In diagnosing hyperlipoproteinemia, a normal plasma TC level is difficult to define. Prospective studies have shown that the incidence of CAD rises continuously with plasma TC and that values previously considered normal in the USA are higher than those found among populations with a low incidence of atherosclerosis. However, these limits are clearly excessive because of the known high risk of cardiovascular disease at these levels. LDL is again calculated, as described above. HDL levels are positively correlated with exercise, moderate alcohol intake, and estrogen replacement therapy and are inversely correlated with smoking, obesity, and the use of most progestin-containing contraceptives.
Enzymatic methods are usually most accurate and are standard in virtually all clinical laboratories.
Plasma triglyceride is usually measured as glycerol by either colorimetric, enzymatic, or fluorometric methods after alkaline or enzymatic hydrolysis to glycerol and formaldehyde. HDL levels are measured enzymatically after precipitation of VLDL, IDL, and LDL from plasma. Lipoprotein electrophoresis is useful only in dyslipidemia and has generally been supplanted by analysis of the apolipoproteins. More severe hyperlipidemia is due to a heterogeneous group of disorders differing in clinical features, prognosis, and therapeutic response. A high plasma level of any lipoprotein can result in hypercholesterolemia.