Low-density Lipoprotein Cholesterol (Direct LDL)

Overview:

For the direct determination of LDL cholesterol in nonfasting patients or in patients whose fasting triglycerides are >400 mg/dL, where the estimation of LDL by calculation may not be possible or may lead to inaccuracies. LDL cholesterol measurement, in conjunction with other lipid measurements, has been shown to be useful in assessing the risk of coronary heart disease (CHD). The National Cholesterol Education Program (NCEP)1 has stated that LDL cholesterol should be the “key index” in determination of CHD risk. Laboratory estimation of LDL cholesterol is most commonly determined by the use of formulas, such as the Friedewald formula.2 Use of this formula is limited to fasting samples with triglycerides <400 mg/dL. Triglyceride values between 250−400 mg/dL may also be associated with errors in LDL cholesterol estimation by calculation which, in turn, can lead to misclassification of the patient in regard to the NCEP guidelines.3

NCEP guidelines for interpretation (see Reference Interval) are based on serum values, and when classifying patients, serum or serum equivalent values should be used. For this direct LDL method, a factor of 1.06 should be used to convert EDTA plasma values to serum values. There is no significant interference from hemolysis up to 10.0 g/L hemoglobin, from bilirubin up to 30 mg/dL, and from triglycerides up to 1200 mg/dL. Abnormal liver function affects lipid metabolism; consequently, HDL and LDL results may be of limited diagnostic value in patients with hepatic disorders.

1. Report of the National Cholesterol Education Program Expert Panel on Detection, Evaluation, and Treatment of High Blood Cholesterol in Adults. The Expert Panel.Arch Intern Med. 1988 Jan; 148(1):36-69. PubMed 3422148

2. Friedewald WT, Levy RI, Fredrickson DS. Estimation of the concentration of low-density lipoprotein cholesterol in plasma without the use of the ultracentrifuge. Clin Chem. 1972 Jun; 18(6):449-502. PubMed 4337382

3. McNamara JR, Cohn JS, Wilson PW, Schaefer EJ. Calculated values for low-density lipoprotein cholesterol assessment of lipid abnormalities and coronary disease risk. Clin Chem. 1990 Jan; 36(1):36-42. PubMed 2297935