Candidate reference methods for determining target values for cholesterol, creatinine, uric acid, and glucose in external quality assessment and internal accuracy control. I. Method setup.

In Germany, the target values for External Quality Assessment (EQA) and internal accuracy control are determined by Reference Methods for several analytes, including cholesterol, creatinine, uric acid, and glucose. We present candidate Reference Methods for these compounds, based on isotope dilution-gas chromatography--mass spectrometry methods that have been developed at INSTAND, one of the two official Germany EQA reference institutions. Each Reference Method target value is calculated from six independent measurements performed on three different days. The mean method CVs ranged from 0.66% for glucose to 0.96% for creatinine. The inaccuracy (bias) of the methods is < 0.7%, as compared with the Standard Reference Material 909 of the National Institute of Standards and Technology. The maximum total error of a Reference Method value, including the 95% confidence interval and systematic errors, is < 2.3%. The presented candidate Reference Methods have been successfully used to set target values in the German EQA scheme and the internal accuracy control of routine laboratories.