Screening - not always a force for good

All this week I've had patients coming in to ask me about private screening. It tends to go through phases - presumably when one of the private screening companies has been doing a mailshot in my patch.

I spend a lot of time talking about the fact that the NHS has limited money. If people want to save the NHS money and spend their own cash on getting themselves screened, surely that's a good thing, right? Wrong. Because screening carries risks as well as benefits - and if a private screening service wants to maximise their income, it's in their interest to downplay the risks of screening and scare people into having tests that, at best, they don't need, and that may do more harm than good.

Last year I wrote a blog about a review of the national NHS breast screening service, which invites all women aged 50-70 (soon to be extended to 47) to have a mammogram to pick up breast cancer. While the review suggested that the service, which screens over 2 million women a year, saves 1,300 lives a year, it also results in women being over-diagnosed. That's because some of the changes picked up would never go on to cause invasive breast cancer, but the present screening can't tell which is potentially deadly and which is harmless. As a result, 4,000 women a year are treated unnecessarily - three times more than the number of lives saved.

Overall, the review concluded that the service was saving lives and women should continue to be offered regular screening. However, it has highlighted to both doctors and patients that patients need to be in possession of all the facts before they make a decision about screening. Only then can they make a really informed choice.

PSA (prostate specific antigen) screening for prostate cancer is a case in point. Prostate cancer kills about 10,000 men a year in the UK, (1,2) and a screening test which reliably teased out patients with an early, accurate test which saved lives without increasing unnecessary treatment would be a huge advance. Unfortunately, PSA isn't such a test. Two thirds of men who have a raised PSA level don't have prostate cancer (2) - yet they will be put through weeks of worry and further tests which may carry complications. These tests are so-called 'false positive' tests.

Worse, even if you do have prostate cancer, PSA is not a very reliable way of working out which cancers will go on to become invasive. That means that men may be given cancer treatment which carries risks of complications including incontinence, erectile problems and damage to the bowel. All that for a cancer which, in some cases, they would never have known about without screening and which would never have done them any harm. One review of PSA screening suggested that for every life saved, 48 men would have to go through unnecessary treatment. (3)

And there's more. Along with 'false positives', some patients may have 'false negative' tests. If you have a private screening test to check your liver function and the result is normal, you may believe that, say, your high level of alcohol intake isn't damaging your health. If your lung function is normal, you may believe you've got away with smoking for years. In fact, smoking or drinking too much is a much better indicator of whether you're going to have problems in the long term than a single screening test. Yet with a false negative test, you may be lulled into a false sense of security, and believe you're immune from the consequences of your lifestyle.

Many private screening companies offer CT scans for screening. There's little or no evidence that this increases the chance of picking up changes at a stage when treatment would make a difference to overall long-term outcomes. Yet depending on how much of your body is scanned, a CT scan exposes you to 50-500 times more radiation than a chest X-ray. (4) As an investigation for people known to be at high risk of problems, the benefits of a single risk are likely to outweigh the risks - but what happens when you get another invitation in a year's time, and a year after that? The radiation will all add up.

The NHS offers several screening programmes for conditions such as cancer and heart disease. For each of these, the risks and benefits are carefully weighed and a decision is made about whether the benefits outweigh the risks at a population level for a particular group. But you're not a population - you're a person. If you're offered screening by a private company, ask yourself why the NHS doesn't think it's worthwhile. If you're offered screening by the NHS, get the facts and make the decision that's best for you.


1) National Institute for Clinical Excellence. Prostate cancer. NICE Clinical Guideline 58. NICE, London February 2008

2) Burford DC, Kirby M, Austoker J. Prostate Cancer Risk Management Programme: information for primary care; PSA testing in asymptomatic men. NHS Cancer Screening Programmes, 2009

3) S chroder FH, N Engl J Med 2009; 360:1320-1328

4) Public Health England information -

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