There has always been a need for simple, objective tests in medicine for various purposes, e.g. to distinguish between intercurrent viral infections and pneumonia in patients with fever. The obvious reason is that similar symptoms and signs may be seen in diseases with different aetiologies. The use of simple tests, such as the erythrocyte sedimentation rate (ESR) or C-reactive protein, has served to single out for further diagnostic procedures those patients with high fever who might have bacterial infections. It would be a waste of money and abuse of limited resources in our healthcare system if every febrile child were to have chest X-rays and bacterial cultures taken. Similarly, expensive, uncomfortable or even risky procedures should not be used on every patient with abdominal discomfort. Somehow, only a fraction of the patients must be selected for further examination. Is it possible that determination of faecal calprotectin concentrations may serve as the ESR of the gut? Furthermore, can this assay also be used for objective assessment of disease activity and response to treatment in inflammatory bowel disease (IBD)?
In this issue of the journal, Summerton et al.  present data that, together with previous studies, suggest that the answers are yes to both questions. Their data derive from a study of a patient population typical of many gastroenterology centres and are therefore highly relevant.
Increased levels of calprotectin in plasma, cerebrospinal fluid, synovial fluid, saliva or urine have been found in infectious, inflammatory and malignant diseases in relevant organs, presumably reflecting the activation of monocytes and neutrophil granulocytes . Large amounts of calprotectin are present in such cells in blood from healthy individuals and, if released into plasma, the level would increase from 0.5 to 50 mg/l, i.e. by a factor of 100. Even higher concentrations will be expected locally in tissues during pathological processes.
Recently, the biological properties of calprotectin have attracted great interest. It can inhibit zinc-dependent enzymes , of which our body contains many hundred different types. By competing for zinc, calprotectin can kill microbes [4,5] and induce apoptosis in both normal and malignant cells . Zinc-dependent metalloproteinases are needed for activation of cytokines like tumour necrosis factor alpha (TNF-α) and for invasive growth of tumours. Calprotectin may therefore participate in the regulation of inflammatory processes and inhibit cancer cell proliferation. On the other hand, excessive concentrations of this protein may induce cell and tissue damage. In view of the massive accumulation of neutrophils at sites of active IBD (as visualized by 111indium scanning), cytotoxic concentrations (above 5 μM) of calprotectin may be reached locally. This protein may therefore play an active role in IBD in addition to being a useful faecal marker.
Since the development of a method for determining faecal calprotectin levels , many studies have shown that this protein is a reliable marker of the presence of infectious, inflammatory or malignant disease in the gastrointestinal tract, from the oesophagus to the rectum. This should not be surprising since large numbers of neutrophils are attracted and activated in response to infections, cell/tissue damage, or increased permeability of the mucosa. Neutrophils migrate towards increasing concentrations of microbial chemotactic substances or lipopolysaccharide (endotoxins) present in large amounts in the gut content. In the tissues, release of calprotectin from neutrophils may give a positive feedback since it causes increased adherence to and migration of neutrophils through the vascular endothelium. Calprotectin is remarkably stable in stools , which may be due in part to the fact that the concentration of calcium in the gut content is high enough to make it resistant to proteolytic degradation .
A strong correlation between the excretion of 111indium from labelled autologous leucocytes into stools and the faecal concentration of calprotectin in patients with IBD [9,10] supports the theory that the protein derives from such cells migrating into the gut lumen.
Although clinical indices for disease activity in IBD are commonly used, they are based in part upon subjective criteria and often are not reliable. An objective, simple and non-invasive test would therefore be very welcome. It was promising that the faecal calprotectin test compared favourably with the 111indium test, which has been called the gold standard for assessment of disease activity in Crohn's disease. But methods using isotopes have severe limitations related to the risk of radiation damage, complexity and costs. In particular, they cannot be used repeatedly for clinical purposes, particularly in children and fertile women. In contrast, the non-invasive calprotectin test can be run as often as needed and without any preparation of the patient. Calprotectin levels also showed a strong correlation to disease activity in IBD as determined by endoscopy, biopsy or clinical indices [11–13], of which the former two are also demanding for the patient, are expensive, and carry a small but definite risk of severe complications. Experiences in children [12,13], in whom endoscopy normally requires general anaesthesia, are favourable and suggest that a faecal calprotectin test should be considered seriously in the routine work-up.
Taken together, the data published from various centres during recent years seem to warrant the conclusion that this new test fulfils most of the criteria relevant for an objective test [12,14].
The intriguing finding by Tibble et al.  that faecal calprotectin levels can predict relapses in IBD patients in clinical remission raises the question: what should be the criteria for a successful treatment, and could relapses have been prevented in some of the patients by continued and/or altered treatment until calprotectin levels were brought closer to the reference range? This should be addressed in further studies.
The correlation between IBD disease activity and faecal calprotectin has, up to now, been analysed mostly by comparison of patient groups, but experiences from sequential analyses of individual patients are now emerging. Figure 1 shows two examples of this where the levels in Crohn's disease patients treated with infliximab were followed over a few months. Calprotectin levels reflected the clinical assessment of disease activity. These cases may also illustrate how the effects of new drugs can be monitored objectively.
For routine follow-up of IBD, Aker University Hospital in Oslo, Norway provides patients with many small, screw-capped containers and preaddressed envelopes so that they can send random 1–2-g faecal samples to the laboratory on a regular basis. When patients in remission develop symptoms again, they are asked to send samples for objective testing of a possible relapse before being seen by a specialist again. Although some people handle stools very reluctantly, in our experience IBD patients in general prefer taking stool samples for calprotectin tests as an alternative to endoscopic examination. Using the new procedure [17,18], only about 0.1 samples are extracted with 5 ml buffer in a closed tube.
With regard to detection of gastrointestinal cancer, several studies [7,15–18] have shown that about 90 per cent of colorectal cancer patients have increased calprotectin levels. This represents a sensitivity twice that of commonly used tests for faecal occult blood. The latter have a somewhat higher specificity. This means that more cancer will be found by the calprotectin test at the expense of some extra endoscopies with negative results. Whether this is acceptable is a political question on which patients, gastroenterologists and hospital administrators may disagree. Perhaps the capacity for those extra endoscopies can be provided by avoiding them in patients with a negative calprotectin test. The diagnosis of IBD can be excluded in patients with a negative test and fulfilling the Rome criteria for irritable bowel syndrome.
Since calprotectin is stable in stools, the test can be established in a few laboratories that can serve relatively large areas and populations. In such a way, investments and reagents can be used more efficiently. Samples from all parts of Norway have, for example, been sent to one laboratory in Oslo since 1993, and presently about 2000 samples are tested per one million inhabitants each year.
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