How this instrument works
In 1972, Friedewald, Levy, and Fredrickson at the NIH compared calculated LDL against direct ultracentrifuge measurements in several hundred blood samples and found a simple shortcut that held up: total cholesterol is the sum of what's carried by three particle classes — LDL, HDL, and VLDL. HDL is measured directly on the panel. VLDL cholesterol isn't measured directly, but at ordinary triglyceride levels it tracks close to one-fifth of the triglyceride figure, so triglycerides divided by 5 stands in for it. Subtract HDL and that VLDL estimate from the total, and what remains is LDL.
Before this shortcut, getting an LDL figure meant spinning a blood sample in an ultracentrifuge to physically separate the particle classes by density — slow, costly, and out of reach for a routine clinic visit. Being able to calculate LDL from three numbers a basic blood draw already produces turned it into the value nearly every patient could get, which is a large part of why LDL, rather than total cholesterol alone, became the figure most cardiovascular risk conversations and treatment targets are framed around.
The shortcut has a real limit. Above roughly 400 mg/dL of triglycerides, or with conditions such as chylomicronemia or type III dysbetalipoproteinemia, the particles carrying triglycerides shift in composition and stop tracking that fixed one-fifth ratio, so the subtraction stops isolating LDL correctly. In those situations a direct LDL measurement, or a newer regression-based equation such as Martin-Hopkins or the Sampson/NIH formula, is the better read — a real, ongoing shift in lipid-testing practice since the original 1972 shortcut was published.
- Enter Total cholesterol in mg/dL from a fasting lipid panel.
- Enter HDL cholesterol in mg/dL.
- Enter Triglycerides in mg/dL.
- Read LDL cholesterol; if triglycerides sit above about 400 mg/dL, treat the result with caution and ask about a direct measurement instead.
Worked example — three lipid panels
Total cholesterol 200 mg/dL, HDL 50, triglycerides 150. Estimated VLDL share: 150 ÷ 5 = 30. Subtract HDL and that estimate from the total: 200 − 50 − 30 = 120 mg/dL of LDL, inside the range many guidelines label borderline-high.
Raise total cholesterol to 240, drop HDL to 40, and push triglycerides to 200: 200 ÷ 5 = 40, and 240 − 40 − 40 = 160 mg/dL — now solidly in high territory.
A cleaner panel — total 180, HDL 60, triglycerides 100 — gives 100 ÷ 5 = 20, and 180 − 60 − 20 = 100 mg/dL, sitting right at a treatment target many clinicians use for higher-risk patients.
Questions
Why divide triglycerides by 5 specifically?
In the 1972 dataset, VLDL particles' cholesterol content averaged close to one-fifth of their triglyceride content across the fasting samples compared against ultracentrifuge results. That ratio came from measurement, not a fixed law of chemistry, which is exactly why it only holds at ordinary triglyceride levels and drifts once triglycerides run unusually high.
Why does the formula break down at high triglycerides?
Above roughly 400 mg/dL, or with disorders such as chylomicronemia and type III dysbetalipoproteinemia, the triglyceride-carrying particles change composition and no longer carry cholesterol in that same one-fifth proportion, so subtracting triglycerides ÷ 5 overstates or understates the true VLDL share. Most labs flag or suppress a calculated LDL above that triglyceride level and recommend direct measurement instead.
Do I need to fast before this blood draw?
Traditionally yes — triglycerides climb sharply after a meal, which inflates the divided-by-5 term and throws off the subtraction. Many labs now accept non-fasting panels for general screening under newer guideline updates, but if triglycerides come back unusually high on a non-fasting sample, a fasting recheck gives the more trustworthy number for this specific calculation.
What are Martin-Hopkins and the Sampson equation?
Both replace the fixed one-fifth ratio with something that adjusts for the actual sample. Martin-Hopkins, from a 2013 JAMA study, substitutes a factor drawn from a large lookup table that varies with triglyceride and non-HDL cholesterol levels. The Sampson/NIH equation, used on the NIH's own online calculator, fits a similar adjustable relationship algebraically. Both stay closer to a direct LDL assay than the fixed-5 approach, especially once triglycerides climb, though the original 1972 version remains the most common line printed on lab reports.
Is a calculated LDL as trustworthy as a direct measurement?
For most people with triglycerides under roughly 400 mg/dL, yes — the calculated figure correlates closely with direct assay results and is what most cardiovascular risk guidelines were built and validated around from the start. Direct measurement earns its keep specifically when triglycerides run high or lipid metabolism is otherwise unusual.
References
- Friedewald WT, Levy RI, Fredrickson DS. 1972, Clin Chem — original formula (PubMed)
- Martin SS et al. 2013, JAMA — Martin-Hopkins method (PubMed)
Read this first: This instrument computes a screening figure from population formulas — it is not a diagnosis, and it cannot see the whole picture a clinician can. Use it to inform a conversation, not to replace one.