Controlled prospective study of faecal occult blood screening for colorectal cancer in Bury, black pudding capital of the world

Colorectal cancer is the second most common cause of death from cancer in the United Kingdom, claiming 20 000 lives each year. Most cancers originate from malignant transformation of adenomas, and screening the general population to detect asymptomatic adenomas and early cancers shows the greatest potential to reduce mortality. In the most evaluated screening protocol, small volumes of blood loss from such lesions are detected by a faecal occult blood test, and people testing positive then have a colonoscopy. This reduces mortality in two ways: detection and endoscopic removal of adenomas prevents later transformation and may reduce the incidence of cancer by 16%¹; and early detection of carcinomas allows treatment of less advanced lesions, resulting in a better prognosis. A large trial of such screening showed that a 15% reduction in mortality from colorectal cancer can be achieved.²

Haemoccult is the faecal occult blood test most widely used in screening trials. It uses the peroxidase-like activity of haemoglobin derivatives passed in the stool to detect gastrointestinal bleeding. False positive results from vegetable peroxidases and red meats in the diet are less likely than with more sensitive tests, minimising the number of patients undergoing inessential colonoscopy.

A major screening trial carried out in England did not request dietary restriction by participants.² Instead, patients with borderline faecal occult blood test results were retested with restricted diet, and those remaining positive had colonoscopy. A future national screening programme may well follow this design. However, a recent meta-analysis showed that moderate dietary restriction does not reduce compliance with the test.³

The Lancashire town of Bury is famed for black pudding, a regional delicacy consisting of congealed pigs' blood, fat, and rusk, encased in a length of intestine. Variations such as German blutwurst, French boudin noir, and Spanish morcilla are enjoyed in other parts of Europe. We postulated that a diet containing black pudding might result in a high rate of false positive Haemoccult test results and thus interfere with a population screening programme.

Methods and results

We selected 10 healthy volunteers below the age of 35 with no family history of colorectal cancer to participate in the study. Each participant completed a Haemoccult test requiring six specimens to be taken from stools passed over three consecutive days. Participants then eagerly ate a locally produced 7 oz black pudding and then had a further Haemoccult test. The tests were analysed at Bury General Hospital. We defined a positive test result as the occurrence of one or more positive specimens from the six provided in a complete test.

Initially all 10 volunteers returned negative tests, but after consumption of black pudding four people tested positive. Ingestion of black pudding resulted in a significantly higher proportion (Fisher's exact test, P<0.05) of positive Haemoccult test results.

To calculate the effect of this on a population screening programme, we questioned 100 orthopaedic outpatients aged 45-74 years about their black pudding consumption. Of the people surveyed, 63% succumbed to black pudding on occasion (figure).

Comment

Gourmets should be advised to avoid black pudding during screening for faecal occult blood. In Bury about 58 000 people are within the 45-74 year age group eligible for colorectal cancer screening. Approximately 1200 (2%) of these would be expected to test positive for faecal occult blood.²,⁴ However, during a three day Haemoccult screen, roughly 4775 (8.2%) of the Bury population would normally consume black pudding, and our data suggest that 1909 (40%) of these will test positive. The local availability of almost irresistible black pudding increases the likely number of positive tests to 3109 (5.4%). As more than twice the number of people than expected would need further investigation, dietary restriction would undoubtedly be beneficial when screening this particular population.

Figure.

Frequency of black pudding consumption

Figure.

COURTESY OF R S IRELAND

Lancashire is famous for black pudding