| MEASUREMENT OF LIPOPROTEINS
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| When we measure 'total plasma cholesterol' or 'plasma triglycerides' in clinical laboratories, we measure just one of their components across all classes of lipoproteins. (Note that clinicians use the term 'triglycerides' rather than 'triacylglycerol'; you will have noticed that here we use both to familiarize the reader with both conventions.) It is very difficult to measure the concentration of lipoprotein particles in plasma because of their heterogeneous composition. To get around the problem, we exploit the fact that most of the cholesterol present in plasma is in LDL, and therefore total plasma cholesterol concentration is an approximation of the LDL concentration. Similarly, plasma triacylglycerol (triglyceride) concentration is a marker of the sum total of VLDL and IDL. To obtain a more accurate picture of lipid metabolism we measure, or calculate, the cholesterol present in different lipoprotein fractions that are first separated by precipitation or ultracentrifugation. The results of such measurements are expressed as, e.g., LDL-cholesterol or HDL-cholesterol. Specialist laboratories separate all lipoprotein subfractions employing the technique of ultracentrifugation (see Fig. 17.5).
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| Figure 17.5 (A) The diagnostic lipid measurements. The measurement of total cholesterol is a first-line screening test. The next step, fasting lipid profile, provides information about the main lipid markers of cardiovascular risk, i.e the concentrations of of total cholesterol, LDL-cholesterol, and HDL-cholesterol and triacylglycerol (triglycerides). The analysis of lipoprotein subfractions by ultra-centrifugation, and the measurement of apoproteins are specialist tests. The total-cholesterol-to-HDL ratio indicates the balance between cholesterol transport to and from peripheral tissues. A ratio above 5 indicates an increased cardiovascular risk. |
| Figure 17.5 (B) The calculation of LDL-cholesterol concentration in plasma. LDL-cholesterol can be calculated from the value of total cholesterol, triglycerides, and HDL-cholesterol using the Friedewald formula. |
| Please note that the laboratory measurements are made either in plasma or in serum (plasma devoid of fibrinogen, see Chapter 3), depending on the substance measured and the method employed. For the sake of simplicity we will refer here to measurements in plasma.
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