Screening harms and overdiagnosis, and how to weigh them

What the harms of screening are

Screening looks for disease in people who feel well, so every harm falls on someone who had no complaint. The UK National Screening Committee names the harms it weighs as overdiagnosis, overtreatment, false positives, false reassurance, uncertain findings and complications [1].

The World Health Organization notes that people are often unaware of the potential harm of screening, and that for many screening tests and health checks a clear evidence base for effectiveness is missing [2]. Knowing the harms is part of making a good choice, and many people still choose to be screened once they know them.

False positives

A false positive is a result that suggests disease when there is none. It usually leads to more tests, and the wait can be hard. In breast screening, for every 100 women screened, 4 are called back for more tests and 1 of those 4 is found to have cancer [3].

Over many rounds, false alarms add up. In US data, about 38.1% of women screened with digital mammography every 2 years had at least one false positive over 10 years [4]. Most are settled with further images, and some need a biopsy.

Overdiagnosis, in real numbers

Overdiagnosis is finding a real disease that would never have caused symptoms in a person’s lifetime. It cannot be seen in any one person, only counted across a screened population [4].

An independent UK review estimated that for every 10,000 women aged 50 invited to breast screening for 20 years, 43 deaths from breast cancer would be prevented and 129 cancers would be overdiagnosed and treated [5]. The panel concluded that screening reduces deaths from breast cancer and should continue, while making clear that some overdiagnosis occurs [5].

False reassurance

A normal result is not a guarantee. No screening test is completely reliable, and a cancer can develop between screening rounds, so changes in your body still need a doctor [3].

The risk is that a normal result makes someone wait. A symptom that appears a few months after a normal screen deserves the same attention as one that appears without any screening at all.

How the UK decides what to screen for

The UK National Screening Committee’s criteria build on principles first set out for the World Health Organization by Wilson and Jungner in 1968 [6]. Among them, there should be evidence from high quality randomised trials that screening reduces deaths or illness, and the benefit should outweigh the harms [1].

This is why some popular tests are not offered. Ovarian cancer screening is not recommended because in a large UK trial it did not reduce deaths [7]. PSA screening is recommended only for a small group of men with a BRCA2 variant and a family history, because for men as a whole it is more likely to cause harm than good [8]. Trials of general health checks found little or no effect on deaths overall [9].

Why screening is still worth having

Harms do not cancel benefits. Bowel screening with FIT from 50 reduces deaths from bowel cancer and may help prevent it [10], and breast screening is judged to prevent many deaths each year [5]. For screening that meets these criteria, keeping up with your invitations is a sound choice.

The harder decisions are tests outside the programmes, such as scans and blood panels for well people. There the evidence is often thinner, and the weighing is yours to do with a doctor.

How to weigh a screening offer

These questions turn a screening offer into numbers you can compare.

• Out of 1,000 people like me, how many would this test help, and how?
• How many would get a false alarm, and what happens next for them?
• Could it find something that would never have harmed me?
• Is this test recommended for my age and risk, and by whom?
• What would I do differently with the result?

How The Wellness can help

A GP appointment gives you time with a GMC registered doctor to go through the screening you are due, and any test you have been offered, in plain numbers for someone of your age and history. When a test is worth having, it is arranged through registered partner clinics and accredited laboratories, and a doctor explains every result.

For a wider conversation about prevention and which advanced tests are worth it for you, the longevity consultation is free and held by video. Our clinics are on Baker Street in Marylebone and Eccleston Street in Belgravia.