Saturday January 12 2008
So we’re all going to get screened for our health problems, by some geezers who’ve bought a CT scanner and put an advert in the paper maybe, or perhaps off Gordon Brown: because screening saves lives, and it’s always good to do something rather than nothing. I think you’ll find – and I fancy having this on a t-shirt – that it’s a tiny bit more complicated than that.
Screening is a fascinating area, mainly because of the maths of rare events, but also because of the ethics. Screening isn’t harmless, because the tests aren’t perfect. You might get a false alarm, causing stress and anxiety (“the worst time in my life” said women in one survey on breast screening), or have to endure more invasive medical investigations, like maybe biopsies. Or you might get false reassurance from a false negative result, which can delay diagnosis of a genuine problem.
And here’s an interesting ethical issue. One of the proposed screening programmes is to catch “abdominal aortic aneurysms” earlier. An AAA is a swelling of the main blood vessel trunk in your belly: they can rupture, and when they do, people can die both quickly and frighteningly. But if you do the repair operation at your leisure, before they rupture, survival is far better. Screening and repairing has been shown to reduce mortality by around 40%, looking at the whole population, so it is a good thing.
But remember, you will operate on some people – as a preventive measure, because you picked up their aneurysm on screening – who would never have died from their aneurysm: it would have just ticked away, quietly, not rupturing. And some of the people you operate on unnecessarily (and remember there’s no crystal ball to identify these guys) will die of complications on the operating table. They only died because of your screening programme: it saves lives overall, but Fred Bloggs, loving husband of Winona Bloggs, who would have lived, remains dead, thanks to you.
That’s Vegas, you could say. But it’s tricky, and the sums are often close. For example, mammogram screening for breast cancer every two years has been estimated to prevent two deaths in 1000 women aged 50-59 over 10 years: that is good. But to do that takes 5000 screens, 242 recalls, and 64 women to have at least one biopsy. Five women will have cancer detected and treated. Again, this isn’t an argument against screening, we’re just walking through some example numbers.
Although interestingly that’s not something everybody is keen to do with screening. Researchers have studied the invitation letters sent out for screening programmes, along with the websites and pamphlets, and they have repeatedly been shown to be biased in favour of participation, and lacking in information
Where figures are given, they generally use the most dramatic and uninformative way of expressing the benefits: the “relative risk reduction” is given, the same statistical form that journalists prefer, like “a 30% reduction in deaths from breast cancer”, rather than a more informative figure like the “number needed to screen”, say, “two lives saved for every 1,000 women scanned”. Sometimes the leaflets even contain some borderline porkies, like this one from
These problems with clear information raise interesting questions around informed consent, although seductive letters do increase uptake, and so save lives. It’s tricky: on the one hand, you end up sounding like a red-neck who doesn’t trust the gub’mint, because screening programmes are often valuable. On the other hand, you want to be thinking this through.
And the amazing thing is, in at least one large survey of 500 people, even when presented with the harsh realities of the tests, people made what many would still think are the right decisions. 38% had experienced at least one false-positive screening test; more than 40% of these individuals described the experience as “very scary” or the “scariest time of my life.” But looking back, 98% were glad they were screened. Most wanted to know about cancer, regardless of the implications. Two thirds said they would be tested for cancer even if nothing could be done. Chin up.