Objective Treatment decisions in inflammatory bowel diseases are increasingly based on longitudinal tracking of calprotectin results. Many hospital laboratories measure calprotectin levels in sent-in stool samples with an enzyme-linked immunosorbent assay (ELISA). Several manufacturers introduced a lateral flow–based test with software application that turns a smartphone camera into a reader for quantitative measurements. We compared three home tests (IB Doc , QuantonCal and CalproSmart) and companion ELISA tests (fCAL, IDK-Calprotectin and Calprotectin-ALP) to see if measurement pairs agreed sufficiently. Design A method comparison study was conducted with stool samples from patients with active or quiescent inflammatory bowel disease. Medical students without any specific laboratory training carried out the home tests with two iOS (iPhone 6 and 7) and two Android devices (Samsung Galaxy S6 and Motorola Moto G5 Plus). Two experienced laboratory technicians measured the calprotectin concentration with the ELISA method. Primary outcome was test agreement (defined as percentage of paired measurements within predefined limits of difference). Secondary outcome included reading error rate (RER) per smartphone type. Results We performed 1440 smartphone readings and 120 ELISA tests. In the low calprotectin range (≤500 μg/g) IB Doc , QuantOnCal and CalproSmart showed 87%, 82% and 76% agreement with their companion ELISAs. In the high range (>500 μg/g) the agreement was 37%, 19% and 37%, respectively. CalproSmart and QuantOnCal had significantly higher RERs than IB Doc (respectively 5.8% and 4.8%, versus 1.9%). Forty-three percent of reading errors was on the Motorola device, in particular with the QuantOnCal application. Conclusions All three calprotectin home tests and companion ELISAs agreed sufficiently when concentrations are ≤500 μg/g. To minimize wrongful interpretation of calprotectin changes over time it is essential to always use the home test and companion ELISA of one and the same manufacturer. Manufacturers should explicitly evaluate and report the suitability of commonly used smartphones for quantitative calprotectin readings.

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