Quantification of plasma lipids, lipoproteins, and apolipoproteins often is required for screening of apparently healthy individuals and for follow-up of patients with dyslipoproteinemias. We studied the performance of two new techniques for direct measurement of low-density lipoprotein cholesterol (LDL-C); likely the most atherogenic cholesterol fraction in humans. The Sigma direct LDL-C method is based on removal of high-density lipoprotein (HDL) and very low-density lipoprotein (VLDL) by immunoabsorption and measurement of total cholesterol in the absorbed serum, whereas the Helena direct cholesterol method involves quantitative assessment of cholesterol by a coupled enzyme reaction following electrophoretic separation of various lipoprotein fractions in agarose gel. Thus, the Helena method concomitantly measures cholesterol in all major lipoprotein fractions such as HDL, lipoprotein(a) (Lp[a]), VLDL, LDL and chylomicrons. While both direct cholesterol methods were capable of providing analytically acceptable results, some limitations were revealed in both. LDL-C measured by the Sigma method also contained trace amounts of VLDL-C (and, as part of its design, Lp[a]-cholesterol in its entirety), whereas the Helena method was technically sensitive to perform. A major advantage of both methods is that they can be carried out in lipemic specimens and in patients with Type III hyperlipoproteinemia (so-called broad beta diseases) when use of the Friedewald formula as an indirect estimate of LDL-C is inappropriate. Since hypertriglyceridemia does not interfere with these new methods, non-fasting specimens can also be used for both diagnosing cholesterol disorders and monitoring therapeutic interventions.
