Towards routine high-throughput analysis of fecal bile acids: validation of an enzymatic cycling method for the quantification of total bile acids in human stool samples on fully automated clinical chemistry analyzers.

Objectives: Bile acid diarrhea is a common but underdiagnosed condition. Because the gold standard test (⁷⁵SeHCAT) is time-consuming and not widely available, fecal bile acid excretion is typically assessed by chromatography and mass spectrometry. Although enzymatic cycling assays are well established for the rapid and cost-effective analysis of total bile acids (TBA) in serum or plasma, their full potential has yet not been extended to stool samples in clinical routine.

Methods: The performance of the 'Total bile acids 21 FS' reagent (DiaSys) was evaluated in fecal matrix according to CLSI guidelines and EU-IVD Regulations (2017/745), and compared to an established microplate-based kit (IDK^®) by measuring patient stool samples (n=122). Method agreement was assessed by Passing-Bablok and Bland-Altman analysis. The quantification of eight individual BAs was assessed using HPLC-MS/MS as reference method.

Results: The DiaSys assay showed linearity between 3.5 and 130 μmol/L, good repeatability, total precision, and reproducibility with CVs of 1.7 %, 3.5 %, and 3.0 %. Limit of blank (LoB), detection (LoD), and quantitation (LoQ) were ≤0.17, ≤0.3, and 3.5 μmol/L, respectively. No significant interference from endogenous substances was observed. The methods showed good correlation up to 140 μmol/L (r=0.988), despite differences in the quantification of individual BAs, with mean deviations of 7 % (DiaSys) and 31 % (IDK^®), respectively.

Conclusions: The advantages of enzymatic TBA analysis on fully automated clinical chemistry platforms can be exploited for the routine analysis of stool samples. However, cycling assays may benefit from reference standards that take into account the composition of the fecal BA pool.