Comparison of CRMs and value-assigned quality controls: urea and uric Acid in human serum or plasma (CCQM-K142)

The CCQM OAWG 2017 CCQM-K142 "Comparison of CRMs and Value-Assigned Quality Controls: Urea and Uric Acid in Human Serum or Plasma" is the third Model 2 Key Comparison coordinated by the OAWG directly testing the chemical measurement services provided to customers by National Metrology Institutes (NMIs) and Designated Institutes (DIs) through certified reference materials (CRMs). CRMs certified for urea and/or uric acid content in human serum or plasma were compared using measurements made on these materials under repeatability conditions. Four NMIs/DIs submitted 10 CRMs certified for urea; five NMIs/DIs submitted 12 CRMs certified for uric acid. These materials represent most of the higher-order reference materials available then for these clinically important measurands. Uncertainty-weighted generalised distance regression was used to establish the Key Comparison Reference Function (KCRF) relating the CRM certified values to the repeatability measurements. The urea results for all 10 CRMs were considered to be equivalent at the 95 % level of confidence and were used to define the KCRF for urea. The uric acid result for one of the 12 CRMs was found to be non-equivalent: the submitting NMI re-evaluated the result and withdrew the material from use in defining the KCRF for uric acid. The remaining 11 CRMs were used to define the KCRF for uric acid. Monte Carlo methods were used to estimate 95 % level-of-confidence coverage intervals for the relative degrees of equivalence of materials, %d +/- U95 (%d), and of the participating NMIs/DIs, %D +/- U95 (%D). For the urea materials, the %D +/- U95(%D) intervals were within (-3 to 5) % of the consensus results. For the uric acid materials from four of the five NMIs/DIs, the %D +/- U95(%D) intervals were within (-4 to 5) % of the consensus results. These results demonstrated that with the exception of one material, the participating institutions could value-assign CRMs for urea and/or uric acid in human serum and plasma.