaller prostate (PSA of 4.2 in a 17 gram prostate).
One of the most accurate ways of diagnosing prostate cancer is looking at the rate of rise of PSA. A PSA should never go up more than 0.7 points per year. For example, it would probably be better to have a PSA count over 3 checks go from 5.0 to 5.1 to 5.2 then it would be to have your PSA count go from 1.0 to 2.0 to 4.0.
Prostate Specific Antigen is only a tool. It is far from perfect but it is useful when the patient and their physician work together to make shared decisions. Since the onset of PSA screening the death rate from prostate cancer has almost been cut in half. Prostate Cancer is still the number two cancer killer of men in the United States. There is still much to be learned. Getting as much information as possible and making shared decisions with your doctor will help you understand the risks and benefits of checking your PSA and what to do with the information.
From an article I wrote originally for about.com