Why isn't more screening always better?

Because screening tests people who feel well, and that changes the arithmetic completely — a test that is useful for someone with symptoms can do net harm when applied to a whole healthy population.

What screening is. Systematically testing an asymptomatic population to find disease earlier, when treatment may work better. Cervical, breast, bowel and abdominal aortic aneurysm screening are established UK programmes, each introduced after evidence that the benefit outweighs the harm.

The harms, which are real and frequently invisible:

False positives. No test is perfect. Screen a large population where the disease is uncommon and most positive results will be false, simply because there are so many more healthy people than affected ones. Each produces anxiety and further investigation, some of it invasive.

Overdiagnosis. The subtlest and most important harm. Some conditions found by screening would never have caused symptoms or death in that person's lifetime — slow-growing or non-progressive disease. Those people are then treated, with all the risk and side effects of treatment, for something that was never going to harm them. They cannot be identified individually, which is what makes this so difficult.

Overtreatment, following from the above.

False reassurance, where a negative result leads someone to dismiss later symptoms.

The two biases that make screening look better than it is:

Lead-time bias. Finding a disease earlier means you know about it for longer, so survival time from diagnosis increases even if the date of death is completely unchanged. Survival statistics alone cannot distinguish this from genuine benefit.

Length-time bias. Screening preferentially catches slow-growing disease, simply because it spends longer in a detectable pre-symptomatic state. Aggressive disease is more likely to appear between screening rounds. So screen-detected cases look better even without treatment.

This is why mortality in a randomised trial is the standard of proof, not survival rates.

The UK National Screening Committee assesses programmes against explicit criteria, which is why some widely requested tests are not offered.

General information, not individual medical advice.