Part two. Screening reduces bowel cancer deaths across a population. That is not quite the same statement as “screening will tell you whether you have bowel cancer” — and the gap between those two sentences can cause confusion.
In the first part of this piece I described the experience of getting a positive bowel screening result. Going through the roller coaster of emotions from thinking I’m fit and healthy to then being convinced I had cancer to the ultimate relief of getting the all clear made me think about the process and what each stage means. On a professional basis I have always seen the rationale for screening for bowel cancer – detecting a pre-cancerous condition which if treated prevents cancer but how does that translate into the real world? Does it work for a population? Then what about on an individual level what does a positive test mean? What are the chances I have cancer? And the converse – if my test is negative, am I in the clear?
1. Does screening work?
All of the evidence indicates that it does reduce bowel cancer deaths in a screened population by about 10%. In Scotland the number of people dying of bowel cancer has fallen by 8% since the roll out of the national screening programme in 2007. When you consider that not everyone invited to participate returns their stool sample this is a significant benefit – indeed when bowel cancer mortality is compared between those who do participate in screening against those that don’t the difference if more like a 27% reduction in bowel cancer death rates.
So screening works for a population and this is what it is designed to do. The bowel screening programme is a public health instrument aimed at the health of a population. It aims to maximise participation and identify high risk groups that need further investigation. It is not a personal diagnostic test, but what are the implications of a positive and negative test for me as an individual?
2. My test is positive. Do I have cancer?
It is unlikely. In the most recent Scottish figures, 2.8% of completed tests came back positive. Of the people who then went on to have a colonoscopy, 5.2% were found to have a bowel cancer. Turn that around: roughly 19 out of every 20 people with a positive screening test do not have cancer.
The group in the middle of that diagram deserves more attention than it gets. Those people did not have a cancer detected. They had a cancer prevented — or at the very least the most likely route to one closed off, and useful information gained about how closely they should be watched in future.
A positive result is not a diagnosis. It is an invitation to find out.
2. My test is positive. Do I have cancer?
A negative screening test is good news but it is not a clean bill of health. It means you are not at high risk of having a bowel cancer. Unfortunately some cancers will be missed by the test – how many can be estimated from the number of cancers diagnosed after a negative screening test and before the next invitation (2 years later). This is known as an interval cancer and the results from UK screening indicate that perhaps as many as half of all cancers are interval cancers.
None of this is a criticism of screening but it illustrates the difference between a public health initiative – the screening program and an individual diagnostic test – which screening is not. The programme is doing exactly what it was designed to do reducing death rate in the community from bowel cancer but it does not tell any individual if they do or do not have a cancer.
A positive result is not a diagnosis. It is an invitation to find out.
What this means in practice
Three things follow, and they are all simple.
Do the test. Around two-thirds of people invited return the kit; a third do not. Participation roughly doubles the benefit you get compared with the headline programme figure. It remains the single most effective thing you can do.
If it is positive, go — and don’t spend the wait assuming the worst. Nineteen out of twenty positive tests will not have cancer. Many will have something removed that could have become one.
If it is negative, don’t file your symptoms under “already checked”. A negative screening test two months ago does not explain a change in bowel habit, bleeding, unexplained weight loss, or iron deficiency anaemia today. If something has changed, that is a conversation with your GP, and a previous screening result should never be the reason you delay it.
Screening asks: how do we find more cancers earlier, across millions of people, with the resources available? It answers that question well, and the evidence that it saves lives in the UK is solid.
The question “what is my risk, and what should I do about it?” is a different question. It is a good one. It just needs to be asked somewhere else.
[Ken Park is a consultant surgeon and Chief Medical Officer at NexEndo Healthcare.]