Test accuracy of faecal calprotectin for inflammatory bowel disease in UK primary care: a retrospective cohort study of the IMRD-UK data

Abstract

Objective: To estimate the test accuracy of faecal calprotectin (FC) for inflammatory bowel disease (IBD) in the primary care setting using routine electronic health records.

Design: Retrospective cohort test accuracy study.

Setting: UK primary care.

Participants: 5970 patients (≥18 years) without a previous IBD diagnosis and with a first FC test between 1 January 2006 and 31 December 2016. We excluded multiple tests and tests without numeric results in units of µg/g.

Intervention: FC testing for the diagnosis of IBD. Disease status was confirmed by a recorded diagnostic code and/or a drug code of an IBD-specific medication at three time points after the FC test date.

Main outcome measures: Sensitivity, specificity, and positive and negative predictive values for the differential of IBD versus non-IBD and IBD versus irritable bowel syndrome (IBS) at the 50 and 100 µg/g thresholds.

Results: 5970 patients met the inclusion criteria and had at least 6 months of follow-up data after FC testing. 1897 had an IBS diagnosis, 208 had an IBD diagnosis, 31 had a colorectal cancer diagnosis, 80 had more than one diagnosis and 3754 had no subsequent diagnosis. Sensitivity, specificity, and positive and negative predictive values were 92.9% (88.6% to 95.6%), 61.5% (60.2% to 62.7%), 8.1% (7.1% to 9.2%) and 99.6% (99.3% to 99.7%), respectively, at the threshold of 50 µg/g. Raising the threshold to 100 µg/g missed less than 7% additional IBD cases. Longer follow-up had no effect on test accuracy. Overall, uncertainty was greater for specificity than sensitivity. General practitioners' (GPs') referral decisions did not follow the anticipated clinical pathways in national guidance.

Conclusions: GPs can be confident in excluding IBD on the basis of a negative FC test in a population with low pretest risk but should interpret a positive test with caution. The applicability of national guidance to general practice needs to be improved.

Keywords: gastroenterology; inflammatory bowel disease; primary care.

Figure 1

Flow diagram of inclusion criteria of faecal calprotectin (FC) tested patients into test accuracy study. *Results not coded invalid or indeterminate in THIN. †Fifteen different units of measurements were used to record FC levels. µg/g is the most commonly used in publications and laboratories. ††FC level of >30µg/g ambiguous at 50µg/g and 100µg/g thresholds

Figure 2

Receiver operating characteristic curve of sensitivity and false positive rate (1-specificity) for thresholds of 33–300 µg/g for the clinical question inflammatory bowel disease (IBD) versus non-IBD for tests with at least 6 months (n=5970), 12 months (n=4793) and 24 months (n=2662) follow-up available after the faecal calprotectin test date.

Figure 3

Sensitivity and specificity at thresholds of 33–300 µg/g for inflammatory bowel disease (IBD) versus non-IBD and IBD recorded within 6 months following the faecal calprotectin (FC) test date (n=5970).