Why are Some Diseases Screened for But Not Others?

GPs can screen for a number of diseases, but not all are offered to everyone. Photograph: Alamy

Screening for disease has the potential to save lives, but only in very specific circumstances​.Why isn’t prostate screening offered in the UK? Why do we only start screening for cervical cancer after age 25? These questions are often leveled at the government or the NHS, who are accused of withholding them due to cost. But although financial reasons play a factor, the screening programme in the UK is most definitely evidence driven.A new report from Sense about Science details how these decisions are made, and busts some screening myths that seem to exist in the public mind.

What does screening mean?

A screening test usually isn’t the same as a diagnostic test (it is for Hepatitis B, but this is the only disease where that’s the case). If you go to the doctor with symptoms of a disease, they’ll do a diagnostic test to see whether that’s the illness you have. Screening is different – it’s done in healthy people who don’t show any symptoms of the disease. Not only that, but the test itself isn’t a diagnosis. Screening tests look for markers of a disease, that is, things that predict the likelihood of a person going on to get ill. A positive screen isn’t ‘proof’ that you have a disease, rather that you may be at elevated risk.

In what circumstances is screening useful?

Despite what might seem intuitive, there’s no point in screening everyone for everything. For a start, some diseases only affect certain groups in the population, so there’s no point in screening people who aren’t at any risk. Also, if there’s no good treatment for a disease, there’s not much point in screening people, as they won’t be able to benefit from early treatment.Perhaps more importantly, screening doesn’t come without risks. Sometimes the screening procedure itself isn’t risk free – for example using x-ray to check for breast lumps as a marker for cancer is fine occasionally, but would be dangerous to do frequently. Given the scale that screening occurs on, tests must be quick and cheap, and as a result, screening findings are not completely accurate. This means sometimes a person will receive an all-clear when in truth they have the marker for the disease, but the test didn’t pick it up. If a disease is rare, the chance of this happening is also low. But many people who are not at high risk will be given an incorrect ‘positive’ result from a screening test.Maybe this is just an inconvenience, and its ‘better to be safe than sorry’? Possibly not. Getting a positive screen can be pretty scary, particularly in the wait to find out what it means. And often the risk of disease in those with a positive screen still isn’t that high. If a disease affects 2% of the population, and the screening is 95% accurate, then out of 1000 people, then more people who receive a positive screening will be wrongly identified (49) than correctly so (19) - explained in more detail here.Not only that, but all positive screening results will have to be further investigated, and this can mean potentially dangerous procedures with no benefit. For example a positive screen for colon cancer would mean a colonoscopy was necessary, which has a 1 in 1000 chance of causing a perforated bowel.This isn’t to do down screening – in its right place, it is a powerful tool that can save lives. But the above examples show that it’s just not appropriate, value-for-money, or beneficial in all situations. But what about the specific examples I mention above?

Why isn’t prostate screening offered in the UK?

In order to screen for prostate cancer, a protein called Prostate-specific antigen (PSA) is measured in the blood. Higher levels of this are indicative of prostate cancer. But it’s not that simple. Firstly, treatment for prostate cancer is not very effective at prolonging life, and it’s very invasive, with increased risk of incontinence and impotence. So identifying more people who have it could result in loss of quality of life for many, without increasing longevity.Not only that, but prostate cancer seems to exist in two forms – one that is aggressive and very dangerous, and the other that is far less so. There’s no way of telling whether an elevated PSA level predicts one or the other. When all the current evidence is weighed together, the potential harms from screening in healthy men outweigh any potential benefit. It’s better for men to monitor themselves and seek medical help at the first sign of symptoms.

Why isn’t cervical screening offered to under 25s?

Cervical screening takes a sample of cells from the cervix and checks them for abnormalities. This means it’s a pretty invasive process. Abnormal cells are more common in younger women, but less often associated with cancer in that age group. This means if younger women were screened, far more of them would show abnormal cells without being at risk of cancer. This overdiagnosis would lead to them having to have unnecessary examinations and further smear tests. There is also no evidence that screening in under 25s would reduce the number of deaths from cervical cancer. This is why screening isn’t offered.Sense About Science’s report ‘Making Sense of Screening’ is available for free download here.