How LDL is calculated
Direct LDL measurement requires ultracentrifugation, so it is usually estimated from a standard fasting lipid panel using the Friedewald equation, which assumes the ratio of triglycerides to VLDL cholesterol is constant.
LDL = Total cholesterol − HDL − (Triglycerides ÷ 2.2) [mmol/L]
The equation is not valid when triglycerides exceed 400 mg/dL (≈4.5 mmol/L) — at that point the assumed TG:VLDL ratio breaks down and the estimate becomes unreliable. A directly measured LDL (or non-HDL cholesterol) should be used instead in that situation, and in non-fasting samples.
LDL categories (NCEP ATP III)
| LDL (mg/dL) | LDL (mmol/L) | Category |
|---|---|---|
| < 100 | < 2.6 | Optimal |
| 100 – 129 | 2.6 – 3.3 | Near optimal/above optimal |
| 130 – 159 | 3.4 – 4.0 | Borderline high |
| 160 – 189 | 4.1 – 4.8 | High |
| ≥ 190 | ≥ 4.9 | Very high |
Clinical use
- LDL is the primary target for statin and other lipid-lowering therapy decisions in cardiovascular risk management.
- Because the Friedewald equation underestimates LDL as triglycerides rise, a directly measured LDL is preferred when precision matters — very high cardiovascular risk, statin titration near a treatment target, or elevated/borderline triglycerides.
- The sample should generally be fasting (9-12 hours) for the triglyceride value — and therefore the calculated LDL — to be reliable.
Frequently asked questions
Why isn't LDL measured directly?
Direct LDL measurement (ultracentrifugation or specialised direct assays) is more expensive and not routinely available everywhere, so it is usually estimated from the total cholesterol, HDL, and triglycerides already measured on a standard lipid panel.
Why does the Friedewald equation fail at high triglycerides?
The equation estimates VLDL cholesterol as triglycerides divided by 5 (mg/dL), which assumes a roughly constant ratio of triglycerides to VLDL cholesterol. That ratio becomes unreliable above 400 mg/dL, so the estimated LDL is no longer accurate.
Does this calculator work with a non-fasting sample?
Triglycerides rise after eating, which can make the calculated LDL less accurate. A fasting sample (9-12 hours) is preferred whenever the Friedewald equation is being used.
What should be done if triglycerides are too high for this calculator?
Request a directly measured LDL, or use non-HDL cholesterol (total cholesterol minus HDL) as an alternative treatment target — non-HDL cholesterol does not depend on the triglyceride level.
References
- Friedewald WT, Levy RI, Fredrickson DS. Estimation of the concentration of low-density lipoprotein cholesterol in plasma, without use of the preparative ultracentrifuge. Clin Chem. 1972;18(6):499-502.
- Expert Panel on Detection, Evaluation, and Treatment of High Blood Cholesterol in Adults (NCEP ATP III). JAMA. 2001;285(19):2486-2497.