Which equation is commonly used to estimate LDL-C when triglycerides are available?

Which equation is commonly used to estimate LDL-C when triglycerides are available?

• A. LDL-C = Total cholesterol minus HDL-C minus (Triglycerides divided by 5)
• B. LDL-C = Total cholesterol minus HDL-C minus (Triglycerides divided by 3)
• C. LDL-C = Total cholesterol minus (HDL-C)
• D. LDL-C = HDL-C plus Triglycerides

Answer: (A)

The key idea is estimating LDL-C by removing the cholesterol tied up in HDL and VLDL from the total cholesterol. In practice, VLDL-C isn’t measured directly in routine panels, so it’s approximated from triglycerides. When triglycerides are measured in mg/dL and the person is fasting, VLDL-C is estimated as TG/5. This leads to the Friedewald equation: LDL-C ≈ Total cholesterol − HDL-C − (Triglycerides/5). That’s why using TG divided by 5 is the correct approach. The other forms either miss the VLDL component, use an incorrect divisor, or add instead of subtract, which doesn’t reflect how LDL-C is derived. Note that this estimation works best when triglycerides are below about 400 mg/dL.

The key idea is estimating LDL-C by removing the cholesterol tied up in HDL and VLDL from the total cholesterol. In practice, VLDL-C isn’t measured directly in routine panels, so it’s approximated from triglycerides. When triglycerides are measured in mg/dL and the person is fasting, VLDL-C is estimated as TG/5. This leads to the Friedewald equation: LDL-C ≈ Total cholesterol − HDL-C − (Triglycerides/5). That’s why using TG divided by 5 is the correct approach. The other forms either miss the VLDL component, use an incorrect divisor, or add instead of subtract, which doesn’t reflect how LDL-C is derived. Note that this estimation works best when triglycerides are below about 400 mg/dL.