To help determine your risk of developing heart disease and to monitor lipid-lowering lifestyle changes and drug therapies; to accurately determine your low-density lipoprotein cholesterol (LDL-C)
When your triglycerides are significantly elevated (above 400 milligrams per deciliter, mg/dL) or you have another condition that affects the accuracy of calculated LDL-C
A blood sample drawn from a vein
The direct low-density lipoprotein cholesterol test (direct LDL-C) measures the amount of LDL cholesterol, sometimes called "bad" cholesterol, in the blood. Elevated levels of LDL-C are associated with an increased risk of hardening of the arteries (atherosclerosis) and heart disease.
Usually, your LDL-C level is calculated using the measured values of the components of a standard lipid panel: total cholesterol, high-density lipoprotein cholesterol (HDL-C), and triglycerides. Using a mathematical equation, the amount of LDL-C can be determined from the three measured values. Calculated LDL-C is about as accurate as direct LDL-C when triglyceride levels are normal and can be done at no additional cost as part of a lipid profile.
In most cases, calculated LDL-C is a good estimate of the LDL-C, but it becomes less accurate with increasing triglyceride levels. When triglycerides are significantly elevated (above 400 mg/dL), the equation is no longer valid. Other conditions such as severe cirrhosis can also affect the accuracy of calculated LDL-C. In these situations, the only way to accurately determine LDL-C is to measure it directly or with special testing techniques (e.g., a beta quantification test).
High triglycerides may be due to a metabolic disorder affecting lipids. However, anyone may have high triglycerides after eating. In either situation, the direct LDL-C test can determine the amount of LDL in your blood.
A direct LDL-C may be ordered by your healthcare practitioner when prior test results have indicated high triglycerides. In some laboratories, the direct LDL test will automatically be performed when the triglyceride levels are too high to calculate LDL-C. This saves the healthcare practitioner time by not needing to order another test, saves you time by not needing to have a second blood sample drawn, and speeds up the time to provide the test result.
Whether calculated or measured directly, LDL-C values are used to assess your risk for heart disease and help guide decisions about what treatment may be best if you are at borderline, intermediate, or high risk. The results are considered along with other known risk factors of heart disease to develop a plan of treatment and follow up. Treatment options may involve lifestyle changes such as diet and exercise or lipid-lowering medications such as statins. LDL-C tests can also be used to monitor your response to therapy to lower cholesterol.
Read the article on LDL Cholesterol to learn more about LDL-C and what results might mean.
Sources Used in Current Review
Matthias Nauck, G. Russell Warnick, Nader Rifai. Methods for Measurement of LDL-Cholesterol: A Critical Assessment of Direct Measurement by Homogeneous Assays versus Calculation. Clinical Chemistry Feb 2002, 48 (2) 236-254. Available online at http://clinchem.aaccjnls.org/content/48/2/236. Accessed October 2019.
© Mayo Medical Laboratories. LDL Cholesterol (Beta-Quantification), Serum. Available online at https://www.mayocliniclabs.com/test-catalog/Overview/89652. Accessed October 2019.
© Mayo Medical Laboratories. LDL Cholesterol, Direct. Available online at https://www.mayocliniclabs.com/test-catalog/Overview/75160. Accessed October 2019.
© 2019 ARUP Laboratories. LDL Cholesterol, Direct. Available online at https://ltd.aruplab.com/Tests/Pub/0020257. Accessed October 2019.
Sources Used in Previous Reviews
Thomas, Clayton L., Editor (1997). Taber's Cyclopedic Medical Dictionary. F.A. Davis Company, Philadelphia, PA [18th Edition].
Pagana, Kathleen D. & Pagana, Timothy J. (© 2007). Mosby's Diagnostic and Laboratory Test Reference 8th Edition: Mosby, Inc., Saint Louis, MO. Pp. 602-605.
Clarke, W. and Dufour, D. R., Editors (2006). Contemporary Practice in Clinical Chemistry, AACC Press, Washington, DC. Winter, W. and Harris, N. Chapter 21 Lipoprotein Disorders. Pp. 251-259.
Wu, A. (2006). Tietz Clinical Guide to Laboratory Tests, Fourth Edition. Saunders Elsevier, St. Louis, Missouri. Pp. 684.
(© 1995-2010). Unit Code 200269: Direct LDL, Serum. Mayo Clinic, Mayo Medical Laboratories [On-line information]. Available online at http://www.mayomedicallaboratories.com/test-catalog/Overview/200269. Accessed December 2010.
Roberts, W. (Updated April 2010). Cardiovascular Disease (Traditional Risk Markers) - Risk Markers - CVD (Traditional). ARUP Consult [On-line information]. Available online at http://www.arupconsult.com/Topics/CVDRiskMarkerTrad.html?client_ID=LTD#tabs=0. Accessed December 2010.
Desvigne-Nickens, P. (Updated 2009 February 2). Frequently Asked Questions, Heart Disease. U.S. Department of Health and Human Services, Office on Women's Health [On-line information]. PDF available for download at http://www.womenshealth.gov/faq/heart-disease.pdf. Accessed December 2010.
Faix, J. (2009 Summer). Read My Lipids: LDL Cholesterol: Calculated vs. Direct. Stanford University Medical Center [On-line information]. PF available for download at http://www.stanfordlab.com/images/PDF/2009Summer.pdf. Accessed December 2010.