Why I'm saying no to a smear

Renew travel card. Book haircut. Go for screening test. Buy cinema tickets. Meet friends." You may have spotted the odd one out – for this is not a list of things to do at the weekend, but the front cover of the NHS Scotland's "Cervical Screening Test" information booklet.

Here's a confession. I am a GP, and I don't go for cervical smears. Nor do I know what my cholesterol level is, and when I reach 50 and am invited to mammography screening, I won't be turning up. I haven't decided about bowel-cancer screening, but at the moment, on the evidence, I'm unconvinced.

Why? Part of the problem is the trivialisation of the choices that are offered to us when it comes to screening tests. It's important to be clear – screening tests are for people who are well and who have no symptoms for disease. So if you have bleeding between periods, or a change in your bowel habit, you don't need screening tests – you need diagnostic tests.

Screening tests are different. Because they aim to find disease you don't know about, the trade-off between benefits and harms tends to be more nuanced compared with tests done to investigate symptoms. Cervical screening does prevent deaths from cervical cancer. But to get that reduction, you have to follow up and/or treat all the women who have cell changes on their screening test. However, most cervical-cell changes found at screening will not lead to cervical cancer. The problem is we can't predict which will, so all need further monitoring or treatment. A study from Bristol in 2003 found that 1,000 women have to be screened for 35 years to prevent one death from cervical cancer; and to prevent that death, 80 women have to have further investigation, with 50 women having treatment to their cervices. Four out of five women found at screening to have "high-grade" changes in their cervix did not go on to develop invasive cancer.

Clearly, there is a benefit – but overall, it's small. That potential for good has to be weighed against the risks of treatment. It's known that having a cervical biopsy – which is done to get more information about the degree of abnormality – raises the risk of pre-term birth in later pregnancies. And the worry and anxiety that the results cause shouldn't be underestimated.

I'm not against screening, but I am against unthinking screening. I weighed up my personal risk factors for cervical screening (for example, smoking is a risk factor), threw in my own priorities – and decided not to have it. And here's the problem. The NHS persists in sending me red-ink letters despite my written declaration to opt out. I'm made to feel a risk-taker in not having cervical screening – yet I'd also be taking my chances if I had it done.

This is at the heart of the screening problem. Most information sent to us, as potential participants, doesn't make clear that it's a personal choice, and a balance of pros and cons. We can end up being screened without knowing that it was an option, not a foregone conclusion. This is important because it's apparent from research studies that when people get balanced information about screening tests, fewer people want them. Take the prostate-specific antigen (PSA) blood test for prostate cancer screening. When it's explained that it can't distinguish a killer cancer from a "cancer" that behaves benignly, more men opt out – the treatment can lead to impotence and incontinence, and there is no guarantee that PSA screening saves lives. The same goes for bowel-cancer screening; the better the information people are given about their chances of benefit and risk of side effects from screening, the fewer want it.

Yet the screening establishment tends to think that people who don't have their prescribed screening tests are more likely disorganised, feckless or ignorant. The truth is quite different. I've elected not to know my cholesterol level because it's just one cardiovascular risk factor among many others that I can control better – such as with exercise and diet. For someone without cardiovascular disease, there's around a one-in-60 chance of avoiding a heart attack by taking cholesterol-lowering statins for five years. But there is also a one-in-167 chance of developing diabetes because of the drugs. I'd rather not take tablets for the rest of my life with this balance of risks. As for breast screening, we know that if you screen 2,000 women for 10 years, one will have her life prolonged because of it. But to get this benefit, 10 other women will have treatment for breast cancer unnecessarily – when their cancer would never have affected their lifespan. When I put this in context with my own personal risk factors and preferences, I can't see myself signing up.

Of course, all this pales beside the many screening tests in the private sector – from CT scans of vital organs to ultrasounds of the neck and ECGs of the heart. Within the NHS, the UK National Screening Committee diligently researches the benefits and risks of screening. In the private sector, there's no such quality control. And false positives can mean invasive tests are done to get biopsies, themselves risking harm. Are we even sure what "normal" is? We know that about 10 per cent of well people have abnormalities on their brain scans – including aneurysms and benign tumours. Clearly, most of these "abnormalities" mean very little – 10 per cent of the population don't die of these "problems". Screening can throw up all sorts of unanticipated problems; shouldn't we know this before stepping into a scanner?

Many people who endure side effects from screening tests are willing to put up with them because they believe they have been saved by them. I'd be willing to bet that if more people understood the fine balance of risks screening offers, there would be less screening – and more anger. To me, the failure of medicine to explain the pros and cons, and the need to make a properly informed screening choice, is scandalous and unsustainable. But it's potential patients who need to object loudest of all.

'The Patient Paradox: Why Sexed-up Medicine Is Bad for Your Health', by Margaret McCartney, is published by Pinter & Martin (£9.99)