Candidate reference methods for determining target values for cholesterol, creatinine, uric acid, and glucose in...

We describe the testing of transferability of candidate Reference Methods developed by INSTAND for cholesterol, creatinine, uric acid, and glucose. The methods are based on isotope dilution-gas chromatography--mass spectrometry. The study consisted of two parts: setup of the methods and self-evaluation for readiness in the collaborating laboratory, followed by independent measurements in parallel with INSTAND. Criteria used for judging the transferability and general reliability of the candidate Reference Methods were: the accuracy and precision of the collaborating laboratory and the agreement between the two laboratories. The accuracy was judged from the results on the Standard Reference Material 909 from the National Institute of Standards and Technology. For all analytes except glucose the bias from the certified value was < 0.7%. The mean intralaboratory imprecision ranged from 0.66% to 1.24%. The agreement between the results was tested by an advanced linear-regression analysis and Student's t-test. In general, the results demonstrate that the candidate Reference Methods developed by INSTAND can be successfully transferred without loss of their inherent precision and accuracy.