Lipids

All the lipids in plasma exist as complexed with proteins. Lipoproteins, consisting of various proportions of lipids, range from 10 to 1000 nm in size (the higher the percentage of the lipid, lower is the density of the resulting lipoprotein, and larger is the particle size). Chylomicrons (diameter 70-1000 nm, density <0.95 g/mL) are present in plasma after a person ingests a fatty meal. These particles originate in the intestinal epithelial cells and consist mostly of lipids. Chylomicrons are absorbed by the adipose tissue and liver. Liver secretes lipoprotein particles called very-low-density lipoproteins (VLDL, density < 1.006 g/mL), low-density lipoproteins (LDL, density = 1.006-1.063 g/mL), and high-density lipoproteins (HDL, density = 1.063-1.21 g/mL) containing decreasing amounts of lipids in that order. The lipoprotein particles with high lipid contents are micellar and are the main source of assay interference. Unlike bilirubin and hemoglobin, lipids normally do not participate in chemical reactions and mostly cause interference in assays by their turbidity. The micellar particles scatter light, and the amount of light scattered is higher at lower wavelength. Because scattered light does not follow Lambert-Beer law of absorbance, scattering normally reduces absorbance producing false results (positive or negative, depending on the reaction principle) (13). Lipemic interference is most pronounced with spectropho-tometric assays, less important with fluorimetric methods, and rarely interferes with chemiluminescent methods. Thus, assays that use turbidimetry for signal are the ones most affected by lipid interference (14). Lipemia may also interfere with assays for fat-soluble analytes, such as steroids and their derivatives. In such cases, interference arises from solvent partitioning and solute exclusion of the analyte between the lipid and the aqueous phases.

Like bilirubin and hemolysis, package inserts do report the extent of lipid interference in a commercial assay. Lipids, however, present a special problem because of lack of readily available standardized materials. Most manufacturers use IntraLipid, a synthetically produced emulsion containing soybean oil and egg phospholipids, for intravenous administration, to spike specimens to simulate lipemic samples. However, samples with IntraLipid do not perfectly mimic lipemic samples (15). Sometimes native lipemic samples have falsely low results in certain assays, but IntraLipid-spiked samples containing same triglyceride concentration do not. This can be understood from the fact that native plasma lipids are very heterogeneous with a wide variety of micellar particle size distribution. Size, charge, and shapes of the particles influence their light-scattering capabilities. Among the plasma lipoproteins, chylomicrons and VLDL particles only scatter light. VLDL exists in three size classes: small (27-35 nm), intermediate (35-60 nm), and large (60-200 nm). Only the latter two sizes of VLDL scatter light. Chylomicron particles vary greatly among individuals, and even in the same individual, depending upon the time that the sample is collected after the meal (16). Thus, even though Kazmierczak and Catrou (17) argue that interference studies need to be done with specimens from patients with hyper-lipidemia, or hyper-bilirubinemia, the subject to subject variation makes it impossible to guarantee that an assay will not be subjected to interference from these interferents. Therefore, studies performed using IntraLipid-spiked lipemic specimens may not necessarily represent lipemic samples from a patient. These results, at best, may be taken as a guideline, rather than final. Assay results from lipemic samples must be interpreted with caution.

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