Verification of Bühlmann faecal calprotectin test (FCAL TURBO test) on Abbott Architect c8000 analyzer

Background: Feacal calprotectin as a non-invasive marker of inflammatory bowel disease (IBD) is routinely measured in our laboratory with fluoroenzymatic immunoassay (FEIA, Thermo Fisher Scientific) on Phadia 100 analyzer. The aims of this study were: i) verification of fCAL turbo test (Buhlmann) on ARCHITECT c8000 (Abbott Diagnostics) using particle enhanced turbidimetric immunoassay (PETIA) and ii) verification of calprotectin stability specified by manufacturer in faecal extract obtained with CALEX® Cap device extraction tube (Buhlmann). Methods: To evaluate assay imprecision, commercial quality control sample and patient faecal extract were run in triplicate for five days. Comparison of FEIA and PETIA assays was made on 40 faecal extracts with calprotectin concentration throughout entire measuring range. Carry-over was calculated by measuring calprotectin in sample with high concentration (in duplicate) followed with measurement of calprotectin in sample with low concentration (in triplicate). Verification of stability was performed by measuring calprotectin in four faecal extracts (27–1035 mg/kg) stored at 4-8 C during 6 days. Allowable bias was set at 30%, according to Labquality external quality assessment criteria. Results: Interlaboratory imprecision for fCAL turbo test was 2.41% for mean calprotectin concentration of 123.9 mg/ kg and 5.48% for 74.5 mg/kg. Passing-Bablok regression analysis showed statistically significant proportional difference [y=0.51(95%CI -35.66 - 0.61) + 1.83(95%CI 1.33 - 3.41)x] and Bland-Altman analysis showed both proportional and constant difference between two methods. Calprotectin proved to be stable in faecal extracts kept at 4-8 C during 6 days in samples with concentration at cut off value (50 mg/kg) and higher, with mean biases 2.67–21.63%. Sample with concentration lower than cut off had mean bias 35.1% but it has no influence on clinical decision. Calculated carry-over effect was 0.10%, in accordance with manufacturer's claim. Conclusion: fCAL turbo test has satisfactory imprecision and minimum carry-over effect. Calprotectin in faecal extract was proved to be stable according to manufacturer specification. Comparison study showed that FEIA and PETIA methods cannot be used interchangeably.