Mrs L.K., a 43-year-old homemaker from Bury in Lancashire, came to see me for some tests to detect possible early signs of malignancy in her gut. She was particularly cautious because her father, mother and brother had all died of bowel cancer in their early 50s.
She underwent a faecal occult blood (FOB) test, in which chemicals are used to detect possible traces of blood in the stools. If these are found, it could indicate bowel malignancy. Aside from gastrointestinal tumours, blood in the faeces can come from a number of other sources, including ulcers, haemorrhoids, colonic polyps, inflammatory bowel disease, and lesions in the gastrointestinal tract caused by medications such as non-steroidal anti-inflammatory drugs (NSAIDs), recent GI surgery or trauma.
Mrs L.K.'s results came back positive. She was understandably alarmed and insisted on immediately undergoing an endoscopy and a colonoscopy, as well as a variety of other relevant urine and blood tests for potential cancer.
These tests are designed to look for so-called 'tumour markers', substances produced by cancer cells. On average, they are more than 90-per-cent accurate. As their target areas and organs overlap a great deal, it is customary to undergo several of these tests to confidently establish either a strong suspicion or a definite all-clear.
The tumour-marker tests, and both the endoscopy and colonoscopy, all came back negative.
Mrs L.K. did another FOB test - but, again, it was positive.
This was a conundrum as she had no symptoms of colitis or any other digestive disorder - nor had she ever had any.
I decided to start again. I went through her diet in minute detail. Her eating habits were unremarkable except for one peculiarity - she adored the Bury black pudding, so much so that she ate it virtually every day. The renowned sausage delicacy is made in her hometown, and its principal ingredient is congealed pig's blood.
Finally, I had a clue as to what Mrs L.K.'s problem may be, and I eventually found a published study that confirmed my suspicion (BMJ, 2002; 325: 1444). The report stated that, although the FOB test can detect the presence of blood in human stools, it cannot distinguish human blood from that of a pig.
I suspect that the people of Bury very likely hold the British record for undergoing the greatest number of unnecessary gut investigations.
There are many other ways in which FOB-test results can be distorted. Consuming liver, or large quantities of beef or mutton, a day or two before the test can produce false-positive results. Nosebleeds, coughing up blood from the lungs, bleeding gums and recent dental procedures (particularly tooth extractions) are other non-gastrointestinal-related factors that could cause false positives. Toilets that have automatically dispensed toilet-bowl cleaners can also affect test outcome, and should be removed from the toilet (followed by two flushes) before collecting the stool samples.
On the other hand, many fruits and vegetables containing vitamin C can lead to false negatives. Some fruits contain chemicals that prevent the test reagent from reacting with blood altogether.
Antacids, corticosteroids, colchicine, anticoagulants (blood-thinning drugs), iodine, bromides, boric acid, reserpine and iron supplements should all be avoided prior to the investigation. Foods such as raw broccoli, raw turnip, cauliflower, melons (including cantaloupe), parsnips, raw radish and horseradish should not be eaten during the 72 hours prior to the FOB test. It should also be remembered that not all polyps bleed, and not all bleeding polyps bleed all the time.
The bottom line is that there are many factors that can result in false-positive and false-negative findings. That is why only 50 per cent of colon cancers test positively in FOB tests (JAMA, 2001; 28: 12).
Harald Gaier is a registered naturopath, osteopath, homoeopath and herbalist. He has recently moved to: The Health Equation, 11 Harley Street, London W1G 9PF; tel: 020 70612 9800.