Direct Assay of LDL Cholesterol: Comparing Measurement and Calculation

Low-density lipoprotein (LDL) cholesterol is a critical lipid subfraction because it is linked directly with the risk of coronary heart disease (CHD). Current approaches to diagnosing and monitoring CHD emphasize the significance of accurate, precise LDL values. Due to the inherent complexity of measuring LDL by the reference method, ultracentrifugation, it usually is estimated by a calculation that uses measured values for total cholesterol, high-density lipoprotein (HDL) cholesterol, and triglycerides. Calculated LDL (CLDL) results may be inaccurate, especially if a patient's triglyceride level is elevated, and it requires a specimen from a patient who has fasted. New assays allow for the direct measurement of LDL (DLDL) analogous to assays for HDL and do not require the patient to fast. While CLDL typically compares well with ultracentrifugation and DLDL, it may provide a result that is different in some instances; that difference may have clinical significance. A method comparison study of CLDL and DLDL resulted in the following linear regression equation: DLDL = 1.067 CLDL + 1.597, r = 0.974, n = 36 .