A possible model for accuracy control of determination of serum cholesterol with use of reference methods. A NORDKEM...

A candidate definitive method for determination of total serum cholesterol developed at the Center for Analytical Chemistry, National Bureau of Standards, USA, has recently been compared with a reference method based on isotope dilution-mass spectrometry, developed at Huddinge Hospital. There was no significant difference (0.2%) in results obtained with the two methods. The Huddinge method was used here for assessment of inaccuracy of a defined enzymatic method, set up and used at four laboratories, one in each of four Nordic countries. The results obtained in the analysis of 21 patient samples were not significantly different from those obtained with the isotope dilution method. In the analytical range 1.7-14.2 mmol/1, the regression equation for the defined enzymatic method (y) versus the isotope dilution method (x) was y = 0.994x - 0.06 and the correlation coefficient 0.9999. The mean between-laboratory variation for the defined enzymatic method was 2.3%. The possibility is discussed that the defined enzymatic method can be used as candidate secondary reference method for assessment of inaccuracy of field methods in national quality control activities. In such a control system, the accuracy can be traced back to the definitive methodology through the hierarchical system.