Medical screening involves targeting a specific population, by age or sex etc, due to their perceived increased risk of disease. It is based on tests performed on an asymptomatic population, with the aim of classifying people as likely or unlikely to have or develop a disease.
Bowel cancer is the second most common cause of cancer mortality and 85% of cases occur in people aged over 60. Men and women between the ages of 60 to 69 are sent faecal occult blood (FOB) test kits every two years. Those with abnormal results are then referred for further investigation. 1.9% of tests are positive; 10.9% of these will have a cancer and 35% an adenoma.
In addition to the FOB test, the NHS is planning to offer an additional one-off flexible sigmoidoscopy screening to men and women aged 55-59.
There are benefits to the patient of screening for bowel cancer:
- the majority of cancer cases detected by the screening test are early-stage cancers, only 1% were found to have metastasised at diagnosis. This increase in the early diagnosis of patients improves their prognosis. There are estimates which suggest that FOB reduces mortality associated with colon cancer by about 25%,
However, there are also problems with the screening test:
- There are false positives: only 10% of those with a positive results will have a cancer. This can lead to anxiety, and will also mean unnecessary investigations such as colonoscopy, which has risks associated with it.
- There are false negatives as colorectal cancers (especially early cancers) can bleed intermittently;
- There may also be overdiagnosis of pre-cancers, which may never have developed to cause any harm, although this is less of a risk than with other cancers.
Bowel cancer screening can reduce mortality associated with bowel cancer, but is not perfect.
