Calprotectin as a Reliable Surrogate Marker for Bowel Inflammation in SpA?

Increased serum calprotectin and C-reactive protein (CRP) levels are independently associated with microscopic bowel inflammation in spondyloarthritis (SpA).  Fecal calprotectin levels are also increased in patients with SpA and coexistent microscopic bowel inflammation as compared to those without comorbid intestinal inflammation.  This data was reported by researchers from the Department of Rheumatology at Ghent University in Belgium and published in the Annals of Rheumatic Diseases. 

Patients diagnosed with SpA have a higher risk of developing intestinal inflammation, and often are found to have subclinical, microscopic bowel inflammation. Thus, screening for intestinal involvement in these patients represents a challenge as symptoms are not always reported by patients.  

Adding to the need for noninvasive screening methods in SpA is the fact that a small percentage of patients with SpA develop inflammatory bowel disease (IBD) over the course of disease, with the risk of progression related to the presence of microscopic bowel inflammation. To address the need for screening tests for bowel inflammation in SpA, researchers evaluated serum and fecal levels of a protein involved in early phases of the immune response in patients diagnosed with SpA, with and without bowel involvement.  

Calprotectin, a protein expressed in phagocytic myeloid cells, is released from activated monocytes and granulocytes as part of the inflammatory response cascade.  It exerts its proinflammatory effects via activation of toll-like receptor 4-dependent mechanisms.  Fecal levels of calprotectin have been used as biomarkers of IBD disease activity, and researchers speculated that its levels may be viable markers for bowel involvement in SpA as well.  

Researchers evaluated 125 patients with newly diagnosed axial and/or peripheral SpA  as defined by the Assessment of SpondyloArthritis international Society (ASAS) criteria from a prospective follow-up study (GIANT cohort).  Information related to disease activity, drug intake, GI symptoms, tender and swollen joint counts, enthesitis, and evaluation of axial mobility were taken.   

Serum samples were also taken from patients with SpA, as well as 39 healthy study participants and 23 patients with rheumatoid arthritis (RA) who served as controls.  Forty four fecal samples were taken within 7 days of colonoscopy prior to bowel preparation from patients with SpA later in the course of the study. 

Ileocolonoscopy was performed in all patients with SpA (total n=125; axial SpA n=104, ankylosing spondylitis n=45, non-radiographic axial SpA n=59) to determine whether microscopic bowel inflammation was present.  Biopsies were classified as normal, acutely inflamed, or chronically inflamed by a blinded, experience pathologist.  Serum and fecal calprotectin levels were analyzed by monoclonal antibody enzyme linked immunosorbent assay analysis. 

Microscopic bowel inflammation was found in 42.4% of patients (n=53) diagnosed with SpA, all with subclinical bowel inflammation who did not report any gastrointestinal complaints. Macroscopic lesions were found in 31% of patients. Serum calprotectin levels were significantly increased in patients with SpA and RA as compared to healthy controls (P<.001 and P=.036, respectively).  Differences were not found between patients with RA and SpA (P=.417). Compared with non-smokers,  smokers had significantly higher serum calprotectin (P=.007), however the differences between controls and patients with SpA or RA remained statistically significant even after correcting for smoking in linear regression analyses (P<.001 and P=.031, respectively). 

Levels of serum calprotectin and CRP were significantly higher in patients with microscopic bowel inflammation as compared to patients with normal bowel histology (P=.033 and P=.036, respectively). After adjusting for associated factors including serum CRP, nonsteroidal antiinflammatory drug use, time between serum samples and colonoscopy, and smoking, serum calprotectin levels remained linked to bowel inflammation.  

Receiver operating characteristic (ROC) analysis was used to define cut-offs for serum calprotectin and CRP levels for the detection of bowel inflammation (3340 ng/mL and 0.4 mg/dL, respectively).  Fecal calprotectin levels were higher in patients with microscopic bowel inflammation as compared to those with none (P=.036). Fecal calprotectin correlated with CRP (rS=0.481; P=.002), but not with serum calprotectin (P=.384). Fecal calprotectin cut-off values for optimal sensitivity (64.3%) and specificity (73.3%) for detection of bowel inflammation was 85 mg/g.

The researchers then sought to develop a predictive model for bowel inflammation in SpA using CRP and calprotectin.  A combination of serum CRP and calprotectin were used since both appeared to be independently associated with microscopic bowel inflammation.  The added value of fecal calprotectin levels was then evaluated with the combination of other tests.  They found that when both serum CRP and calprotectin were increased, adding fecal calprotectin did not contribute further to diagnosis. However, in patients with either CRP or serum calprotectin levels elevated, the addition of fecal calprotectin testing did add information about the frequency of bowel inflammation. 

“When CRP and serum calprotectin were low or when both were high, determination of fecal calprotectin did not provide added value for detection of bowel inflammation, although it did provide further differentiation when only one of these serum markers was elevated..we propose a two-step screening approach using CRP and serum calprotectin, and only if necessary, fecal calprotectin for risk assessment of microscopic bowel inflammation in SpA,” the study authors stated. 

Summary and Clinical Applicability

Serum and fecal calprotectin levels appear to be reliable biomarkers of SpA, suggesting that they may be helpful in identifying patients with SpA that have elevated risk of subclinical bowel inflammation who may most benefit from further screening. The screening approach for bowel inflammation in SpA detailed in this study generated an area under the ROC curve of 74.4%.