Medical technology is bringing tests previously confined to the laboratory into our homes, allowing us to search for signs of disease well before any obvious symptoms appear. Is this new patient power - or another health risk? Peter Baker investigates.

If a mole changes colour, a strange lump appears, or our bowel or bladder habits change, then we know we should visit a doctor. But we can now use more than our eyes, fingers and general sense of well-being to detect potential health problems.

Most pharmacies now stock a range of home-test products. Cholesterol tests and blood pressure monitors have been available for several years but several new devices have recently come on to the market. These enable us to analyse our stools for traces of blood (a possible symptom of a wide range of bowel disorders, including cancer), probe our urine for excess glucose (which could indicate diabetes) and measure our body fat to check if it is a risk factor for heart disease.

If we lived in the United States, we could buy more diagnostic kits than might be found in many British hospitals. Anxious Americans can check themselves for HIV, chlamydia, helicobacter pylori (the bacteria linked to peptic ulcers), tuberculosis, hepatitis, pneumonia and prostate specific antigen (a marker for prostate cancer). They can even inspect their children's urine for evidence of illegal drug use.

Although home-testing for HIV is specifically banned in the UK, there is no regulatory framework that prevents other home tests being marketed here - and they almost certainly will be. "We're all so greedy for information that this is an unstoppable process," says Dr David Murfin of the Royal College of General Practitioners.

But while home tests may well seem like a hypochondriac's dream come true, should the rest of us be investing in them? Common-sense suggests that any product capable of detecting health problems at the earliest possible stage must be beneficial. After all, the sooner a problem is detected, the greater the likelihood of successful treatment.

Cecilia Yardley of Colon Cancer Concern argues that "18,000 people a year die of bowel cancer while research suggests screening could detect up to 18 per cent of cases before they would otherwise have been picked up and when they can be successfully treated".

Doubts remain about the reliability of some of the tests, however. While it is tempting to assume that any device incorporating chemicals, test tubes and micro-chips must be accurate, this is by no means always the case. An independent evaluation of three blood-pressure monitors found only one met standards specified by the British Hypertension Society.

Meanwhile, an investigation of Boots' cholesterol test by the Government's medical devices directorate concluded that it was "capable of giving accurate results when used by a trained professional [but] when used by the consumer the potential for obtaining incorrect results is increased".

Boots amended their test after this report but the British Heart Foundation remains concerned about the potential for mistakes. The biggest danger is a so-called "false negative" response. This means that a test has missed signs of potential disease and incorrectly suggested the person is healthy. It is not difficult to imagine someone carrying out a home test for signs of diabetes, misreading the results and, believing themselves to be in the clear, ignoring symptoms like fatigue or increased thirst. In this scenario, the result of the test could actually be to delay effective treatment rather than bring it forward.

The tests can also offer false reassurance. A cholesterol test could, quite accurately, show that someone has an acceptable level. "But cholesterol is just one risk factor for heart disease," says Dr Murfin. "A good result could make people think they're not at risk even though they're eating badly, not exercising enough and smoking 40 a day."

Then there is the anxiety inevitably caused by a positive result. "The problem is that people can be testing themselves for what could be serious conditions," explains a spokeswoman for the British Medical Association.

"They could get frightening or worrying results and they don't have any information or support."

This is a key reason why HIV home-testing is prohibited in the UK. In some cases, moreover, the result will ultimately prove to be a "falsepositive" - in other words, further tests will reveal the person to be healthy. Although self-testing seems certain to grow and can be seen as another way in which we can take greater control over our own health, serious questions remain about its advisability. Until these are resolved, whether we choose to test ourselves or not, the best medical advice remains: if we are worried about our health, we should see a doctor.

dread disease detectors

Cholesterol Test (Boots, pounds 7.99)

A high cholesterol level is a risk factor for heart disease. To use the device you prick your finger with a lancet and squeeze blood into a meter. However an inaccurate low result could provide false reassurance, so the British Heart Foundation says initial tests should be carried out only by a GP.

Blood pressure monitor (Digital monitors start at around pounds 50)

Only one is approved by the British Hypertension Society - Omron's HEM 705 CP, pounds 169.95.) High blood pressure, usually symptomless, is a risk factor for heart disease. You attach a cuff to your wrist and read the display. The British Heart Foundation believes a monitor may be of use to people already known to have high blood pressure.

Urine test (Kent Pharmaceuticals, pounds 7.95)

Urine abnormalities may signify diabetes, kidney and liver disease or urinary tract infections. The test, based on a hospital version, is done with a card held in the urine stream then compared to a coloured chart. The British Diabetic Association believes kits can be a preliminary way of detecting glucose intolerance but should not be relied upon for a diagnosis

Blood in stools test (Kent Pharmaceuticals, pounds 9.95)

Invisible traces of blood in faeces can signify bowel cancer and other diseases. A stick is dipped in a sample, placed in a bottle and the result is seen. The manufacturer claims 99 per cent accuracy. Colon Cancer Concern says a test will let individuals identify a problem needing treatment.

Body-fat monitor (Tanita, pounds 89.99)

High levels can be a heart-disease risk factor. You stand on a device like a scales and it passes a minute current through your body, when your proportion of fat is displayed. Accuracy said to be plus or minus 5 per cent