The psa test - Just what is it good for?

Screening for prostate cancer is surrounded by controversy - and with good reason. Although deaths due to prostate cancer have gone down since 1985, when the PSA test - which measures levels of prostate-specific antigen in the blood - was first introduced, it is not certain that the decline in mortality is thanks to this particular screening test.

PSA is a protein produced by the prostate. When the prostate gland enlarges, PSA blood levels tend to rise. But, like all blood tests, this test produces a high number of results that are false-positives - indicating cancer when there is none - and false-negatives - giving the all clear when there is cancer (Urologe A, 2000; 39: 22-6). False-positives appear mostly in men aged 50 or older, those most likely to be undergoing regular screening. In this age group, 15 out of every 100 men will have elevated PSA levels. Of these, 12 will be false-positives and only three will actually have cancer.

There is disagreement over what level of PSA is significant. The usual cut-off point is 4 ng/mL, but a lower level does not guarantee no cancer is present. As much as 25 per cent of men with prostate cancer have PSA levels below this.

PSA levels can be raised by benign prostate enlargement, inflammation, infection, age and race - black men appear to have naturally higher PSA levels than white men. Having ejaculated two days prior to the test can also raise PSA levels (Urology, 1996; 47: 511-6). Indeed, PSA levels alone are not enough to distinguish between cancer and other conditions.

At the heart of the matter is the fact that detection doesn’t always save lives. Even though the PSA test combined with rectal examination can detect small tumours, finding a small tumour does not necessarily reduce the chances of dying from prostate cancer. Also, PSA testing may not help in the case of a fast-growing, aggressive cancer that has already spread to other parts of the body.

PSA testing can identify very slow-growing tumours that are unlikely to be life-threatening. In fact, most prostate cancers are slow-growing and may be present for decades before they cause symptoms.

It is not clear whether the benefits of PSA screening outweigh the risks of the follow-up diagnostic tests and cancer treatments. For example, biopsy to determine the presence of cancer can itself cause significant side-effects such as bleeding and infection (J Sci Am, 2000; 6 [Suppl 2]: S188-92), and can render up to 15 per cent of men who undergo the procedure impotent (J Urol, 2001; 165: 445-54). Worse still, biopsies can also be inaccurate, detecting only 40 per cent of the cases that will go on to develop cancer (Br J Urol Int, 2000; 85: 1078-84).

For these reasons, it is important that the benefits and risks of the PSA test - and all the procedures that follow it - be carefully taken into account before embarking on this course of action.