This is not so much a test for IBS but a test for not-IBS. Faecal Calprotectin (FC) is a marker released from white blood cells (leukocytes) that are exuded from the inflamed bowel. It is perhaps the most sensitive test for inflammatory bowel disease, which although present in only one in a hundred patients with symptoms resembling IBS, is a significant and treatable cause of severe chronic ill health. Thus, if patients with long term symptoms of diarrhoea have low titres of FC (50 microgram per gram of stool sample), doctors can be 80% confident that they do not have Ulcerative Colitis or Crohn’s Disease. This is important because this might reduce the need for referral to gastroenterologists and unnecessary endoscopy. FC was originally introduced as a test to be used in specialist centres, but now there are tests that can be simply conducted by your practice nurse or even using test kits and home with the result scanned and sent to your doctor using your mobile phone.
It’s a no-brainer, you might think, but testing for FC have not always shown a reduction in endoscopy. There is some controversy about the cut off levels of FC, below which IBS is more likely or above which, patients need to be endoscoped to confirm inflammatory bowel disease. Some studies and tests indicate these are 50 and 200 microgram per litre. Others put the levels a little higher. On many occasions FC titres are intermediate, neither too low to rule out Crohn’s and colitis nor high enough to be absolutely sure of the diagnosis. Such patients tend to be referred for endoscopy anyway just to make sure. Before the advent of FC, doctors would make a clinical judgement on whether to refer patients for endoscopy and in most instances would be right.
But chronic inflammation of the bowel can be caused by conditions other than Crohn’s and Ulcerative Colitis. These include microscopic colitis, which can be associated with quite severe diarrhoea and is said to be as common as Crohn’s and Ulcerative Colitis combined. Although titres of FC can be elevated in active MC, they are not as high as in Crohn’s and Ulcerative Colitis and may not exceed the cut off range. The cause of Microscopic Colitis is unknown, but it responds well to Budesonide, a poorly absorbed steroid that acts directly on the colon and has few general side effects. Other conditions that cause IBS-like symptoms and inflammation include malabsorption of irritant bile acids (BAM), though it is possible that bile acid malabsorption may be a cause of microscopic colitis. Coeliac Disease can also cause bowel inflammation, but there are other more sensitive tests for CD. Although these conditions all release white cells into the bowel, none of them necessarily generate abnormally high titres of FC. This is because they release the wrong white cells; instead of leukocytes, they release lymphocytes which do not produce FC. The same applies to post gastroenteritis IBS, which is also associated with significant bowel inflammation. Finally cancer of the bowel may occasionally release leukocytes into the bowel and cause elevated titres of FC, particularly if the surface of the cancer is ulcerated and infected, though if that hasn’t occurred, titres may be quite normal. So FC is not a reliable screening test for cancer.
As for as IBS is concerned, the message seems to be: low titres of FC may provide the doctor with a greater index of confidence that a patient with long term symptoms of diarrhoea has IBS as long as microscopic colitis, coeliac disease, bile acid malabsorption and bowel cancer have been screened out. In other words, IBS is still a diagnosis of exclusion, but FC might add greater precision to the exclusion of other conditions. Will it reduce rates of endoscopy? Only time will tell.
In view of the controversies and confusions that surround the nature of what we call IBS, the lack of any specific pathology or diagnostic marker, it is not surprising that there is no reliable positive test of IBS. Simple screening tests may be helpful to exclude other conditions, but cannot be said to be totally reliable.