Faecal immunochemical testing (FIT): Sources of result variation based on three years of routine testing of symptomatic patients in English primary care.

James, T; Nicholson, B D; Marr, R; Paddon, M; East, J E; Justice, S; Oke, J L; Shine, B

Introduction: We aimed to determine the analytical capabilities of a commonly used faecal immunochemical test (FIT) to detect faecal haemoglobin (Hb) in symptomatic people attending primary care in the context of the English NICE DG30 guidance.Materials and Methods: Faecal specimens referred from primary care patients were utilised to undertake a series of studies to provide an Data obtained from independent verification studies and clinical testing of the HM-JACKarc FIT method in routine primary care practice were analysed to derive performance characteristics.Results: Detection capabilities for the FIT method were 0.5 µg/g (limit of blank), 1.3 µg/g (limit of detection) and 3.0 µg/g (limit of quantitation). Of 33 non-homogenised specimens, 31 (93.9%) analysed in triplicate were consistently categorised relative to 10 µg/g, compared to all 33 (100%) homogenised specimens. Imprecision was higher (median 27.8%, (range 20.5% to 48.6%)) in non-homogenised specimens than in homogenised specimens (10.2%, (7.0 to 13.5%)). Considerable variation was observed in sequential clinical specimens from individual patients but no positive or negative trend in specimen degradation was observed over time (p=0.26).Discussion: The FIT immunoassay evaluated is capable of detecting faecal Hb at concentrations well below the DG30 threshold of 10 µg/g and is suitable for application in this context. The greatest practical challenge to FIT performance is reproducible sampling, the pre-analytical step associated with most variability. Further research should focus on reducing sampling variability, particularly as post COVID-19 guidance recommends greater FIT utilisation.