Did You Know Your "LDL" Number Was Never Actually Measured?

When you get a cholesterol test, LDL, the "bad" cholesterol, usually isn't measured directly. Instead, it's calculated from other numbers on your blood panel. Here's why, and why that matters more than most people realize.

1. Direct Measurement Costs More

Measuring LDL directly requires specialized lab equipment and extra processing steps. That's manageable for a single patient, but not practical when millions of people get routine blood tests every day. So for decades, labs have used a formula instead of a direct measurement.

2. The Formula: Friedewald Equation

This formula was developed in 1972 and became the standard way labs calculate LDL for the next several decades:

mg/dL: LDL = Total Cholesterol − HDL − (Triglycerides ÷ 5)

mmol/L: LDL = Total Cholesterol − HDL − (Triglycerides ÷ 2.2)

(The number differs between units because triglycerides and cholesterol convert to mmol/L differently, so it isn't the same conversion factor in both versions.)

Why Divide by 5?

Most triglycerides travel through the blood inside particles called VLDL. On average, VLDL cholesterol works out to roughly triglycerides divided by 5. This isn't exact, it's an average based on studying many people. So instead of measuring LDL directly, the formula estimates it: take total cholesterol, subtract HDL, then subtract this estimated VLDL. What's left is reported as "LDL."

3. When the Formula Doesn't Work Well

Because LDL is calculated rather than directly measured, it can be inaccurate in certain situations:

• Triglycerides at 400 mg/dL or higher: the formula breaks down badly and generally isn't used at all
• Triglycerides between 150 and 400 mg/dL: research has found the formula starts underestimating LDL well before it officially "breaks," not just at the 400 mg/dL cutoff most people assume
• Not fasting before the test: this can throw off triglyceride levels and, in turn, the calculated LDL
• Low LDL cholesterol, especially below 70 mg/dL: the formula tends to underestimate LDL in this range, which matters a lot for people on statins or other LDL-lowering treatment being monitored closely

4. A More Accurate Formula Now Exists

In 2013, a research team led by Dr. Seth Martin at Johns Hopkins analyzed lipid data from more than 1.3 million patients and developed an improved version of the calculation, now known as the Martin-Hopkins equation. Instead of using a fixed divisor of 5 for everyone, it adjusts the divisor based on each person's individual triglyceride and non-HDL cholesterol levels, since the fixed "divide by 5" assumption turns out to be a poor fit for a large share of the population.

In a study of more than 5 million patients, the Martin-Hopkins equation correctly classified about 90 percent of LDL cholesterol values, compared to roughly 83 percent for the original Friedewald equation, with the biggest improvement seen in exactly the patients who need accuracy most: those with low LDL cholesterol or elevated triglycerides. Current cholesterol management guidelines now specifically recommend using the Martin-Hopkins equation, particularly when LDL cholesterol is below 70 mg/dL, and many major labs have already adopted it.

Formula Comparison at a Glance

Formula	Year Introduced	Best For	Limitation
Friedewald	1972	General population, normal triglycerides	Underestimates LDL when TG is high or LDL is low
Martin-Hopkins	2013	Low LDL, elevated triglycerides	Slightly more complex calculation, not yet universal
Direct LDL measurement	N/A	Triglycerides ≥400 mg/dL, or when precision is critical	Higher cost, not always ordered by default

Frequently Asked Questions

Q: How do I know which formula my lab used?
Most standard lab reports don't specify which equation was used unless you ask. If your triglycerides are elevated or your LDL is very low, it's reasonable to ask your doctor or lab directly whether the Martin-Hopkins equation or a direct LDL measurement was used.

Q: Should I be worried that my LDL number isn't a direct measurement?
For most people with typical triglyceride levels, the calculated LDL is a reasonably accurate estimate. The main groups who should pay closer attention are people with high triglycerides, very low LDL targets, or diabetes, since these are the situations where calculated LDL is most likely to be off.

Q: Can I request a direct LDL test instead of the calculated version?
Yes, direct LDL testing is available and can be requested, particularly if your triglycerides are high or your calculated LDL results seem inconsistent with your overall risk profile. Talk to your doctor about whether it's appropriate for your situation.

Key Takeaway

Most LDL results on a standard lab report aren't directly measured, they're calculated, historically using the Friedewald formula from 1972. It's fast, inexpensive, and reasonably accurate for most people, but it can meaningfully underestimate LDL when triglycerides are elevated or LDL is already low. A more accurate alternative, the Martin-Hopkins equation, is increasingly being adopted by labs and is now specifically recommended in cholesterol management guidelines for exactly these situations. If your numbers seem borderline or inconsistent with your risk profile, ask your doctor which formula was used, or whether a direct LDL test makes sense for you.

References

1. Friedewald WT, Levy RI, Fredrickson DS. Estimation of the Concentration of Low-Density Lipoprotein Cholesterol in Plasma, Without Use of the Preparative Ultracentrifuge. Clinical Chemistry. 1972;18(6):499-502. doi.org/10.1093/clinchem/18.6.499
2. Martin SS, Blaha MJ, Elshazly MB, et al. Comparison of a Novel Method vs the Friedewald Equation for Estimating Low-Density Lipoprotein Cholesterol Levels From the Standard Lipid Profile. JAMA. 2013;310(19):2061-2068. doi.org/10.1001/jama.2013.280532
3. Johns Hopkins Medicine. Martin/Hopkins Method to Calculate LDL or 'Bad' Cholesterol Outperforms Other Equations, Study Shows. hopkinsmedicine.org/news/newsroom/news-releases/2023/06