The dark side of early diagnosis

Cancer screening can catch problems early. But it also raises the spectre of false positives. Photo: Prospect composite

What can add five years to your life without lengthening it by a single day, improve the performance of the NHS while wasting its resources, and make people grateful for unnecessary suffering?

The answer is early diagnosis.

Early diagnosis sounds like a no-brainer. The NHS has targets for speedy diagnosis and treatment of cancer, and offers screening for a wide range of conditions, in babies, children and adults. Private medicine and charities offer even more.

Much of this early diagnosis can, without doubt, be a very good thing. What’s surprising is that in the wrong hands or the wrong circumstances, it can also be a very bad one. To make matters more complicated, sometimes it can be hard to tell the good early diagnosis from the bad. The story of our attempts to put the logic of early diagnosis into practice is a hard lesson in the human costs of failing to communicate that uncertainty, and the dangers that lurk in the intuitively obvious.

To see how easily we can be misled, start with a definitional trick that makes early diagnosis appear a good thing, simply by moving the goalposts. Imagine your cancer is diagnosed at a late stage, aged 66, and you die from it four years later aged 70. Your cancer might have been sitting unrecognised for years. If only you had known.

Then, in another life, your wish is granted. This is the parallel world which, in a recent speech about how AI would soon tackle cancer, Theresa May held up as her vision of the future. “Every year, 22,000 fewer people will die within five years of their diagnosis compared to today.” So thanks to screening, you get your diagnosis earlier at, say, 62.

But now suppose that’s all it is—an early diagnosis. And suppose there’s no treatment for it or the treatment is unsuccessful (as we’ll see, this turns out to be a common scenario). You still die at 70, having lived not a day more. But fear not; because you become one of those who do not “die within five years of their diagnosis,” you can still be counted as someone who lives longer.