One day I'm looking at my face in the mirror when I notice some tiny pearly-white spots around my eyes. They're barely noticeable, but still, they worry me. They've appeared at the same time as some other facial lesions, little cyst-like bumps, and when I'm increasingly conscious of greying hair, thickening waist, diminishing muscle tone and all those other banal signifiers of early middle age. I mention the spots to a dermatologically aware friend at work and immediately she says: 'They're probably cholesterol deposits. Cholesterol collects around your eyes. Have you done a test recently?'

A cholesterol test? No.

'Have you ever done one?'

No.

'How old are you?'

Thirty-five.

Sorrow and shock collide on her face. 'You're 35, and you've never done a cholesterol test? You should do one every year once you reach 30.'

At lunchtime I go to Boots. There's a whole wall full of test kits. It takes me a little while to find the cholesterol one. While I'm looking I spot one for blood glucose. I decide to buy it, too - you never know, I might be diabetic. Perhaps that's why I'm tired all the time. Come to think of it, I'm quite thirsty at the moment, and thirst is a symptom of diabetes, isn't it? The blood-glucose test has a digital monitor and is called a FreeStyle Lite. Apparently the results are really easy to read. So that's good.

When I get home I go upstairs and empty the contents of the cholesterol test kit on to the bed. There's a test card, a lancet and a plaster. I read the instructions. I remove the test card from its foil sachet and prime the lancet. I'm more nervous than I expected. Maybe I'll have a quick coffee to calm my nerves. Or does coffee raise your cholesterol level? I'm sure I read somewhere that it does.

The first drop of blood I let fall on to the test area isn't very big. As I try to squeeze out another, my fingertip accidentally touches the test area. This is a no-no, but never mind.

I wait three minutes and rip off the strip so that I can match the green shade of the test area to the green shade on the colour chart. But the test area is all mottled. Some bits are light green, some bits are dark. There's a bit of red in there, too. The result seems to show that I have a cholesterol level of 3.9mmol/l (millimoles per litre of blood), well within acceptable limits. But it's hard to say for certain ...

In ancient Egypt, women tested themselves for pregnancy by urinating on wheat and barley seeds. If, over the course of several days, the barley grew, it meant the woman was carrying a boy. If the wheat grew, the baby was a girl.

The first home-test kits for pregnancy went on sale in the US in 1978. They were a right old faff. 'For your $10,' an article in Mademoiselle magazine noted, 'you get pre-measured ingredients consisting of a vial of purified water, a test tube containing, among other things, sheep red blood cells... as well as a medicine dropper and clear plastic support for the test tube, with an angled mirror at the bottom.'

Nearly 30 years on, pregnancy test sticks are commonplace, as are home tests for stomach ulcers (or rather the bacteria that can cause them, Helicobacter pylori), diabetes, allergies and sundry STDs, as well as more obscure conditions such as bowel polyps. It's a booming industry. The market for these kits will reach £99m this year and has risen by 30 per cent in five years. It's estimated that it will reach £158m by 2012.

Our attitude to our health has changed radically in the past decade, and doctors don't always know how to respond. 'Ten or 15 years ago, people were coming in and asking us to take their pulses and read their auras and tongues,' says Dr Rajendra Sharma, medical director of the Diagnostic Clinic in London, which provides health-screening services as well as selling home-test kits. 'Now, they expect more scientifically credible testing.'

The internet has made it easier to inform - and misinform - ourselves. Sometimes sites double up as medical encyclopaedias and diagnostic service providers: at www.netdoctor.co.uk we can post questions which a 'real' doctor answers. And, of course, the reach of Google makes it thrillingly easy to access academic papers about drug trials and obscure cancer treatments, or at least condensed abstracts of them. Doctors hate this ad hoc acquisition of pseudo-expertise - so much so that a couple of years ago they retaliated by launching their own site, www.besttreatments.co.uk (in association with the British Medical Journal), which sifts through the flotsam to bring you information from accredited sources. But do you know anyone who's used it?

The people responsible for this surge in self-empowerment are said to be the 'worried well' - a quietly savage euphemism for 'hypochondriacs'. But although the word hypochondria is tossed around casually, it's actually a very specific complaint. The American Psychiatric Association's Diagnostic and Statistical Manual of Mental Disorders defines it as a preoccupation with the fear or idea of having a serious illness, based upon a misinterpretation of bodily sensations. Crucially, this fear or idea must persist for at least six months despite appropriate medical examination and reassurance.

Professor Paul Salkovskis is clinical director of the Centre for Anxiety Disorders at the Maudsley Hospital in south London. For him, classic hypochondria is a serious illness in its own right - a type of obsessive-compulsive disorder (OCD) best treated using cognitive behavioural therapy. Does he think the popularity of these tests is a sign that we're becoming more hypochondriacal?

