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This study examines the performance and clinical use of a commercial immunoseparation assay for low-density lipoprotein (LDL) cholesterol in a sample population of normolipidemic and hyperlipidemic adult volunteers. Using paired fasting and nonfasting samples, we compared the direct LDL assay with the beta quantification method and the Friedewald calculation. Overall, the direct LDL assay correctly classified 82% and 60% of fasting and nonfasting subjects, respectively, into National Cholesterol Education Program risk groups. The Friedewald method correctly classified 84% of subjects. The fasting direct LDL assay has comparable positive and negative predictive values to the Friedewald method, except at an LDL cholesterol of 100 mg/dl. The nonfasting direct LDL assay demonstrates unacceptable positive predictive values when LDL cholesterol decreases to the 130 to 159 and > or = 160 mg/dl categories. Overall, direct LDL assay demonstrates limitations in the nonfasting state and at the LDL cholesterol level of 100 mg/dl used for patients with established coronary heart disease. [ABSTRACT FROM AUTHOR]