In many clinical laboratories, the LDL-Cholesterol value listed in lipid profile test results is often not a result of direct measurement, but an estimated result from three other parameters: Total Cholesterol, HDL, and Triglycerides. This estimation method is known as the Friedewald formula, and it has been widely used since it was first published in 1972.
This article discusses how the Friedewald formula works, its limitations that are important for Medical Laboratory Technologists (ATLM) to know, and how to verify the plausibility of results before they are reported.
What Is the Friedewald Formula?
The Friedewald formula calculates LDL-Cholesterol indirectly, assuming that VLDL-Cholesterol can be approximated by the Triglyceride value divided by 5 (specifically for mg/dL units):
From this equation, LDL can be isolated as:
This approach is popular because it is inexpensive and does not require additional reagents—it simply utilizes three parameters that are routinely checked in a standard lipid panel.
Based on the Friedewald equation: Total Cholesterol = LDL + HDL + (Triglycerides / 5). Unit: mg/dL.
Results are estimates for educational purposes and do not replace clinical interpretation.
Important Limitations: When This Formula Should Not Be Used
The assumption "VLDL ≈ Triglycerides / 5" only applies under conditions where the VLDL cholesterol-triglyceride ratio is relatively stable. The main limitations that must be noted are:
- Triglycerides > 400 mg/dL — in this condition, the VLDL ratio becomes disproportionate, making the LDL estimation inaccurate and tending to significantly underestimate.
- Non-fasting samples — Triglycerides tested without fasting can provide fluctuating values, affecting the accuracy of the estimation.
- Type III dyslipidemia (dysbetalipoproteinemia) — certain clinical conditions render the basic assumptions of this formula invalid.
Under these conditions, it is recommended to use a direct LDL assay method instead of the Friedewald estimation.
The Role of ATLM in Verifying Results
Before lipid profile results are reported, ATLM can perform several simple verification steps:
- Check the Triglyceride value first. If it is above 400 mg/dL, mark the calculated LDL result as invalid and consider a direct LDL test.
- Pay attention to results that are mathematically nonsensical, such as a negative estimated LDL value — this indicates a combination of results that need to be re-examined, either from the pre-analytical side (hemolysis, severe lipemia) or analytical side (daily QC control of the three parameters).
- Ensure the QC of the three basic parameters (Total Cholesterol, HDL, Triglycerides) is performed according to Westgard rules, because a small error in any of them will have a direct impact on the accuracy of the estimated LDL.
Modern Alternative: Martin-Hopkins Method
As an additional note, some modern laboratories are starting to switch to more adaptive estimation methods such as Martin-Hopkins, which uses a VLDL divisor factor adjusted based on a combination of Triglyceride and non-HDL Cholesterol values, rather than a fixed "divided by 5" factor. This method is claimed to be more accurate, especially at high Triglyceride or low LDL levels, although Friedewald remains the most commonly used method due to its simplicity.
Important Note
The calculator in this article is provided for educational purposes and initial estimation. LDL estimation results via the Friedewald formula do not replace direct measurement in cases of high Triglycerides or other specific clinical conditions.