'In terms of the patients I treat, I haven't noticed a surge in hypochondria since these tests have gone on the market,' he says. 'It's the same with health information on the internet. If people have a susceptibility to that sort of phobia, it doesn't make any difference where they get their information from. If it wasn't the internet it would be a medical encyclopaedia. The problem is the underlying anxiety, not what's feeding it.'

The worried well, then, are mid-spectrum hypochondriacs adept at framing their fears in terms which suggest they reflect not irrational self-absorption but a desire to live healthy, happy, unproblematic lives. Often, they're educated middle-class people who pride themselves on their ability to tell good information from bad. They carry in their heads a Platonic ideal of wellness and believe that that is how they should feel the whole time.

Banning smoking in public and issuing stern alcohol limits are part of the government's drive to get us to take greater responsibility for our health - to 'manage' it, as if we were all on the board of MyBody plc, so that we don't have to 'consume' more healthcare than necessary. This is fair enough: it's certainly the only way the NHS is going to cope in 25 years' time, when the number of adults of state pensionable age is expected to exceed the number of children by 34 per cent.

But when does rational concern for one's health spill over into irrational obsession? Every year for the past 20 years, three times Formula One world champion Sir Jackie Stewart has flown his wife and two sons to the Mayo Clinic in Minnesota for in-depth health checks in which every inch of their bodies is scanned. In March 2000, thanks to these tests, his eldest son, Paul, was found to have a potentially fatal type of cancer - non-Hodgkin's lymphoma of the colon. A few months later his wife, Helen, was diagnosed with breast cancer. And Sir Jackie himself was found to have a malignant melanoma on his right cheek. In the light of this, is it fair to brand him a 'somatiser', excessively preoccupied with the state of his body?

Let's take another example. In the month following Kylie Minogue's diagnosis with breast cancer in May 2005, referrals to the Breast Unit at Royal Gwent Hospital in Newport, South Wales, jumped 61 per cent. The fact that, despite this, there was no actual increase in the number of cases led one of its staff to conclude: 'The extra press attention causes anxiety. This group of women have symptoms that were completely innocent.' Hang on, you want to say - how did they know their symptoms were innocent? And isn't increased breast awareness something the NHS has been trying to cultivate for years? What are you complaining about?

'People want to be at peak health and limit the effects of ageing,' says Nadine Masseron, a nutritionist and naturopath based at the Highgate Health Centre in north London. She says very few of the patients she sees are hypochondriacal. Most are people who have had no success with conventional medical treatments, or who have devised treatment programmes for themselves which they want her to verify. These programmes often involve taking large amounts of 'smart' supplements, or 'nutraceuticals' - things like Panax ginseng (the ginsenocides can relieve fatigue and stress and lower blood pressure) and L-carnitine (which helps to convert fatty acids into energy).

Is there any real value in taking these supplements? Masseron thinks so. 'Even small vitamin and mineral deficiencies can have a knock-on effect and be the cause of underlying health issues,' she says.

I go to Boots and buy some selenium. I keep reading about it on health websites - and besides, I like the name, which makes me think of Jane Seymour in the original Battlestar Galactica. Dr Sharma says selenium is very important: 'Without selenium you don't make glutathione, so your allergic defence goes wrong. Selenium deficiency is the reason we've had a sixfold increase in allergies since the Sixties.'

My wife finds my blood glucose monitor on the kitchen table. 'What's this?' she asks.

'It's my FreeStyle Lite,' I say. 'Do you like it?'

She frowns. 'It looks like an MP3 player. One of those really cheap ones that hold about 10 songs. How does it work?'

'Ah,' I say. I say this because I don't really know. The instructions are quite long and complicated - and there's a whole separate set of them for the lancing device! There are also loads of little plastic bits and a pot of something, possibly the test strips the instructions keep banging on about.

'Why do you need it anyway? You're not diabetic.'

'I might be,' I say, vaguely hurt. 'And even if I'm not, it'll be useful to have in the medicine cupboard. Just in case.'

I sit down again with the instructions. My head hurts. Obviously, I hope I'm not diabetic, not yet. Diabetes is something I want to save for my forties, like whisky and getting into jazz.

I insert the test strip and wait for the machine to switch on. Once I've obtained a suitably small blood sample (the size of a pin head), I allow the strip to absorb it from my finger. The machine beeps. There's a five-second pause and then the result appears. My blood glucose level is 5.7mmol/l. Completely normal.

Brendan Duigenan and his wife Terri founded www.selfdiagnosis.co.uk in 2004. Based in Stockport, it sells all the home-test kits you'd expect, and some you might not, like Drink Detective, which detects the three drugs most commonly used to spike drinks - benzodiazepines (including Rohypnol, Valium, Klonopin and Xanax), GHB and Ketamine.

