Having your cholesterol checked -- especially your LDL cholesterol -- is important in order to address high cholesterol levels and assess your risk for heart disease. While other lipids, such as HDL, total cholesterol and triglycerides, are directly measured using laboratory instruments, LDL may be measured directly or indirectly, using a calculation or a measurement via an instrument.
Indirect measurement of LDL is performed by using the Friedewald equation. The Friedewald equation is a mathematical formula developed by William Friedewald to calculate the concentration of LDL, or “bad” cholesterol, in the bloodstream. This equation calculates the concentration of LDL based upon the presence of total cholesterol, HDL and triglyceride levels:
LDL = total cholesterol – HDL – (triglycerides/5)
Although this equation is fairly accurate and widely employed by a number of laboratories, there are certain factors that could cause your LDL cholesterol levels to be incorrect by using this calculation. These would include triglyceride levels over 400 mg/dL and consuming a meal before having your cholesterol levels -- and LDL cholesterol -- checked.
Direct measurements of LDL are determined without a calculation. Although not as commonly performed, direct LDL measurements can be used in cases where triglycerides are very high. In some cases, these test kits can measure other lipid particles in addition to LDL, such as LDL subtypes and apolipoproteins. However, not everyone needs a more extensive lipid profile tested. Additionally, these tests are usually more expensive than using the Friedewald equation to calculate LDL. Because of this, they are not as widely used.
The National Cholesterol Education Program does not specify which method you should use to measure your LDL cholesterol. However, regardless of the method your healthcare provider chooses to measure your LDL, it should be performed at least once every five years. By knowing your LDL number, you can take the appropriate measures needed to lower it, if needed.
Lindsey CC, Graham MR, Johnston TP, et al. A clinical comparison of calculated versus direct measurement of low-density lipoprotein cholesterol level. Pharmacotherapy 2004;24:2.
Nauck M, Warnick GR, Rifai N. Methods for measurement of LDL cholesterol: a critical assessment of direct measurement by homogenous assays versus calculation. Clin Chem 2002;48:236-254.
Dipiro JT, Talbert RL. Pharmacotherapy: A Pathophysiological Approach, 7th ed 2008.