Serum cholesterol during 27 years: Assessment of systematic error and affecting factors and their role in interpreting population trends
Introduction
In the estimation of the population-attributable risk of coronary heart disease mortality, blood lipids play a central role [1]. Longitudinal risk assessment is done by regular monitoring of serum lipids in population health surveys spanning several decades [2]. Knowing the systematic error of serum cholesterol for each time point is important for minimizing the error in risk calculations and for interpreting changes in serum cholesterol on a population level.
Laboratories usually confirm and adjust the analytical performance of their methods by participating in external quality assessment (EQA) programs. Activity in this field for lipid measurements started in the early 1970s in clinical chemistry in the Nordic countries [3], [4] as well as in other countries and organizations, e.g. WHO [5] and Center for Disease Control (CDC, Atlanta) [6].
There are many variables that may cause problems in maintaining satisfactory precision and accuracy of methods from year to year. New methods and analysers replace old ones and these changes may affect the analytical bias [7]. There are also many other factors that add to the measurement uncertainty in assays. These include method of calibration, use of external and internal quality assessment materials, calibration and control materials. The reference sample materials (fresh, frozen, lyophilized) may vary between different EQA surveys. Recently it has been shown that lyophilized sera gave 6% lower bias results for cholesterol compared with fresh–frozen sera [8]. The major problem is that during manufacturing these lyophilised materials, changes occur in lipoproteins and other serum components causing a different reactivity in chemical reactions compared to that observed for native serum samples. This artificial matrix effect is well documented in the literature [7], [9], [10], [11]. Because most of the commercial calibrators are also lyophilized sera, erroneous conclusions concerning the accuracy of lipid measurements may be drawn [8]. In a clinical setting a higher systematic error can be allowed compared to that necessary in epidemiologic research.
Serum cholesterol and other cardiovascular risk factor levels in the Finnish population have been followed over a 30-year period. Risk factor measurements began with the North Karelia Project [12] in the province of North Karelia and in the reference area Kuopio. Since 1972 risk factors have been monitored regularly every 5 years in six different areas from a random sample of about 10,000 Finnish adults.
In the present work we show how we have succeeded in maintaining the accuracy of the serum cholesterol assay by participating regularly in several different EQA programs during a quarter of a century. We evaluated the effect of modifications of the methods, types of calibrations and implementation of new analysers on bias. In addition, to estimate the trueness of mean cholesterol values in large Finnish population health surveys, we have calculated the bias correction for mean cholesterol values of the Finnish population during Finrisk health survey years.
Section snippets
External quality assessment
Since 1978 the Laboratory of Analytical Biochemistry at KTL has taken part in five different EQAs, altogether 438 rounds. Quality assessment samples have been received from the WHO Lipid Reference Program (Prague) two–six times a year in 1978–1997 (three concentration levels), from the ShortTerm A surveys of the External Quality Assessment Scheme (LabQ1, Labquality Ltd, Helsinki) twelve times a year in 1978–2004, from the Lipids and Lipoprotein surveys (LabQ2, Labquality Ltd, Helsinki) twice a
Changes in the method
In the 1970s total serum cholesterol was determined by the modified Liebermann–Burchard (LB) method [14]. The automated cholesterol method was developed for use on a Technicon AutoAnalyzer II system (Technicon Instruments Corporation, Tarrytown, NY) [15]. The enzymatic CHOD cholesterol method had been under development using also AutoAnalyzer II (AAII) equipment and tested for some years before finally taking into routine use in 1980 (Table 2). Using fresh sera the mean serum cholesterol
External quality assessment
Efforts to eliminate systematic errors of especially lipid methods by EQA in clinical chemistry laboratories in Scandinavia have a long tradition [16], [17], [18]. The comparability of results for many methods between clinical chemistry laboratories was assured by founding of the EQA organization (Labquality Oy) in the 1970s in Finland. Without this national external quality program it would have been impossible to trace reliably changes in the serum cholesterol levels of the Finnish population
Conclusions
The systematic error of the cholesterol methods has had only a minor effect on estimating and interpreting changes in serum cholesterol in our population surveys. To be able to detect ever smaller changes in serum cholesterol on a population level we conclude from our experience that 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.
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