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Your LDL moved. Here’s the most likely reason
If your LDL cholesterol changed between two lab reports and nothing about your health did, the likeliest explanation is arithmetic. LDL is not measured in your sample — it is worked out from three values that are, using one of three different equations. They do not all produce the same number from the same blood.
If this is the first you have heard that LDL is calculated, start with how three of your lipid numbers are measured and the rest are worked out. Comparing two of a parent’s reports? The same questions apply.
If both reports came from the same lab
Laboratories change methods and rarely announce it. A switch shows up as a number that moved on its own.
If they came from different labs
Two labs can report different LDL values from identical blood without either being wrong. Section 7 covers how to check yours.
ℹ️ Medical Disclaimer: This is general education about how laboratories calculate LDL cholesterol. It does not diagnose, interpret your results, recommend or adjust medication, or advise on any procedure, test, or insurance decision, and no figure here is a target for you. Before acting on it, consult a board-certified physician — your primary care clinician, or a cardiologist or lipid specialist if one is involved.
What both equations start with is identical
Every equation in current use starts from the same three measured values: total cholesterol, HDL cholesterol, and triglycerides. Each subtracts HDL from total cholesterol identically, and that step is not disputed.
Some cholesterol is carried by triglyceride-rich particles, and that portion is estimated rather than measured. How much to subtract for it is the only thing the equations argue about.
The three values your blood actually gave
See which values on the panel are measured directly and why sources disagree on how many tests it contains.
Where the estimating begins
🔬 How It Works: Your lab can measure total cholesterol and the share carried by HDL particles. It cannot cheaply measure the share carried by triglyceride-rich particles, so it estimates that from your triglyceride level. That estimate is the whole difference between the equations.
Three equations, one disputed number
The Friedewald equation, Martin-Hopkins, and NIH Equation 2 all subtract an estimate of triglyceride-carried cholesterol. They differ in what they divide your triglycerides by.
| Equation | Triglyceride-to-VLDL ratio used | Built from | Not valid above | Key clinical detail |
|---|---|---|---|---|
| Friedewald (1972) | A fixed factor of 5, applied to everyone | Beta-quantification on 448 samples | Above 400 mg/dL | The long-standing standard, used by NHANES since 1996 |
| Martin-Hopkins (2013) | An adjustable factor from your own non-HDL and triglyceride values | Ultracentrifugation data from 1.35 million patients | Above 400 mg/dL | NHANES uses a 360-cell table to pick the factor |
| NIH Equation 2 (2020) | A different structure, not a simple divisor | Beta-quantification on 8,656 patients, 18,715 samples | Above 800 mg/dL | The most accurate at triglycerides of 400 mg/dL and above |
Source: the federal survey documentation for all three equations, CDC NCHS, NHANES 2017–March 2020.
Friedewald: divide triglycerides by five
Friedewald assumes one ratio holds for everyone. That was reasonable in 1972, and it is still right most of the time.
Martin-Hopkins and NIH equation 2: newer, and shaped differently
Martin-Hopkins picks the divisor from your own values, which is why your calculated LDL can move while the non-HDL number that factor is keyed to stays identical. NIH Equation 2, released in January 2020, holds up at triglyceride levels where the other two are no longer used.
⚠️ Clinical Warning: The triglyceride figures above are limits on the equations, not warnings about your triglycerides. Above 400 mg/dL most labs report no calculated LDL rather than an unreliable one; a blank line is a method limit, not a result.
How much can the number actually move?
In a study comparing all three equations in one national sample, the average difference between Friedewald and Martin-Hopkins was 3.17 mg/dL, with a median of 2 mg/dL. That sample was 4,556 US participants aged 12 to 80 in the 2017–2020 National Health and Nutrition Examination Survey.
Against NIH Equation 2, the average Friedewald difference was 2.08 mg/dL. For most people the equation change moves the number by two or three points.
The typical gap is a few points
📊 Clinical Data Point: Mean absolute difference, Friedewald vs Martin-Hopkins: 3.17 mg/dL (median 2 mg/dL) in 4,556 US participants aged 12–80. — Source: a study comparing all three equations in the same NHANES sample, Journal of Lipid and Atherosclerosis, 2023.
Where the two equations disagree most
The differences varied by sex and by triglyceride level, so the gap is not spread evenly. An average is a starting point, not a prediction about you.
When the change crosses a category line
In the same sample, Friedewald and Martin-Hopkins placed 93.0% of people in the same LDL category, and agreement against NIH Equation 2 was 95.4%. For the minority whose category did change, what a genuinely higher result changes is worth reading alongside this.
When the difference is big enough to matter
For a small group, the equation choice is not academic. The federal documentation is direct: Friedewald underestimates LDL cholesterol below 70 mg/dL and when triglycerides are high, with the potential consequence of undertreatment.
An underestimate reads as better news than the sample supports. If you are in that group, this is a conversation to have rather than a conclusion to draw alone.
Low LDL numbers move the most
In the FOURIER trial, among patients already on intensive lipid-lowering therapy with low LDL levels, the two equations were compared against the laboratory reference method. Martin-Hopkins values sat a median of 2 mg/dL from that reference and Friedewald values 4 mg/dL. In that already-treated group, 22.9% of Martin-Hopkins values differed from the reference by over 5 mg/dL, against 40.1% of Friedewald values.
Higher triglycerides widen the gap
The 70 mg/dL and 400 mg/dL figures here are study strata and method-validity limits. Neither is a danger threshold, and neither is a target for you.
