HDL laboratory test

High Density Lipoprotein Microchapters
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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]; Associate Editor(s)-in-Chief: Rim Halaby, M.D. [2]

Chemical Measurements

HDL, as discussed above, forms from two large proteins, predominantly apo A-I and apo A-II, positioned, back to back. Charged amino acids on the outer surface attract water, making the particles both water-soluble (so as to carry fats within the blood) and able to associate with HDL receptors on in the surface of cells, e.g. the macrophages of which plaque is predominantly composed, at least in the early stages. Cholesterol is carried within and between the two particles. If the particles pick up more cholesterol, then the particles enlarge and a third or fourth apo A protein joins the grouping, termed large HDL. Chemical measurements can be used to estimate HDL concentrations present in a blood sample, though such measurements may not indicate how well the HDL particles are functioning to reverse transport cholesterol from tissues. HDL-cholesterol is measured by first removing LDL particles by aggregation or precipitation with divalent ions (such as Mg++) and then coupling the products of a cholesterol oxidase reaction to an indicator reaction. The measurement of apo-A reactive capacity can be used to measure HDL cholesterol but is thought to be less accurate.

Electrophoresis Measurements

Since the HDL particles have a net negative charge and vary by size, electrophoresis measurements have been utilized since the 1960s to both indicate the number of HDL particles and additionally sort them by size. Larger HDL particles are carrying more cholesterol.

NMR Measurements

The newest methodology for measuring HDL particles, available clinically since the late 1990s[24] uses Nuclear Magnetic Resonance fingerprinting of the particles to measure both concentration and sizes. This methodology was pioneered by researcher Jim Otvos and the North Carolina State University academic research spinoff company[25] and dramatically reduced the cost of HDL measurements.