Accuracy of calculated serum low-density lipoprotein cholesterol for the assessment of coronary heart disease risk in NIDDM patients

OBJECTIVE - To evaluate the accuracy of LDL cholesterol calculated with Friedewald's equation in the assessment of cardiovascular risk in NIDDM patients. RESEARCH DESIGN AND METHODS - The calculation of LDL cholesterol according to Friedewald's formula was compared with the measurement of LDL cholesterol separated by ultracentrifugation in 151 NIDDM patients with fairly good metabolic control (HbA(1c) less than or equal to 10%) and in 405 nondiabetic subjects. RESULTS - Measured and calculated LDL cholesterol was found to be well correlated in both diabetic (r = 0.95) and nondiabetic (r = 0.97) subjects. Compared with measured LDL cholesterol, the calculated LDL cholesterol differed by greater than or equal to 10% in 34% of samples from diabetic patients and in 26% of samples from nondiabetic subjects (chi(2) = 3.885, P < 0.05), The percentage of error increased when the serum triglyceride (TG) level was greater than or equal to 200 mg/dl (2.26 mmol/l) and when the ratio of VLDL cholesterol to TG was <0.20 or >0.29 in both groups of subjects. Although the percentage of error from calculated LDL cholesterol was greater in diabetic than in nondiabetic subjects because of the greater prevalence of hypertriglyceridemia in the former group, the misclassification of coronary heart disease risk, according to the cutoff points of the National Cholesterol Education Program (NCEP), was similar in the two groups (25% in diabetic and 22% in nondiabetic subjects). In both groups of patients, the misclassification of coronary heart disease risk was higher when calculation of LDL cholesterol produced values near the cutoff points. CONCLUSIONS - Although accuracy in the estimation of LDL cholesterol is less than ideal, Friedewald's equation seems to be of value in the correct assignment of coronary heart disease risk classes in the great majority of diabetic as well as nondiabetic subjects. Caution must be exercised for subjects in whom calculated LDL cholesterol is close to the cutoff points of the NCEP guidelines.