To what extent should you be tested for illnesses you show no sign of having?

This autumn, the science and technology committee at Westminster will publish a report trying to answer that question.

If it means you get picked up early and treated, then testing seems sensible and cost-effective. But the committee has been taking evidence for its National Health Screening Inquiry for more than six months, against a backdrop of increased screening of all types. Breast cancer, hypertension, bowel cancer, dementia. We test for many things nowadays, but the contention is that there isn't enough evidence that it helps.

Routine screening is often based on limited attempts to target at-risk groups, or on no targeting at all. It can lead to women with pre-cancerous or non-cancerous lesions in their breasts having unnecessary treatment, up to and including mammograms, "just in case". It sees hundreds of thousands of patients treated with drugs to control hypertension, often experiencing problematic side effects. The expense can mean other conditions go untreated.

Meanwhile the private sector is getting in on the act. Last week Groupon was offering members a 55 per cent discount on a full body MRI scan.

It isn't just Westminster MPs who are concerned. The Centre for Evidence-Based Medicine in Oxford, backed by doctors' journal the BMJ, held a sold-out conference on overdiagnosis last week. One of the speakers was Glasgow GP and author Dr Margaret McCartney, who argues much better information is needed for patients about potential benefits and harms of scanning and screening.

It can be complicated. For example, if 43 in every 10,000 women screened for breast cancer have their lives saved as a result, does it matter that another 129 women have treatment unnecessarily?

Patients' wishes should matter. But other factors come into play. Defensive medicine by doctors anxious not to miss anything. Calculations related to cost and budgets rather than quality of life. Global healthcare corporations "selling" sickness.

The key is surely evidence, and it's likely the inquiry will say we need far better evidence for screening programmes. That could save vast numbers of patients from invasive treatments and save the NHS money not just in paying for tests, but the expense of the sometimes entirely unnecessary treatment they lead to.