Direct LDL Cholesterol Measurement vs. the Friedewald Equation
Other than the ultracentrifugation reference method which is expensive, time consuming and requires specialised equipment there are two other commonly used methods to determine the concentration of LDL Cholesterol; the friedewald equation and direct LDL measurement.
The Friedewald equation is widely used in clinical laboratories to estimate the concentration of LDL. The equation takes into consideration the concentration of HDL Cholesterol, Total Cholesterol and Triglycerides to provide an estimated value for LDL.
There are several limitations associated with the use of the friedewald equation meaning it cannot be used under the following circumstances:
When triglyceride levels are higher than 400mg/dl (4.5mmol/l)
When Chylomicrons are present in the sample
When the sample contains high levels of VLDL
When the patient suffers from diabetes, liver dysfunction or obesity
Often the patient samples presented for analysis will have high levels of triglycerides; ruling out possible use of the Friedewald equation. Elevated triglyceride levels will result in an under estimation of LDL if the Friedewald equation is used.
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The graph below shows the effect of triglyceride levels greater than 400mg/dl on LDL cholesterol determination by different methods.
More recent studies have suggested that triglyceride concentrations above 250mg/dl will adversely affect the estimation of LDL using the Friedewald equation. The graph below shows the mis-estimation of LDL cholesterol using the Friedewald formula as triglyceride concentrations increase.
Due to the limitations of the Friedewald equation the National Cholesterol Education Programme (NCEP) now recommend the direct measurement of LDL cholesterol.
The Randox direct clearance method unlike most commercially available direct LDL assays has an advanced reagent formulation enabling rapid clearance of turbidity resulting in reduced interference from bilirubin and triglycerides.
The assay works by removing all non-LDL components in the first step of the reaction allowing LDL cholesterol to be accurately and specifically measured in the second step.
Despite being based on the clearance method the detergents and buffering systems used in most other direct assays vary leading to significant differences in assay performance.
No sample preparation
Suitable for use with serum and plasma
Reduced interference from Bilirubin and Triglycerides
Highly specific for LDL Cholesterol
Not affected by VLDL, VLDL is removed in the first step of the reaction leading to a more accurate estimation of LDL
Excellent correlation with ultracentrifugation methods even in discrepant samples
Liquid ready to use reagents
Headquartered in the United Kingdom, Randox Laboratories Ltd. is a market leader within the in vitro diagnostics industry, manufacturing high quality diagnostic products for laboratories worldwide. Our extensive product portfolio offers complete solutions within the fields of forensic toxicology, clinical chemistry, cardiology, veterinary, drug residues, life sciences, oncology, molecular diagnostics and internal and external quality control. A continuous strategy of innovation has allowed Randox to revolutionise technology for the forensic toxicology market. Significant re-investment in Research and Development has seen the novel Biochip Array Technology adapt and evolve, with four analysers available for various settings. Twenty international offices and over one hundred distribution partners, ensure a high level of customer service worldwide.
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