Background: Low-density lipoprotein (LDL) is measured by its cholesterol content (LDL-C), but it has been suggested that LDL triglyceride (LDL-TG) may also be related to coronary artery disease risk. LDL-TG can be measured after ultracentrifugation or electrophoresis, but these are labor intensive methods, indicating the need for an automated homogeneous assay. Methods: TG-rich lipoproteins (TRLs), LDL, and HDL were isolated by ultracentrifugation and used to determine optimal characteristics of surfactants and various enzymes for assay development. We analyzed assay precision and linearity, and compared results with those obtained after ultracentrifugation. Serum samples from a large population study (n = 12 284 subjects) were used to generate reference intervals for LDL-TG and to determine levels in various types of hyperlipidemia. Results: An assay for LDL-TG has been developed by use of surfactants 1 and 2, and enzymes to measure LDL-TG directly on an automated analyzer. There was an excellent correlation between results obtained with this assay and after isolation of LDL by ultracentrifugation. When the assay was applied to serum samples from normal and hyperlipidemic subjects, median normal values were 0.09 mmol/L, with significant median elevations observed in subjects with increased LDL-C, hypertriglyceridemia, combined hyperlipidemia, and hyperchylomicronemia of 0.19, 0.18, 0.28, and 0.43 mmol/L, respectively, as compared with mean LDL-C values in these subjects of 2.25, 4.01, 2.66, 3.96, and 2.43 mmol/L, respectively. Conclusions: We have developed an automated homogeneous assay for LDL-TG for potential use in research and clinical laboratories, and documented that the TG molar content of LDL is about 5% of its cholesterol content. IMPACT STATEMENT LDL is assessed by measuring its cholesterol content. It has been suggested that LDL triglycerides are also related to cardiovascular disease risk. We have developed an automated homogeneous LDL-TG assay with use of an optimized combination of surfactants, lipoprotein lipase, and cholesterol esterase. We investigated LDL-TG distribution in a large population study. This assay readily distinguished LDL-TG from TG in other lipoprotein fractions, and the assay provided results that were highly correlated with LDL-TG measured after isolation of LDL by ultracentrifugation. Thus, our automated LDL-TG assay has potential value in clinical research and practice.
