ABSTRACT
BACKGROUND: WEQAS, one of the largest EQA (External Quality Assessment) providers in the UK, offers a bile acid EQA scheme, with linear serum pools containing cholic acid, deoxycholic acid and chenodeoxycholic acid, reflecting levels observed in obstructive cholestasis. Total bile acids are currently measured routinely by non-specific enzymatic methods. Traceability of results to the SI unit utilizing reference target values is the preferred method of comparison of returned results where available, ensuring the transfer of accuracy from definitive methods to routine methods. METHODS: Target values have been assigned to EQA material utilizing isotope dilution gas chromatography mass spectrometry (ID-GCMS). The methodology was based on published routine methods and adapted for use as a ID-GCMS target method. The total bile acid target value was reported as the sum of the three major bile acids measured: cholic acid, deoxycholic acid and chenodeoxycholic acid. RESULTS: The produced target values have been used to assess the performance of total bile acid methods. A degree of variability was observed between the third-generation enzyme-formazan methods and the fifth-generation thio-NADH methods. Additionally, the Sentinel results showed a positive bias in comparison to their peer formazan method group. CONCLUSION: The use of ID-GCMS target results provides a common comparison for resumed results in EQA schemes, highlighting any method differences. Thus can then aid in the harmonisation of results observed for each of the different methods.