He et al HDL, Cardiovascular Disease, and Diabetes Mellitus e11 (ie, a small cholesterol-poor HDL particle is equivalent to a large, cholesterol-rich particle). Indeed, clinical studies suggest that HDL particle concentration may provide information on CVD status independent of HDL-C. 28, 29 Two principal methods have been described for quantifying HDL particles in human plasma, one based on nuclear magnetic resonance (NMR) 30, 31 and the other based on ion mobility analysis, 32 classifying HDL into large, medium, and small subfractions. One potential limitation of the NMR method is that it is based on measurements of distinctive NMR signals arising from the HDL particle lipids with assumptions made about the physical properties of HDL lipids and HDL volume. The HDL particle concentration is derived from deconvolution of a complex overlapping NMR signal of all lipoprotein classes (including VLDL [very-low-density lipoprotein] and LDL) and has not yet been formally biochemically validated. 33 To quantify HDL by ion mobility analysis, HDL particles are separated on the basis of their differential mobility in a carrier gas and directly counted using laser scattering. 32, 34 The ion mobility analysis method is further calibrated using both nanoparticles and recombinant HDL, yielding molar concentrations of HDL in line with estimated stoichiometry of apoA-I (apolipoprotein AI) on HDL in contrast to NMR. 34 Size exclusion fast protein liquid chromatography or high-performance liquid chromatography systems can then be used separate lipoproteins according to particle size in solution. Furthermore, structure and composition analyses of HDL particles (proteomic/lipidomic methods and immunoaffinity chromatography), provide additional assessment of HDL particles. Because of the use of different methods to classify, isolate, measure, and normalize HDL, it is difficult to compare studies and experimental results centered on analyzing HDL functions. Thus, to assess the issue of HDL function as a potential therapeutic target, standardized robust, simple, and universal analytical methods will be required in the future.