A screening programme to detect bowel cancer is being rolled out from two pilot sites in England and Scotland. In Scotland, every two years, people aged between 50 and 74 years will be invited to have a faecal occult blood (FOB) test. Those that are positive will be offered colonoscopy to identify the source of the blood in their faeces However, although the traditional FOB test is relatively cheap it has a number of drawbacks. Positive tests can, if not collected and tested properly, result from the presence in the diet of animal blood (in black pudding, for example) or foods containing the enzyme peroxidase such as radishes, turnips and broccoli. A diet high in vitamin C can cause false negative results.

Researchers at the Scottish Bowel Screening Centre based in Dundee have investigated whether retesting those with a positive traditional FOB test using a more specific but more expensive immunochemical test could reduce the number who need to have a bowel examination in hospital using colonoscopy. The immunochemical FOB test uses antibodies to human globin, the protein part of the red blood cell pigment haemoglobin. The test is more specific than the traditional test because it does not react with animal blood and is unaffected by peroxidases. People with a positive result with the traditional test volunteered to collect a further two samples to be examined by the immunochemical test.

Dr Callum Fraser and his colleagues reported in The Lancet Oncology online in January that of 795 participants for whom colonoscopy results were also available, only one of the 171 (<1%) with two samples negative by the immunochemical test and one of the 127 (<1%) with one negative and one positive sample were found to have bowel cancer. This compared with 38 of 497 (8%) of those with two positive samples. They found that immunochemical testing of those who are weakly positive by the traditional FOB test would reduce the need for colonoscopy by about 30%. They concluded that their findings are “important in terms of reducing costs, inconvenience and associated morbidity, without compromising the effectiveness of screening”.