Bile acid reference intervals for evidence-based practice

In this edition, Huri et al. (BJOG 2022) have added to the growing literature defining pregnancy-specific reference ranges for total serum bile acid (TSBA) concentrations.

Reference intervals in clinical pathology are typically calculated using an ‘indirect resource’, such as stored laboratory samples. Results are partitioned into biologically relevant groups (typically age and sex), outliers are excluded to remove anomalous results due to disease, and the central 95% of the population is calculated for each group. Two large studies, that of Huri et al. and our own (Mitchell et al. BJOG 2021;128:1635–44), have recently calculated reference intervals for non-fasting TSBA concentrations in the third trimester of pregnancy, finding strikingly similar results for the upper limit (20.2 and 18.3 μmol/l, respectively). Both studies excluded samples with cholestatic pathology before analysis, treating the cohorts as a ‘direct resource’ and obviating exclusion of outliers...