Serum cholesterol during 27 years: assessment of systematic error and affecting factors and their role in interpreting population trends

BACKGROUND: The aim was to assess which factors cause a systematic error in serum total cholesterol measurements and how bias can influence the interpretation of serum cholesterol changes of the Finnish population. METHODS: Data on precision and accuracy during 27 years for serum total cholesterol were documented from participation in 438 rounds of five different external quality assessment (EQA) programs. RESULTS: The mean annual accuracy (bias) of the cholesterol assay using the results from all EQAs during 1978-2004 was -0.74% (95%CI -0.88 to -0.60). An exceptionally large deviation in bias coincided with the introduction of a new serum calibrator lot. New methods or instrumentation had only a minor impact on serum cholesterol bias. The mean serum cholesterol bias during the latest five population studies in 1982-2002 was -0.10% (95%CI -0.60 to 0.40) but comparison of the bias between the last study (CDC EQA in 2002) and the four previous ones (WHO EQA) showed a net difference of 3.32% (p<0.001). Correcting the mean serum cholesterol of men with respect to WHO and CDC EQA bias changed the interpretation for the last two survey years from an increase of 1.8% to no change. CONCLUSIONS: It is necessary to participate in EQA programs, which include target values measured by the CDC cholesterol reference method and then to perform bias corrections on the mean cholesterol values of the populations.