What is the difference between apoB and LDL cholesterol?
Longetic stores each lipid result next to the reference range your own lab printed, because the two numbers answer different questions. LDL cholesterol measures how much cholesterol is being carried inside low-density particles, while apoB counts the particles themselves: the 2019 ESC/EAS guideline says quantitation of ApoB directly estimates the number of atherogenic particles in plasma. Most of the time the two agree, and sometimes they do not.
That line comes from the 2019 ESC/EAS Guidelines for the management of dyslipidaemias, published in the European Heart Journal on 31 August 2019. It is written for clinicians and for laboratories, not for patients, and that matters for everything below. In England and in Spain, almost every official sentence about apoB is addressed to a doctor or to a lab, never to the person whose blood it is.
The same guideline puts a number on how often the two disagree. "In around 20% of patients there may be discordance between measured LDL-C and ApoB levels." It also records that the test itself is not exotic: "Standardized, automated, accurate, and inexpensive methods to measure ApoB are available." The current United States guideline, the 2026 ACC/AHA multisociety guideline on the management of dyslipidemia, published online on 13 March 2026, is more cautious, and the American Heart Association's summary of it for clinicians says that "ApoB measurement helps identify adults with residual elevated lipoprotein-related risk that may be underestimated by the standard lipid profile alone".
Why is the LDL on your report calculated rather than measured?
Most laboratories do not measure LDL cholesterol at all. They measure total cholesterol, HDL and triglycerides, then calculate LDL from those three. The US Centers for Disease Control and Prevention says it plainly in its own survey documentation: "Serum LDL-C levels were calculated from directly measured values of total cholesterol, triglycerides, and HDL-C." Which arithmetic a lab uses is a choice, and labs have made different ones.
There are three equations in general use, and the CDC names all three. The Friedewald equation dates from 1972. The Martin-Hopkins equation was published in JAMA in November 2013 by Seth Martin and colleagues at Johns Hopkins. The third arrived more recently: "In January 2020, the NIH Equation 2 was released (Sampson, et. al., 2020)." That one is usually called the Sampson equation, after its first author at the Clinical Center of the National Institutes of Health, and it was published in JAMA Cardiology in 2020.
Each has a point where it stops working, and the points are different. The CDC states that "The Friedewald equation is not valid for triglyceride results greater than 400 mg/dL." The 2019 ESC/EAS guideline puts the same limit in European units and adds an earlier one: "With high TG values (>4.5 mmol/L or >400 mg/dL) the formula cannot be used", and, before that threshold is reached, "calculated LDL-C has been found to underestimate LDL-C at concentrations of TG ≥2 mmol/L (177 mg/dL)." For the newest equation the CDC says: "The NIH Equation 2 is not valid for triglyceride results greater than 800 mg/dL."
| Equation | First published | Triglyceride limit the CDC prints |
|---|---|---|
| Friedewald | 1972 | Not valid above 400 mg/dL |
| Martin-Hopkins | 2013, JAMA | Limit stated, number not in the line quoted |
| NIH Equation 2 (Sampson) | January 2020, JAMA Cardiology | Not valid above 800 mg/dL |
The CDC does not pick a winner. In the dataset where this is documented it reports that "All three equations are being released in this dataset", and in a December 2025 report it notes that "It is well known that the equations perform differently for varying levels of triglycerides". It also gives the one case where two of them agree exactly: "the Friedewald and Martin equations will be equal when the Martin adjustment factor is 5."
What happens when your lab changes the formula?
Nothing on your report tells you. A pathology page at University Hospitals Sussex, an NHS trust in England, prints the exact arithmetic its laboratory uses for calculated LDL, and that formula is not the 1972 one, yet the page names no equation at all: the words Friedewald, Martin, Sampson and NIH do not appear on it. Anyone comparing an older result with a newer one from that lab is comparing two numbers and has been told nothing.
The page, last updated on 19 May 2026, prints the calculation as: LDL = (TC/0.948) – (HDL/0.971) – [TG/3.74 + (TG x non HDL)/24.16 - (TG²/79.36)} - 0.244. The brackets do not match; that is how it appears, and it is reproduced here unchanged. The same page carries its own stopping point: "Calculation is unreliable when TG >9 mmol/L and not valid if TG >20 mmol/L."
Spain arrives at the same problem from the other end, and writes it down. A 2023 consensus produced by a multidisciplinary group drawn from more than a dozen Spanish scientific societies, published in Revista Clínica de Medicina de Familia, says of the oldest equation: "Dada la inexactitud de la ecuación de Friedewald en pacientes con TG > 150 mg/dL", and then "es preferible realizar la estimación de c-LDL por la fórmula de Martin/Hopkins". It also states the rule that makes a series of results readable at all: "La cuantificación de parámetros lipídicos debe hacerse siempre con la misma metodología."
Whether the profession has settled on one equation is an open question, and it is worth knowing that the people arguing are the people who wrote the equations. Clinical Chemistry published two opposing positions back to back in February 2024. One, co-authored by Seth Martin, concludes that "the Martin–Hopkins equation has consistently emerged as the LDL-C calculation of choice". The other, by Maureen Sampson and colleagues, says that "most clinical laboratories are still using the outmoded Friedewald equation", and adds a caution that matters more than the dispute: "the accuracy of the different equations may depend on the manufacturer of the lipid tests". Both sides agree on one thing, in the second paper's words: "the Sampson-NIH and Martin-Hopkins equations yield similar results for most patients".
Where does apoB sit in England and in Spain?
