Thursday, April 28, 2005

Discordant results for LDL obtained by direct measurement and Friedewald calculation (FW-C)

Henry Gang, Chemistry, Endocrinology, Biochem Technology Lab, Inc, 5622, Broadway, Riverdale, NY, USA

LDL has become the benchmark for lipid measurements and the main determinant and indicator for therapy.Currently LDL is either calculated(LDL-C) by the Friedewald formula (FW-C) or assayed by direct measurement(LDL-D).It is therefore imperative to report LDL as accurately as possible.
In our laboratory we observed that, more often than not, LDL-D did not coincide with LDL-C.

We therefore proceeded to again review LDL-C's accuracy.

The lipid results of 101 consecutive reports selected randomly on a given day were evaluated, specifically total cholesterol (TC), HDL, triglycerides (TG) and LDL-D, and LDL-C calculated. Standard statistical methods were used, and all analyses carried out on a Beckman LX20 analyzer.

Statistical calculations for the complete group ( LDL-C as dependent variable vs LDL-D as independent variable) showed a systematic underestimation of LDL-D by LDL-C ( slope 0.87, r 0.954, p<0.001).Overestimation of LDL-C occurred in 18 cases, with 11/18 at TG<100 mg/dL.
At a general accuracy tolerance level of 8-10% only 41/101 (40.6%) were FW-C compliant; even at a 12% tolerance only 60.5% were FW-C compliant.
Accuracy of FW-C is mostly affected by TG levels. With increasing TG the likelihood and magnitude of deviation increases. At TG<100 mg/dL 18.6% of the results exceeded the 12% tolerance.

This study shows that in daily laboratory practice and in real time medical evaluation, FW-C appears to have limited utility. The NCEP may have a need for a) different cutoff depending on the method; or b) a correction formula ; or c)discontinuation of one of the methods if the present cutoffs are to be maintained; or d) the indication of the method used.

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