Lipoprotein measurement

To address the issues discussed previously in this chapter, the NCEP convened expert panels to consider various aspects of diagnosis and treatment of hypercholesterolemia and develop guidelines for reliable lipid and lipoprotein mea-surements Two separate laboratory panels considered issues related to blood lipid and lipoprotein measurement. The first, the NCEP Laboratory Standardization Panel, was concerned with the measurement of total cholesterol, gnd the second, the NCEP Working Group on Lipoprotein Measurement, addressed the measurement of triglycerides, HDL cholesterol, and LDL cholesterol. The recommendations of both panels include extensive reviews of lipid and lipoprotein methodology, and the interested reader is referred to the original reports for details. Here we summarize the principal considerations and recommendations for clinical lipid and lipoprotein measurements. 

In developing recommendations for lipid and lipoprotein measurement, the NCEP panels considered several basic issues. 

To achieve these aims, development of reference methods was required as accuracy targets for lipid and lipoprotein measurements and guidelines were established for analytical performance. 

Various technologies have been used to measure plasma lipids and lipoproteins and lipoprotein subfractions, including enzymatic, immunochemical, and chemical precipitation reagents, and physical methods, such as ultracentrifugation, electrophoresis, column chromatography, and others. Such methods have been reviewed extensively. As mentioned earlier, however, the cholesterol content of any particular lipoprotein class can vaiy somewhat from individual to individual. Moreover, although different methods of lipoprotein separation may produce similar lipoprotein fractions, they usually do not produce identical fractions, giving rise to systematic biases between methods that purport to measure the same component. The present discussion focuses primarily on methods and procedures commonly used in clinical practice for lipid and lipoprotein measurements. 

Additives including anticoagulants such as citrate and fluoride can have large osmotic effects that cause water to shift from the cells to the plasma. This dilutes the lipoprotein by 10% or more and produces erroneously low values. EDTA, traditionally the preferred anticoagulant for lipoprotein measurements, causes a slight dilution, but has been used because it also inhibits certain oxidative and other changes that can affect some lipoprotein or apofipoprotein measurements. Lipid and Hpoprotein concentrations in EDTA plasma tend to be about 3% lower than in serum, an effect that may not be readily noticeable in HDL cholesterol measurements. EDTA, however, complexes some of the Mn in the heparin-Mn method, and it has been found necessary to use a higher concentration of MnCb (0.092 mol/L, final concentration in the reaction system) when the procedure is... 

The normal physiological component of variation is calculated from the total variation of measurements in serial specimens from the same patients, after adjusting for analytical variation. Such estimates differ somewhat from study to study, but after an extensive review of the literature, the NCEP panels concerned with lipid and lipoprotein measurement assumed average physiological... 

Routine methods. In most cases, lipid and lipoprotein measurements can be made using specimens of either serum or plasma. Measurements in EDTA plasma can be converted to serum-equivalent values using the following equation ... 

Glycerol blanking. The NCEP Working Group on Lipoprotein Measurement endorses the following recommendations, adapted from the Lipids and Lipoproteins Division of the American Association for Clinical Chemistry ... 

TABLE 26-27 National Cholesterol Education Program Recommendations for Analytical Performance of Lipid and Lipoprotein Measurements ... 

It is important to note that the NCEP panels considered that the physician usually does not distinguish between lipid and lipoprotein measurements on the basis of the methodology used to make the measurements. For this reason, the NCEP guidelines do not distinguish between measurements made in the laboratory or those made in alternative settings with desktop analyzers or other methods. 

Myers GL, Cooper GR, L.O. H, Hassemer DJ, Kimberly MM. Standardization of hpid and lipoprotein measurement. In Rifai N, Warnick GR, Dominiczak M, eds. Handbook of lipoprotein testing, VoL Washington DC AACC Press, 2000 717-48. 

Recommendations on Lipoprotein Measurement From the Working Group on Lipoprotein Measurement. National Cholesterol Education Program, Vol. Bethesda NIH/NHLBI NIH Publication, 1995 95-3044. 

The Working Group on Lipoprotein Measurement (Bach-orik PS, Myers GL, Ross JW, Stein EA, and Warnick GR) (1995) Recommendations on Lipoprotein Measurement. National Institutes of Health - National Heart, Lung, and Blood Institute. NIH Publication No. 95-3044. September. 

---