First the bad news about DIY tests...

Am I pregnant? What's my blood pressure? Am I HIV positive? Do I have a rogue gene? These are just some of the things you can potentially find out in your own home. Are we grateful? How often is self-testing a good idea, and how often not?ton

You get a little drunk, a little careless: the result is a one night stand with someone you hardly know. Years later, you meet someone marvellous and fall in love. Things get serious and you'd like to do without condoms. You both decide to get tested for HIV.

A quick excursion on the Internet and you've ordered an HIV test to use in the privacy of your own home. You don't bother with pre-test counselling; you're sure you won't need it. You prick your finger, place the drops of blood on the specimen card and send it off to the lab. You settle down for a five-day wait. Finally, you call for the result. It's positive. Your whole world falls apart, but no one is there to help you pick up the pieces.

Sounds unlikely? Not in the US, where companies like the Home Access Health Corporation now offer home HIV testing for just $29.95, with results back in a week. Although such tests are currently illegal in the UK, there is considerable lobbying for change. Hilary Curtis, executive director of the BMA Foundation for Aids, is one of many people worried about giving people a free rein to find out about their future health.

"If you test positive, you need proper counselling to help you express your feelings and get over the initial shock," she says. "It's a very uncontrolled situation, and a testing company, however responsible, cannot really control the circumstances in which someone phones up for their result. People who are already vulnerable may react very badly."

Yet in some areas, DIY tests are something we all take for granted. Twenty years ago, confirming a pregnancy was a very laborious affair involving a trip to the doctor and an agonising wait for the result; now a woman can pop into any chemist and know the result five minutes later. And the do-it-yourself approach is not limited to pregnancy: ovulation, blood pressure, blood glucose levels and cholesterol testing kits are all available in the high street.

Nor is it likely to stop there. As research uncovers a genetic component for many common diseases, tests to determine susceptibility to a whole variety of conditions are likely to become more common. A year ago, the company University Diagnostics launched a postal testing service for cystic fibrosis (CF). This enables anyone to find out whether they are one of the 2 million people in the UK carrying the gene for this disorder.

CF is a nasty condition. It is an hereditary disease of the lungs and digestive system and usually leads to death within 25 years. Sufferers have to undergo daily physiotherapy to the chest, antibiotics, vitamins and tablets to help digestion. Unfortunately, parents who are carriers will never show any sign of the disorder, and many never know they are affected until they pass the disease on to their child.

The University Diagnostics test involves a simple mouthwash, which is sent to the lab for analysis of the cells. By checking for the 16 most common mutations, the company can tell you within 85 per cent accuracy whether or not you are a CF carrier. If you are, it will recommend your partner is tested, too. If you are both affected, you have a number of options: refrain from having children; get your foetus tested during pregnancy and consider termination; or have your child tested after birth.

So if you're a prospective parent, should you be worried about CF? Where the condition is known to run in the family, parents will be offered free tests on the NHS; the rest of us have a one in 2,500 chance of having an affected baby. To pass on the disease, both parents must be carriers - a one in 625 chance - and even then there is only a one in four risk that you will pass CF on to your child.

According to Dr Paul Debenham, managing director of University Diagnostics, the pounds 65 test is still something future parents should seriously consider. "There's a child born every day with CF to couples who had no idea they were carriers. If there's CF in your family you get tested on the NHS, but if you're not at high risk, where can you go?"

However, the CF test caused some concern, prompting the Department of Health to set up the Advisory Committee on Genetic Testing to regulate companies offering home tests. One of the draft guidelines was a stipulation to provide pre-test counselling; although University Diagnostics provides information on whether you should have the test and what it involves, and offers access to its clinical geneticist, Dr Debenham does not feel counselling is as essential as for, say, breast cancer screening.

"CF is all about risk to the next generation, not yourself. It's different to discovering that you have a predisposition to a life-threatening disease," he argues.

There are plenty of tests around that can tell you whether you have, or are likely to develop, a life-threatening disease but many, such as the genetic tests on offer in clinics to high-risk families for cancers of the breast and colon, are not feasible for the general public, as the problems with the affected genes tend to differ from one family to the next.

But without formal legislation regulating genetic diagnosis providers, tests for susceptibility to other conditions such as Alzheimer's could be put on the market tomorrow. How many people would want to know if they were in danger of developing a life-threatening, and currently incurable condition?

Although University Diagnostics is currently evaluating US tests for osteoporosis, and considering checks for genetic factors that predispose to, for example, adult-onset diabetes and cardiovascular disease, Dr Debenham draws the line at offering tests for incurable conditions.

"People need information they can respond to constructively, so we offer tests only if there is something you can do in terms of lifestyle or medical treatment. Alzheimer's is very different to diabetes, where the right lifestyle can minimise its severity, or cardiovascular disease, where it's important to change one's diet or exercise levels."

But even if we could buy tests that would give us information to make positive changes, how many of us would do so? According to Angus Clarke, a clinical geneticist at the University of Wales, many people have a paradoxical response to risk information. He says that in one cholesterol screening programme, those found to be above average risk became fatalistic and didn't bother to change their lifestyle; those at a low risk thought they simply didn't need to bother.

The biggest problem with offering off-the-shelf testing, however, is making sure we have the support to deal with the results. Dr James MacKay, consultant in cancer genetics at Addenbrooke's Hospital in Cambridge, believes genetic home tests would be unlikely to provide the level of information and counselling offered at the moment to women at high risk of breast cancer, for example.

"You have to think carefully before having tests like those for breast cancer about what you would do if you have the faulty gene," he says. "The biggest danger is that you end up with knowledge you wish you'd never had. You also need information about what would happen if you are clear; you may, for example, not be at a high risk of developing genetic cancer, but you may still get the much more common non-genetic form."

Health experts are also concerned about how home tests would be marketed. In the case of HIV, for instance, most people in the UK are not at high risk, says Curtis. But one way to sell kits is to make us feel more threatened than we really are.

"You have either got to play on the fears of the low-risk majority, or encourage the minority of people at high risk to test themselves frequently," she says. "In those cases we should really be encouraging them to change their lifestyle and reduce their risk"n