'We set it up on the back of personal experience,' explains Brendan, who, like his wife, is an applied chemist by training. 'I was interested in self-testing myself and it struck me that the products were hard to find. We did a bit of exploratory work in the States, and certainly there were some big numbers quoted about market size in the US. On the back of that we started to experiment with a few suppliers.' Business is good, he says, but hampered by low public awareness. The bestselling tests are for chlamydia, followed by those for blood sugar and cholesterol. 'I thought the PSA [prostate specific antigen] tests for prostate cancer would sell well,' says Duigenan. 'After all, every guy over 50 should buy one every year on his birthday. But actually sales of these have been very slow. Perhaps it's a bit too esoteric.'

That tests for STDs should be so popular isn't surprising. Indeed, it's important not to lump in with the cash-rich, time-poor worried well people who are buying these kits simply because they're too embarrassed to go to the doctor. As it happens, sufferers from STDs have their own dedicated site, www.DrThom.com. It's run by Dr Thomas Van Every, an obstetrician and gynaecologist at the Chelsea and Westminster Hospital in London, and is one of the few online medical services to be registered with the Healthcare Commission. 'For some conditions, patients don't need to be sitting in front of a specialist,' Dr Van Every has said. 'The unit costs of providing health checks over the internet are much cheaper than face-to-face consultations and it may be an area the NHS moves towards.'

But again, are patients the best people to judge whether they need to see a specialist? And are these tests reliable enough to base that decision on?

'The problem with these tests is that they're not conducted in an appropriate clinical context,' says Professor Salkovskis. 'The false positive rate is as high as one in three, but people aren't always aware of that. There's also a danger that information which is supposed to be objective acquires a commercial slant. You send off your blood sample and the answer comes back, "Well, you didn't have x, but we found traces of y. Perhaps you'd like a test for that, which we also happen to sell?"'

'The tests should not replace a trip to the doctor,' agrees Duigenan. 'We're the first to admit you can have false positives and negatives, so symptoms should always take priority. But sometimes I think people need a bit of a push to see a doctor, especially guys. And if they find out from a test that their PSA level is high and it encourages them to pick up the phone, then that's fantastic.'

Next, I decide to test my liver. All the recent stuff in the media about middle-class drinking has alarmed me. Who would have thought there was anything wrong with drinking three-quarters of a bottle of wine a night? Not me.

YorkTest's LiverCheck is a home-test kit that allows you to check the health of your liver - to make sure it hasn't been ravaged by alcohol, diabetes, hepatitis or drugs. You send the YorkTest lab a sample of your blood and they test it for the presence of enzymes called alanine aminotransferase (ALT) and aspartate aminotransferase (AST), high levels of which are associated with liver deterioration.

The kit comes in a beautiful cyan blue plastic case. Inside there are two lancets - I'm growing to really dislike lancets - and a collection bottle for the blood. You prick your finger, then hold it next to the little tube sticking out of the bottle and the blood is drawn down by capillary action.

I prick my thumb, but not much blood comes out. Filling the sample bottle is like trying to put toothpaste back in the tube. I squeeze and squeeze my thumb, but what little blood there is has already coagulated. Luckily there's another lancet, so I try again on the other thumb. This time it's a different story: there's a torrent of blood. It goes everywhere - all over the side of the bottle; all over the label I wrote on so carefully. I fill the bottle in about five seconds. But the blood is still pouring out of my thumb. By the time I've finished the kitchen looks like an abattoir.

My three-year-old daughter walks in. She frowns. 'Daddy! You've cut yourself!'

'Yes. No. It's complicated.'

'Do you know why you've cut yourself?'

'Because I'm taking a blood sample.'

'Do you know why?'

'It's a test to make sure my liver's working.'

'I want to do a test to make sure my liver's working.'

'Not now. When you're a bit older maybe.'

When the results come through, I honestly have no idea what to expect. LiverCheck plots your score on a colour-coded chart - green for 'optimal' function, then down through amber, dark amber and finally red for George Best. My score is green; but it's towards the amber end of green. Although my ALT level is only 15 U/L (the normal range is 0 to 41), my AST level is 33 U/L (the normal range is 0 to 40). It's higher than I would have expected; higher than I would like. The results come with a Lifestyle Planner which contains predictable advice about diet and exercise and alcohol consumption. There's also a scary-looking multiple-choice questionnaire called a 'Lifestyle Audit'. (I'm intrigued by the question: 'Do you take large doses of vitamins, nutritional products or herbal remedies without consulting your doctor?' 'Yes, all the time' is the wrong answer. So should I take my selenium or not?)

