Calprotectin constitutes up to 60% of the soluble proteins in the cytosols of human neutrophils. In addition, it is distributed in monocytes, macrophages, and epithelial cells [1-3]. Their release is activated through interaction of activated monocytes with endothelial cells that increase leukocyte recruitment, and through proinflammatory chemokines by which phagocytes further promote extravasation of leukocytes to the sites of inflammation [4]. When measured in feces, calprotectin corelates well with neutrophil infiltration of the intestinal mucosal surface and within the gut lumen, and is a hallmark of digestive inflammatory pathology [5]. This protein is able to bind calcium, zinc and manganese ions. Since these elements are of vital importance for bacterial growth, their chelations by calprotectin contribute to the alteration of the gut microbiota, exerting an antimicrobial effect. Through the sequestration of zinc ions (Zn²⁺), calprotectin is also able to inhibit many zinc-dependent enzymes such as matrix metalloprotease, inducing an antiproliferative effect as well as apoptosis in both normal and transformed human and animal cells [6]. These properties suggest a pivotal role of calprotectin in inflammatory processes through its effect on the survival and growth of cells. Therefore, fecal calprotectin (FC) is useful as a noninvasive test reflecting various pathological processes occurring in the intestinal mucosa of pediatric patients.

In addition, its structure is very stable at room temperature for up to 7 days, and is resistant to bacterial degradation [7]. This makes calprotectin an ideal marker not only for hospitalized patients, but also for outpatient management.

Numerous studies have been published showing the usefulness of FC in the detection and monitoring of several gastrointestinal (GI) disorders, particularly inflammatory bowel disease (IBD) [8-10]. Differentiating patients with organic diseases from those with functional disorders (i.e., irritable bowel syndrome [IBS]) may be difficult in clinical practice. Indeed, abdominal discomfort including pain, bloating and diarrhea is common, which often arises from functional GI disorders, but may also indicate IBD. When measured in feces, FC can be used to differentiate between nonorganic and inflammatory intestinal disorders, especially to identify IBD. However, it should be noted that elevated FC levels could be found not only in IBD, but also in other GI pathological conditions including infective colitis, microscopic colitis, eosinophilic colitis, colorectal cancer and beyond intestinal disease [11-14].

This review aims to explain the role of calprotectin in a range of IBDs and other pathological conditions in pediatric clinical practice.

FC levels may vary with age. Currently, an FC level below 50 μg/g is considered normal for children older than 4 years [15]. To obtain the mean FC level of children under 4 years of age, we studied FC levels in healthy children at four kindergartens for 6 months. Children were excluded if their parents reported any signs of cold, flu, stomach discomfort, or similar problems in the last 2 weeks. Additionally, children with a history of preterm birth, low or large birth weight, large or small weight for their age (<3rd percentile or >97th percentile), and positive results for stool virus or bacterial polymerase chain reaction were excluded. According to a recent study, newborn infants have high calprotectin levels that later decline, and usually reach normal levels by the age of 4 years (Fig. 1) [16]. FC levels in a cohort of healthy children aged between 6 months and 4 years were found to be in the 95th percentile with 135 μg/g in the 7–12 months group, 65 μg/g in the 13–18 months group, 55 μg/g in the 19–24 months group, 40 μg/g in the 25–30 months group, 21 μg/g in the 31–36 months group, and 12 μg/g in the 37–48 months group (Table 1) [16]. The FC level was especially very high and variable in the 0- to 6-month age category [17]. In this age, FC levels are affected by feeding and delivery methods. FC values were higher in infants <6 months old who were breastfed and born by normal spontaneous vaginal delivery.

Monocytes and intestinal epithelial cells can be activated by different inflammatory triggers such as cytokines or bacterial products, leading to the increased expression and secretion of calprotectin, which can be detected in the stool. Moreover, calprotectin interacts with the toll-like receptor 4 (TLR4) activating macrophages and endothelial cells, and enhancing the expression of several adhesion molecules. In turn, TLR4 is also activated on neutrophils, thereby amplifying inflammation through the release of cytokines, reactive oxygen species and calprotectin again. This process, which involves both phagocytic cells and epithelial cells, leads to neutrophil transmigration and mucosal damage [22,23].

There are few data on the potential clinical benefits of FC measurements in groups of individuals with chronic disease at increased risk of GI inflammation outside IBD.

FC is a reliable noninvasive tool for assessing intestinal inflammation and plays a relevant role in clinical practice. However, clinicians must be aware that FC levels could be affected by several nonpathological conditions especially in the younger ages. Thus, the following key messages may be highlighted for pediatric clinicians.

· A cutoff of 50 μg/g is widely accepted in children aged from 5 to 17 years. However, in children of less than 5 years, the laboratory results must always be interpreted according to the clinical context, and in case of inconsistent results, FC measurements should be repeated, since age and diet could affect FC values.

· FC could be used to screen for inflammatory disease, and elevated results support the need to select the patients who need further investigation in primary care (colonoscopy).

· FC could be used to monitor the response to drug therapy in patients with chronic inflammatory conditions such as IBD. However, more research is needed to elucidate the evidence-based clinical practice guidelines in tailoring and monitoring drug therapy in patients with IBD.

· Moreover, additional information is warranted in conditions such as microscopic colitis and other inflammatory diseases of the GI tract associated with autoimmune disorders, or allergies owing to the currently limited database.

The importance of FC in pediatric gastroenterology has increased over time. It provides higher sensitivity, specificity and better information in the evaluation of pediatric IBD than other tests, as well as in the prediction of the clinical course of the disease. Despite many possible functions of FC, its biological function in relation to disease still remains unclear. Further studies are needed to elucidate the clinical relevance of FC in various pathological pediatric conditions and to establish whether FC has any implications beyond its role as an inflammatory mediator.