What is the primary limitation of the Friedewald equation for LDL cholesterol calculation, and when should it not be used?

• A. It becomes unreliable when triglycerides exceed 400 mg/dL or in non-fasting samples; direct LDL measurement or alternative calculations are preferred in those cases.
• B. It provides exact LDL values at any triglyceride level without need for direct measurement.
• C. It only applies to HDL cholesterol and not LDL.
• D. It requires apolipoprotein B measurement to calculate LDL.

Answer: (A)

Estimating LDL-C with the Friedewald method relies on a fixed relationship between triglycerides and VLDL cholesterol, using VLDL-C = TG/5. The primary limitation is that this assumption fails when triglycerides are very high or the sample isn’t fasting. Above about 400 mg/dL, the TG-to-VLDL-C ratio varies more, and/or chylomicrons from a nonfasting state elevate TG, so the calculated LDL-C becomes unreliable. In those situations, direct LDL-C measurement or an alternative calculation method should be used. This is why Friedewald is avoided when triglycerides are high or when samples are nonfasting, and why direct measurement or other formulas provide more accurate results across a wider range of lipid profiles.