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Diagnosis and treatment of prostate cancer (part 1)

Q1: Just how reliable is the PSA test? I have heard reports that it can give false positives, which can worry people unnecessarily. Is a physical examination sufficient? Is there any evidence that there is a genetic factor involved? Thank you. Tony Whyte

A1: The PSA is the best overall test we have but it is not ideal. Its main problem is apparent false positives. That is to say if the normal cut off point is between 2.5 and 4.5, depending on a man's age, and the PSA is above the cut off point but no significant tumour is detected when the patient has more tests the result is then described as a false positive.

The clinical importance of false positives has been, in my opinion, grossly exaggerated for medico political reasons. Nobody wants to have a false positive test as this makes a biopsy likely. From the patient's point of view this is uncomfortable or actually painful procedure especially if the prostate is unusually large or the patient has a bleeding tendency. Very occasionally infection results from a biopsy. Fortunately this is a rare complication and antibiotics are given to lessen the likelihood. The other objection to the false positive is that an anxious patient may have to wait for some days while one is being organised.

The political and administrative objections to PSA testing are firmly held, but are usually hidden as neither politicians, nor administrators, want to admit that considerations of this sort are put before the patient's welfare and the possible detection of a lethal cancer at a time when the tumour might be totally eradicated. The objection to widespread PSA testing from the administrative point of view is that although the PSA blood test is cheap and quick the follow up tests including biopsy for those with a false positive are time consuming and expensive. A transrectal prostatic biopsy with ultrasound requires highly trained staff, relatively expensive equipment and it blocks the examination (outpatient) facilities for half an hour or so. For one reason or another it costs several hundred pounds.

Furthermore, and administrators like to admit this even less than they like to be heard to advocate any policy that might prevent early diagnosis, if the positive tests turns out to be positive and the patient needs treatment the costs are very great. It was once said to me by a high ranking civil servant that if the NHS discovered all cases of prostate cancer that needed operative treatment it would bring the surgical departments in hospitals to a grinding halt and bankrupt the service.

The PSA test can be made more accurate by carrying out two different readings on the blood. Not all the PSA is carried in the bloodstream in the same form. The PSA may be either free or bound to one of two different proteins. In patients with prostatic cancer the amount of unbound PSA in the bloodstream is proportionally less. The cut off point is taken as 18 per cent. Proportions above this indicate that benign prostatic enlargement is more likely than malignancy, percentages below eighteen make it less likely.

Another test for PCA (prostatic cancer antigen 3) is done on urine that has been passed by the man just after his prostate has been thoroughly massaged per rectum. Prostatic massage is not painful but it is physically and emotionally uncomfortable for the patient. It is an embarrassment that it is worth suffering if achieving the correct diagnosis is thereby achieved and a life saved.

The number of false negatives, this of course depends on the level at which the pass mark is set, is much less than false positives but false negatives are greatly more dangerous than false positives. A false positive will involve the patient in discomfort and a minimal risk of infection. On the other hand a false negative may lead to false reassurance and a misdiagnosis that can be lethal. It is little consolation to a patient in whom there has been a false negative to know that the type of cancer that has a false negative is the type of one that is most likely to cause death. A word of warning. I was careful to refer to apparent false positives because it has been found that patients who have a so called false positive but apparently normal follow up tests are appreciably more likely to develop detectable cancer of the prostate at a later date than are those whose PSA has always been normal.

Physical examinations are important but the doctor has to be skilled in doing them and should be doing them all day and every day if his or her opinion is to be a well founded one. The physical examination of a prostate per rectum only allows that proportion of the prostate that can be reached by a finger to be examined. If only one test can be done it is far more important to have the PSA than a rectal examination.