Was evidence on test accuracy from secondary care enough to approve faecal calprotectin testing for primary care use?

Introduction: Decisions about test availability for patient care are often based on limited evidence. Faecal calprotectin (FC) testing has been approved by NICE for the differential diagnosis of inflammatory bowel disease and irritable bowel syndrome in UK primary care in adults with unexplained abdominal complaints. The decision was based solely on evidence from secondary care. However, transferability of test accuracy estimates between settings cannot be assumed when patient populations differ between settings. We aimed to reassess the evidence against a primary care pathway with FC testing to evaluate what we know about test accuracy of FC testing in primary care. Methods: We updated the previous test accuracy review [1] of FC testing with colonoscopy as the reference standard. Meta-analyses in R version 3.4.1 explored heterogeneity. Results: Thirty-eight studies were eligible including five from primary care. The studies’ patient populations, however, resembled a continuum from primary to secondary care. None of the studies sufficiently addressed the research question. Primary care studies either defined the target disease broader than the intended IBD group or did not use the preferred reference standard. The studies were highly heterogeneous in terms of tests and clinical question frequently offering more than one 2x2 diagnostic table for different tests and different clinical questions. Meta-analysing outcomes and investigating setting as a covariate was not feasible as this would have required expressing a preference for a test and clinical question and disregarding others. Separate exploration of test type and clinical question by meta-regression showed that neither can be assumed to be generic. Discussion: We are lacking evidence to ascertain the assumed test performance of FC testing in primary care. Alternative approaches to simply categorising settings into primary and secondary care are needed to assess studies for their plausibility to reflect the performance of FC testing in primary care