According to its supporters, Live Blood Analysis marks "a new era of scientific discovery" which is presently conquering the UK. One of the growing number of websites advocating this latest diagnostic tool describes its impact thus: "As the horse gave way to the horseless carriage, so must science now accommodate a new understanding of the body as a whole."

The principle of LBA is fairly simple: a drop of blood is taken from your fingertip, put on a glass plate and viewed via a microscope on a video screen. Despite the claims made for it, LBA is by no means new; using his lately developed microscope, Antony van Leeuwenhoek observed in 1686 that living blood cells changed shape during circulation. Ever since, doctors, scientists and others have studied blood samples in this and other ways.

What is new, however, is what today's "holistic practitioners" claim to be able to do with LBA. Proponents believe that the method provides information "about the state of the immune system, possible vitamin deficiencies, amount of toxicity, pH and mineral imbalance, areas of concern and weaknesses, fungus and yeast", as another website puts it.

Others dare to be much more concrete and claim that they can "spot cancer and other degenerative immune system diseases up to two years before they would otherwise be detectable"; or say they can diagnose "lack of oxygen in the blood, low trace minerals, lack of exercise, too much alcohol or yeast, weak kidneys, bladder or spleen". All this would amount to a remarkable discovery if it were true. But it's not.

No credible scientific studies have demonstrated the reliability of LBA for detecting any of the above conditions. In what was, to the best of my knowledge, the first attempt to assess the value of this method, a practitioner with several years of experience in LBA tested the samples of 110 patients. Twelve had cancer and the task was to identify their samples without knowing further details. The results could hardly have been more disconcerting - just three of the 12 with confirmed cancer were detected, and the authors concluded that the method "does not seem to reliably detect cancer. Clinical use of the method can therefore not be recommended."

So why are holistic practitioners becoming so fond of it? Seeing one's own blood cells on a video screen is, admittedly, a powerful experience. It gives patients the impression of hi-tech, cutting edge science combined with holistic care. And impressed patients are ready to part with a lot of money. American websites explain how a practitioner can make $100,000 (£57,000) annually by purchasing the equipment necessary for performing LBA. The bulk of this money is made not through charging for the test itself but by selling expensive nutritional supplements to the patient with the promise that these will correct whatever abnormality has been diagnosed.

In other words, patients are potentially cheated three times over. First, you are diagnosed with a "condition" you don't have; then a lengthy and expensive treatment ensues; and finally the bogus test is repeated and you are declared "improved" or "back to normal".

One of the most out-spoken promoters of LBA is James R Privitera, a doctor from California. In 1975, he was convicted for selling an illegal cancer "cure" and sentenced for six months. His medical licence was temporarily suspended, and he was placed on 10 years' probation. During this period, Privitera founded two companies for commercialising LBA.

In the US, LBA is used by about 10,000 doctors and increasingly also by chiropractors and naturo-paths. The authorities are becoming concerned about the widespread use of an ineffectual technique. In June 2005, the state health department of Rhode Island ordered a chiropractor to stop performing LBA, stating that the public should be wary of anyone who offers it. In the UK, the General Medical Council recently investigated the case of a doctor making wild claims about LBA and eventually decided to issue him with a "letter of advice".

Advocates of LBA continue to insist that it is a diagnostic method "valuable for the early detection of serious health conditions". In my view, it is fraudulent; those who promote ineffectual diagnostic methods for financial gain are charlatans, and patients who try them are being ripped off.

· Edzard Ernst is professor of complementary medicine at the Peninsula medical school at the universities of Exeter and Plymouth.