Lipid Testing Inaccuracies

Lipid Testing Inaccuracies

The Friedewald Equation
The ultracentrifugal measurement of LDL is time consuming and expensive and requires equipment not commonly available in the clinical laboratory or physician's office. For this reason, LDL-cholesterol is most commonly estimated from quantitative measurements of total and HDL-cholesterol and plasma triglycerides (TG) using the empirical relationship of Friedewald et al. (1972).

[LDL-chol] = [Total chol] - [HDL-chol] - ([TG]/5)
where all concentrations are given in mg/dL.

The quotient ([TG]/5) is used as an estimate of VLDL-cholesterol concentration. It assumes, first, that virtually all of the plasma TG is carried on VLDL, and second, that the TG:cholesterol ratio of VLDL is constant at about 5:1 (Friedewald et al. 1972). Neither assumption is strictly true.

Limitations of the Friedewald equation:
The Friedewald equation should not be used under the following circumstances:

When chylomicrons are present.
When plasma triglyceride concentration exceeds 400 mg/dL (4.52 mmol/L).
In patients with dysbetalipoproteinemia (type III hyperlipoproteinemia).

In circumstances in which these conditions apply, LDL-cholesterol should be measured directly. Chylomicrons are visible as a floating "cream" layer when the specimen is allowed to stand undisturbed at 4^° C overnight. Samples with triglyceride concentrations exceeding 400 mg/dL (4.52 mmol/L) are generally turbid. The recognition of dysbetalipoproteinemia (type III hyperlipoproteinemia), however, requires the identification of b-VLDL. Since b-VLDL contains proportionately more cholesterol than normal VLDL, the use of the factor [TG]/5 underestimates the amount of cholesterol in the VLDL fraction, and consequently the Friedewald equation overestimates LDL-cholesterol. Use of the Friedewald equation in this case will result in the misidentification of a dysbetalipoproteinemic (type III) patient as having hyperbetalipoproteinemia (type II hyperlipoproteinemia).

Comparison of Calculated
and Direct LDL Cholesterol

How do calculated LDL values compare to Betaquantification values?
The limitations of the Friedwald Equation for the estimation of LDL cholesterol are well known and well documented. Although many of us generally perceive the Friedwald equation to under estimate the LDL-Cholesterol, Warnick et. al. found that in some cases the Friedwald equation actually over estimated the LDL Cholesterol as evidenced by the plus or minus 10% comparison of the calculated LDL when compared to Beta Quantification. The overestimation results from the inherent imprecision of the Total Cholesterol, Triglyceride and HDL measurements. Since the calculated LDL is derived from these three measurements the imprecision of LDL measurement is the total of the imprecision of the three measurements.

What are the clinical reasons for directly measuring LDL?
McNamara et. al. found that "When TG were < 4.00 g/L (400 mg/dl) 86% of estimated LDL cholesterol values were properly classified according to the National Cholesterol Education Program cutpoints ". This also states that 14% of patients with triglycerides below 400 mg/l were misclassified. Furthermore, Warnick et. al.¹ found when TG<200 mg/dl 90% of calculated LDL-C values were within ±10% of values measured by Beta Quantification but only 72% were within ± 10% in the TG range of 200-400 mg/dl. The Lipoprotein Measurement Working Group, the laboratory standardization panel of the NCEP, has determined that calculation of LDL cholesterol will be insufficient to provide LDL cholesterol values within the NCEP recommended precision and accuracy limits³. Because LDL cholesterol concentration becomes the basis for definitive dietary and drug treatment the accuracy of the measurement is especially important¹.

The Working Group on Lipoprotein Measurement of the NCEP has recommended that new methods for LDL-cholesterol measurement be developed. Such methods should be capable of quantitating LDL-cholesterol directly.