What to do with a number that moved across a line
✅ Patient Action: Bring both reports to your next appointment and ask your prescribing clinician — or your cardiologist or lipid specialist, if one is involved: “My LDL was calculated with a different equation this time. Does that change how you read it, and would you want it rechecked by the same method?”
Family history changes how much a borderline result carries; our genetic risk assessment tool covers what to bring.
What the current guideline says about which equation
The 2026 ACC/AHA dyslipidemia guideline, published in March 2026 to replace the 2018 blood cholesterol guideline, states a preference: calculate LDL using the Martin-Hopkins or Sampson/NIH equations, both preferred over Friedewald.
It also places them above direct LDL measurement other than beta-quantification, the laboratory reference method.
The 2026 recommendation, in plain terms
Use one of the newer equations. That is a preference, not a prohibition — a lab still running Friedewald is not producing an invalid result, and as Section 4 showed, the equations usually agree.
Why this changed in 2026
🔬 How It Works: Both newer equations were tested against ultracentrifugation, the reference method that physically separates the particles instead of estimating them. Across a wide span of triglyceride and LDL levels, fasting and non-fasting, they tracked that reference more closely than Friedewald did.
See the current guideline’s stated preference between the equations. If direct measurement is the question you came with, why your lab calculates LDL instead of measuring it answers it in full.
How to find out which equation your lab used
Two things to look for on the report itself. Some label the LDL line “calculated” and a few name the equation, so compare the wording across both reports, not just the number.
If neither names an equation, that is common, and the answer has to be asked for.
Where it appears on the report
Usually beside or beneath the LDL value, if at all. The rest of what’s on the report puts the line in context.
What to ask if it doesn’t appear at all
✅ Patient Action: Call the laboratory named on the report, or ask at your next visit: “Which equation calculated the LDL here, and was the same one used on my previous report?” Until you know, treat LDL values from two labs as two estimates rather than a trend.
While you wait, what to do about results that haven’t arrived and how the 2026 guideline changed the rest of the panel both help.
Common questions about LDL calculation equations
1. Why did my LDL cholesterol change when my diet didn’t?
The most common non-medical explanation is an ldl calculation equation difference between the two reports. LDL is estimated from measured total cholesterol, HDL, and triglycerides, and laboratories use three different equations to do it. A switch moves the number without your blood changing, and your clinician can confirm whether that explains your result.
2. Is the Martin-Hopkins equation more accurate than Friedewald?
Against ultracentrifugation, the laboratory reference method, Martin-Hopkins has tracked more closely than Friedewald across a wide range of triglyceride and LDL levels, in both fasting and non-fasting samples. The 2026 ACC/AHA guideline prefers it, alongside the Sampson/NIH equation. Friedewald remains valid and agrees with the newer equations most of the time.
3. Can I compare LDL results from two different labs?
Not as a like-for-like trend, unless you know both used the same equation. Two laboratories can report different LDL values from identical blood, and in one national sample the average difference between two equations was about 3 mg/dL. Ask each lab which it used, then raise the comparison with your clinician.
4. Which LDL equation does my lab use?
Some reports print the equation name or the word “calculated” beside the LDL line, and many print nothing at all. When it is not stated, the laboratory named on the report can tell you, or ask at your next appointment. Knowing this ldl calculation equation difference matters most when comparing two reports.
5. What is the Sampson or NIH equation 2?
It is the third equation in current US use, released in January 2020 and built from a beta-quantification reference method on 8,656 patients across 18,715 samples. Unlike the other two it stays valid up to a triglyceride level of 800 mg/dL, and it is the most accurate of the three at 400 mg/dL and above.
6. Does the equation matter if my LDL is in the normal range?
Usually less: in a sample of 4,556 US participants, Friedewald and Martin-Hopkins assigned the same LDL category 93.0% of the time. The gap widens at low LDL levels and higher triglycerides, so an ldl calculation equation difference matters most near a line your clinician treats differently. Ask how they read your result.
7. Why does the Friedewald equation underestimate LDL?
It applies a single fixed factor of 5 to everyone’s triglycerides to estimate the cholesterol those particles carry. When that assumption overshoots the amount being subtracted, the LDL left over comes out lower than it should. Federal documentation identifies this at LDL levels below 70 mg/dL and when triglycerides are high.
8. Should I ask for a recheck if my lab changed equations?
That is a reasonable question to raise, not a step to take alone. Bring both reports and ask your prescribing clinician whether the change affects how they read the result, and whether a recheck by the same method would help. Most people land in the same category either way, so often the answer is no.
What to take to your next appointment
Your LDL is calculated, not measured, and three equations are in current use. For most people the choice between them moves the number by two or three points, and 93% land in the same category regardless. At low LDL levels and higher triglycerides the gap widens, and it widens toward underestimating.
Take both reports to your next appointment and ask which equation produced each. That question turns two numbers you cannot compare into two you can.
About this content
How this article was put together: researched from recognised health sources, drafted with the help of AI tools, and edited by hand, with sources linked throughout.
Sameer Patel is the founder and editor of My Medicine Advisor. He is not a doctor or medical professional — before starting this site he worked in banking,…
Medical disclaimer
The content on MyMedicineAdvisor is provided for general informational and educational purposes only and is not a substitute for professional medical advice, diagnosis, or treatment. Health information on this website should not be used to diagnose, treat, cure, or prevent any condition without guidance from a qualified healthcare professional. Always seek the advice of your doctor, physician, or another licensed healthcare provider with any questions you may have regarding a medical condition, symptoms, medications, or treatment decisions.