In both countries apoB exists, but at different heights in the system. No patient page on the NHS website names it, while three NHS hospital laboratories in England list it as an orderable test, and at Leeds it is, in the lab's own words, only routinely available for the lipid clinic. In Spain it is named in national professional guidance instead, and the two countries report it in different units.
| Where apoB appears | England | Spain |
|---|---|---|
| Public patient pages of the health service | Not named | Not named |
| National professional or consensus guidance | Not found on the pages read | Named in the 2023 multi-society consensus |
| Published as an orderable laboratory test | Yes, in hospital pathology directories | Named in the 2023 consensus |
| Unit used for apoB | g/L | mg/dL |
| Unit used for cholesterol | mmol/L | mg/dL |
The absences are as specific as the presences. Four patient-facing pages were read for this article: the NHS high cholesterol page and its getting-tested and cholesterol-levels sub-pages, all reviewed on 13 March 2026, and the equivalent NHS inform page for Scotland, last updated on 7 May 2026. None of them contains the word apolipoprotein. The NHS England cholesterol-levels page goes further than that: it does not mention LDL by name either. What it gives the public is total cholesterol, HDL, and "non-HDL cholesterol – the difference between total cholesterol and HDL". On the Spanish side, the Ministry of Health's national cardiovascular strategy of 2022 contains neither apolipoproteína nor perfil lipídico.
Where apoB does appear in England, it appears three times with three different sets of numbers. North Bristol NHS Trust publishes ">12-60 year old: M=0.49-1.73 g/L, F=0.53-1.82 g/L;". North West London Pathology publishes "0.41 – 1.06 g/L" and "0.47 – 0.99 g/L". Leeds Teaching Hospitals publishes "M: 0.66 to 1.33 g/L" and "F: 0.60 to 1.17 g/L". Three NHS laboratories, one analyte, three reference ranges that do not line up. This is not a mistake by any of them. It is what a reference range is: a statement about that lab's own method and its own population.
Then there are the units, which change at the border. NHS inform, for Scotland, states it directly: "Blood cholesterol is measured in units called millimoles per litre of blood." and "This is often shortened to mmol/L." Spanish lipid documents use mg/dL throughout. Apolipoprotein B adds a second mismatch on top of the first: English NHS laboratories report it in g/L, while the US MedlinePlus encyclopedia entry, reviewed on 21 July 2026, gives it in mg/dL. The same substance, three decimal points apart on the page. If you move results between two countries, this is the reason they look unrelated, and it is covered on its own in why unit conversion matters in health care.
Do you have to fast for a lipid profile?
Usually not, and the answer now differs by country and by test. NHS England tells patients: "You might be asked not to eat anything (to fast) for up to 12 hours before the test." The very next sentence on that page says: "But most people do not need to fast." A Spanish consensus from 2023 goes further and states that apoB specifically is unaffected either way.
The European position is older than both. A joint consensus statement of the European Atherosclerosis Society and the European Federation of Clinical Chemistry and Laboratory Medicine, published in the European Heart Journal on 26 April 2016, recommends that "non-fasting blood samples be routinely used for the assessment of plasma lipid profiles." It keeps an exception, and the exception has to travel with the recommendation: "fasting sampling may be considered when non-fasting triglycerides >5 mmol/L (440 mg/dL)". The same statement adds that "Non-fasting and fasting measurements should be complementary but not mutually exclusive." The 2023 Spanish consensus says the same for a first assessment: "Generalmente, no se requiere ayuno para la determinación de un perfil de lípidos en la valoración del riesgo inicial."
On apoB the Spanish consensus is unambiguous: "Los valores de Apo B no varían por la situación de ayuno." The US patient-facing entry for the same test is not: it says your provider "may tell you not to eat or drink anything for 4 to 6 hours before the test." These two sources disagree, and the disagreement is reported here rather than resolved. What follows from it is practical rather than clinical: because the instruction varies, the only way anyone later knows which applied to your sample is if it was written down at the time. What you did in the day before a draw can move other results too, which is covered in a hard workout before a blood test.
What to keep so your lipid results stay comparable
Keep the report, not the number. The unit, the reference range the lab printed beside it, the date, whether you had eaten, and the name of the laboratory are all part of what the number means. A 2018 position statement from AACC, now ADLM, gives the reason in one line: "different measurement procedures may give different numeric values for the same test".
Six things belong with every lipid result you keep. The full report rather than a single figure, because total cholesterol, HDL, non-HDL and triglycerides are what the calculated figure was built from. The unit as printed, mmol/L or mg/dL. The reference range the lab printed next to the value, because it belongs to that lab. The name of the laboratory, because changing provider changes the method. The date of the draw. And a note of whether you had eaten, because the instruction you were given varies by country and by test.
One thing you cannot keep is the thing that moved your number most, because no lab prints it on a patient report: which equation produced the calculated LDL. If a result looks different from last year's and nothing in your life changed, that is worth saying out loud at your next appointment, in those words. Ask whether the laboratory changed, whether the method changed, and whether the calculated LDL is comparable with the earlier one. A doctor can answer that question in a minute. It is also the reason to resist reading a single move as a trend, which is set out in two lab results show a change, three show a trend.
Longetic keeps each result as it was issued and shows each biomarker's value next to your lab's own reference range, which is the part most often lost when a number is copied out of a report. Two limits are worth saying plainly: Longetic does not convert between units, it leaves the unit and the range exactly as the report printed them, and it does not record which equation a laboratory used to calculate your LDL, because that information is not on the report either.
