Comparison of calculated and direct low density lipoprotein cholesterol determinations in a routine laboratory

Abstract

Background: Low density lipoprotein cholesterol (LDL-C) concentrations form the basis for treatment guidelines established for hyperlipidaemic patients. LDL-C concentrations are commonly calculated using the Friedwald formula (FF) which has several limitations. Recently, direct methods for LDL-C estimation have been developed which are suitable for routine laboratories. Objective: To compare serum LDL-C concentrations determined by a direct assay and the Friedwald formula. Design: Cross-sectional study. Setting: Mater Hospital Laboratory, Nairobi, Kenya. Methods: The clinical performance of the two methods was evaluated by analysing 211 fresh plasma samples from fasting adult patients. The samples were divided into four groups-normolipidaemic; and Types IIa, IIb and IV hyperlipidaemias. Results: The Friedwald formula (FF) correlated best with the direct assay in the normolipidaemic samples (r = 0.879; y = 0.468 + 0.852x). Direct LDL-C values were significantly lower than the FF in the Type IIa hyperlipidaemia samples (paired differences 0.38 ± 0.62). There was only 65% agreement between the two methods in the borderline high LDL-C group of the National Cholesterol Education Program (NCEP) classification (LDL-C 3.36 - 4.14mmol/L). Conclusion: There is lack of agreement between the FF and the Abbott direct LDL-C assay. If the two methods are used interchangeably, there may be confusion in the classification and control of lipid lowering medication for patients with hyperlipidaemia.