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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See more of The 2005 Institute for Quality in Laboratory Medicine Conference