The lifestyle audit is helpful, but it isn't enough. It doesn't contextualise the information for me. I want someone to tell me why, when I don't drink to excess, my AST level is edging out of the normal range, and whether it matters. None of the literature helps me out. Instead, a disclaimer in tiny print advises: 'A normal ALT or AST does not necessarily mean you have normal liver function, or that you are free of all liver disease. If you have any concerns regarding your results please see your GP.'

Oh. OK, then.

Heart FM DJ Toby Anstis is crying. He's crying because Dr Paul Jenkins, medical director of a company called Genetic Health, has told him he has a high risk of developing colon cancer.

I'm watching a documentary on ITV called The Killer in Me. Some celebrities are being tested for their inherited predispositions to life-threatening diseases. Because the programme is on ITV and therefore idiotic beyond belief, the science behind the testing is explained only in terms of the subjects possessing 'good' and 'bad' genes. What is actually happening is this: the celebrities' genomes are being scanned for variations in the sequencing of their DNA called single nucleotide polymorphisms (SNPs), which may affect how people develop diseases and respond to environmental pathogens. GMTV presenter Fiona Phillips is worried about developing Alzheimer's like her mother, so in her case Genetic Health's researchers will have been checking for SNPs on the gene apolipoprotein E, which is associated with Alzheimer's.

I know a bit about this because Dr Sharma from the Diagnostic Clinic is very evangelical about a test for polymorphisms of the MTHFR gene which helps the body detoxify itself. He says: 'I think it's unethical not to have genetically tested my children to find out if they're unable to deal with the carcinogens in barbecued food.' Strong words, and they made an impression. I remember thinking it was a test I'd like to have - a test everyone should have. Imagine the saving to the NHS if you could identify your cancer risk, then take measures to banish from your diet and life environmental factors which might heighten that risk!

Genetic Health was started only last year by Brian Whitley, its managing director, and Dr Paul Jenkins, a specialist in endocrine oncology. It's a bit like LiverCheck. You order a test kit from their website (cost: £825 for the full service), then post them your sample - in this case a swab from the inside of your cheek - and a form detailing your medical history.

It sounds fantastic, and Whitley's claim to have had 'some spectacular results' would seem to be borne out by the satisfaction of Genetic Health's customers, among them TV's Dr Chris Steele, who tried to bamboozle them by withholding aspects of his medical history only to find that the tests revealed them.

'It's soft science,' says Dr Fred Kavalier from the British Society of Human Genetics, 'and if you asked any geneticists about it they'd be very sceptical.'

Kavalier anticipates DIY home testing for genetic disorders in the not-too-distant future. 'The technology is there,' he says. 'The costs are coming down and what used to be a tough and unautomated process is now much easier.' What he's less sure about is the point of it all. 'We just don't know enough yet about how genes react with each other and the environment. You're paying a lot of money for information that no one really knows how to interpret. If it's true, then great. But there's no evidence that by changing your behaviour you're going to lengthen your life or stop yourself getting cancer. It wouldn't be money well spent for the NHS to start screening everyone for polymorphisms which are probably insignificant.'

Medicine is currently at an odd, uncertain stage. On the one hand, we'll continue to get terribly, fatally ill - perhaps iller than ever, if the proliferation of superbugs and mutant viruses goes unchecked; but on the other, pre-emptive defence will be as important a strategy in defeating illness as what doctors think of as 'firefighting'. Dr Leroy Hood from the Institute for Systems Biology in Seattle believes that in just 10 years' time a home-testing kit will be available which spots early signs of disease anywhere in the body by measuring each organ's 'protein fingerprint'.

Today's relatively crude home-test kits are tiny but potent symbols of The Way Things Are Going. But their use needs to be systematised, preferably within the NHS, and regulated so that false positives and negatives are picked up on. Some of the news these tests are going to break will be bad, and it's doctors who need to deliver that news, not the companies who manufacture the kits.

George Cheyne, in The English Malady, his 1733 study of hypochondria, thought the condition a by-product of cultural sophistication: 'The present age,' he wrote, 'has made efforts to go beyond former times, in all the arts of ingenuity, invention, study, learning, and all the contemplative and sedentary professions.'

'We're becoming more health-conscious,' says Dr Sharma, 'but that's different from hypochondria. Doctors are finally getting through to the public the necessity not to be obese, not to smoke, to exercise regularly and drink lots of water. It's an enormous leap from 10 years ago. I think people have come to understand that it's no good waiting until you're ill.

'The NHS has become the National Illness Service. Walk in there healthy and no one's going to pay any attention to you. Walk in there ill and you're in one of the best services in the world. It's not time to knock the NHS, but to rethink how we train doctors and educate the public. Chinese foot doctors, who go from village to village treating their patients, don't get paid unless their patients are well. It's the people who are well who pay their doctors!' He laughs as if to say: Imagine!

And I laugh too, peeling the plasters from my sore thumbs.