Small dense LDL (sdLDL) is a subtype of LDL that expresses greater atherogenicity than large buoyant LDL and is characteristic of the dyslipidaemia seen in metabolic syndrome, obesity and type 2 diabetes(Reference Berneis and Krauss1). With a dramatic increase in these conditions in both adults and children worldwide, a rapid and reliable method of estimating sdLDL is of potential value in the identification and subsequent management of ‘at-risk’ individuals.
Separation of LDL subclasses has been achieved by methods including preparative ultracentrifugation or polyacrylamide gradient gel electrophoresis (PGGE); the former has been developed to allow quantification using iodixanol density-gradient media and pre-staining(Reference Davies, Graham and Griffin2, Reference Graham, Higgins, Gillott, Taylor, Wilkinson, Ford and Billington3). While this method is suitable for high-through-put analysis, the procedure is only semi-quantitative. Fully-quantitative ultracentrifugation is more time-consuming, and not therefore suitable for large-scale screening. A simple and rapid method for sdLDL quantification based on Mg–heparin precipitation has been described by Hirano et al (Reference Hirano, Ito, Saegusa and Yoshino4). The present report describes a comparison of the latter method for sdLDL quantification with the iodixanol gradient ultracentrifugation method(Reference Graham, Higgins, Gillott, Taylor, Wilkinson, Ford and Billington3).
Blood sampled into a tripotassium citrate anticoagulant was obtained from nine adults. Plasma removed by centrifugation was separated into two portions that were used for sdLDL analysis by one of the two methods; the procedures were carried out blind by different operators. Ultracentrifugation of one portion in an iodixanol gradient was followed by fractionation and measurement of the cholesterol in twenty fractions, providing a complete lipoprotein profile for each individual from which the sdLDL could be estimated. The Mg–heparin method was performed as described(Reference Hirano, Ito, Saegusa and Yoshino4). Briefly, heparin–MgCl2 was added to plasma to separate VLDL, IDL and large buoyant LDL. sdLDL and HDL remained in the infranatant fraction and LDL-C was determined by the direct LDL-C method on an ILAB 650 autoanalyser (Randox Laboratories Ltd, UK).
On the small sample studied these two methods gave a reasonable correlation (Figure), as indicated by the similarity in fractionated cholesterol profiles and significant correlation between the cholesterol content of sdLDL. The Mg–heparin precipitation method may provide a suitable method for estimation of sdLDL in ‘at-risk’ individuals. This method may allow for quantitative high-through-put analysis for use in large-scale dietary interventions in populations who are dyslipidaemic.
Figure Comparison of sdLDL measurements by alternative methods.
