Indirect calculation of LDL-C becomes inaccurate when TAG exceeds which value?

• A. 200 mg/dL
• B. 300 mg/dL
• C. 400 mg/dL
• D. 500 mg/dL

Answer: (C)

LDL-C is often estimated with the Friedewald equation, which uses the triglyceride value divided by 5 to approximate VLDL-C: LDL-C = total cholesterol − HDL-C − (triglycerides/5). This relies on a relatively constant ratio between triglycerides and VLDL-C, which holds when triglycerides are not very high and the sample is fasting. When triglycerides rise above about 400 mg/dL, this relationship no longer holds because the composition of lipoproteins changes (more TG-rich remnants and possibly chylomicrons), so the TG/5 estimate of VLDL-C becomes unreliable. As a result, the indirect calculation of LDL-C becomes inaccurate beyond that level, and direct measurement of LDL-C is recommended.