More than 10,000 cases of cancer of the prostate, the walnut-sized gland that sits just below the bladder, are diagnosed every year in the UK. It is the second-most common cause of cancer deaths in the US and the fourth in the UK.
What is it?
The controversial prostate-specific antigen (PSA) test detects raised levels of PSA, an enzyme produced by the prostate and thought to be an indicator of prostate cancer. When the gland enlarges, greater amounts of PSA are produced in the blood. The test is a simple blood test, taking a sample from a vein.
Is it accurate?
As we've written before, it's not clear that the test saves lives. At this time, there are no published randomised trials confirming that early detection improves the long-term prospects for patients with the disease (BMJ, 2004; 328: 301-2):
* It produces a high number of false positives - indicating cancer where none is present - and a high number of false negatives - failing to detect cancer when it's present (Urologe A, 2000; 39: 22-6). False positives occur mainly in the over-50s, who are more likely to be undergoing regular screening. In this age group, 15 out of every 100 men will have elevated PSA levels, of which 12 will be false positives and only three a true indication of cancer.
* Doctors can't agree as to what blood level of PSA is significant. The usual cut-off point is 4 ng/mL, but less than that doesn't automatically mean no cancer. As much as 25 per cent of men with prostate cancer have PSA levels below this.
* The test cannot specify the type of cancer present. Prostate cancer has many forms, with variable growth rates and degrees of fatality; the test doesn't distinguish between a fast- or slow-growing cancer, or between a harmless and dangerous type. Also, as research has yet to define what constitutes a clinically important prostate cancer, doctors have no clear idea of what the specific target of screening should be.
* The test cannot distinguish a good prognosis. Current test parameters can identify a few men with prostate cancer who have good prospects for long-term survival and a few with poor prospects, but not the vast majority of men with prognoses in between (Ann Intern Med, 2002; 137: 917-29).
* A positive test is usually confirmed by biopsy, another not particularly accurate procedure. Biopsies detect only 40 per cent of those who will go on to develop cancer (Br J Urol Int, 2000; 85: 1078-84).
* Elevated PSA levels can be caused by a number of other factors, including:
- non-fatal forms of prostate cancer
- common prostatitis (inflammation of the prostate)
- benign prostatic hyperplasia, a natural enlargement of the prostate that often occurs with ageing
- medicines for the prostate and the drug finasteride, used to treat male-pattern baldness
- ejaculation, which can elevate PSA levels for up to 48 hours afterwards
- any physical activity, especially cycling, which can physically stimulate the prostate to produce more PSA.
Is it safe?
Although the procedure itself isn't hazardous, a wrong diagnosis can set in motion the entire modern cancer regime, which can be harmful, if not fatal (BMJ, 2004; 328: 301-2). As many as one in six men will be diagnosed with the disease, compared with about one in 29 who will die from it (Ann Intern Med, 2002; 137: 917-29). Prostatic surgery can result in incontinence and loss of sexual function, and hormone treatment can lead to loss of libido and the development of breasts.
Safer uses of the PSA test
* Make sure your PSA test is carried out before a digital rectal examination as physical manipulation of the prostate can stimulate the gland to produce more PSA.
* Find out your percentage of free PSA. PSA circulates in the blood as free PSA and bound to a protein. Men with prostate cancer tend to have less free PSA than men without prostate cancer (Ann Intern Med, 2002; 137: 917-29).
* Use the PSA test only as part of a more comprehensive test for prostate cancer, and consider grossly elevated levels of PSA as only a red flag for further investigations.