حاسبة كوليسترول LDL

LDL cholesterol is the number most cardiovascular risk decisions are built on, yet on most lipid panels it is never measured. It is calculated from the three values the laboratory does measure: total cholesterol, HDL cholesterol and triglycerides. Understanding which equation produced your LDL matters, because the equations disagree most in exactly the situations where the result changes management, namely low LDL and high triglycerides.

المعادلة

Friedewald (mg/dL):   LDL = Total cholesterol - HDL - (Triglycerides / 5)
Friedewald (mmol/L):  LDL = Total cholesterol - HDL - (Triglycerides / 2.2)
Non-HDL cholesterol = Total cholesterol - HDL

• Triglycerides / 5 estimates VLDL cholesterol in mg/dL, assuming a fixed ratio of triglyceride to cholesterol in VLDL particles
• Non-HDL cholesterol needs no estimation step and stays valid at any triglyceride level

Friedewald WT, Levy RI, Fredrickson DS, Clinical Chemistry, 1972. The fixed divisor was derived from a small sample and assumes a constant VLDL composition, which is the assumption that fails at high triglycerides.

How the calculation works

Total cholesterol is distributed across three main lipoprotein classes: HDL, LDL and VLDL. Laboratories measure total cholesterol and HDL directly, and measure triglycerides, which travel mostly in VLDL. If the ratio of triglyceride to cholesterol inside VLDL particles were constant, dividing triglycerides by five would give VLDL cholesterol, and subtracting HDL and VLDL from the total would leave LDL. That is precisely what the Friedewald equation does.

The weakness is the fixed divisor. VLDL composition is not constant; it varies with triglyceride concentration and between individuals. As triglycerides rise, the ratio drifts and the divisor of five increasingly overestimates VLDL cholesterol, which makes calculated LDL too low. This is why the equation is conventionally treated as invalid above 400 mg/dL of triglycerides, and why it underestimates LDL well before that ceiling.

The Martin-Hopkins method addresses this by replacing the fixed divisor with one selected from a lookup table indexed by the person's own triglyceride and non-HDL values, derived from a very large clinical laboratory dataset. It is more accurate at low LDL and at raised triglycerides, and many laboratories have adopted it. If your report includes an LDL that differs from what the Friedewald equation gives, this is usually why.

How to read your result

The bands below come from long-standing US guidance and are useful for orientation, but current cholesterol guidelines do not treat LDL as a universal target with one cut-off. Treatment decisions depend on overall cardiovascular risk, so the same LDL can be acceptable in one person and an indication for treatment in another.

• An LDL of 190 mg/dL or above in an adult is treated as a specific signal, not just a severe number: it raises the possibility of familial hypercholesterolaemia and prompts family screening.
• Non-HDL cholesterol, simply total minus HDL, requires no estimation, stays valid when triglycerides are high, and is a stronger predictor of risk in people with diabetes or metabolic syndrome.

Fasting, non-fasting and why your number moves

Lipid panels were traditionally drawn fasting because triglycerides rise after a meal, and because the Friedewald equation is sensitive to triglycerides. Current guidance in both Europe and the United States accepts non-fasting samples for most routine assessment, reserving a repeat fasting sample for cases where non-fasting triglycerides come back markedly elevated.

Biological variation also matters when comparing two results. Total cholesterol commonly varies by several percent between measurements in the same stable person, and triglycerides vary considerably more. A change of a few mg/dL between two panels is usually noise rather than signal, and a treatment decision should rest on the trend across repeated measurements rather than on one reading.

حدود الاستخدام

• The Friedewald equation is not valid when triglycerides exceed 400 mg/dL (4.5 mmol/L), and it progressively underestimates LDL at triglyceride levels well below that ceiling.
• Accuracy is worst where it matters most for treatment: at LDL below 70 mg/dL, the level many high-risk patients are treated towards, Friedewald can understate the true value substantially.
• The equation is not valid in type III hyperlipoproteinaemia or when chylomicrons are present, because the assumed VLDL composition no longer holds.
• Errors in any of the three measured inputs propagate directly into the calculated LDL, and each of those measurements carries its own analytical variation.
• If your laboratory uses the Martin-Hopkins method or a direct LDL assay, its reported LDL will not match the Friedewald result, and the two should not be compared across time as if they were the same measurement.
• LDL is one input to cardiovascular risk. Age, blood pressure, smoking, diabetes, family history and lipoprotein(a) all matter, and no single lipid value determines whether treatment is warranted.

الأسئلة الشائعة

Is LDL cholesterol measured or calculated?

On most standard lipid panels it is calculated, not measured, from total cholesterol, HDL and triglycerides. Direct LDL assays exist and are used when the calculation is unreliable, such as when triglycerides are very high.

Why is the Friedewald equation invalid above 400 mg/dL of triglycerides?

The equation estimates VLDL cholesterol as triglycerides divided by five, which assumes a constant composition of VLDL particles. That assumption breaks down as triglycerides rise, so above roughly 400 mg/dL the estimate is unreliable and a direct LDL measurement is used instead.

What is non-HDL cholesterol and why does it appear on my report?

Non-HDL cholesterol is total cholesterol minus HDL. It captures the cholesterol carried by all atherogenic particles, needs no estimation step, and remains valid when triglycerides are high, which makes it particularly useful in diabetes and metabolic syndrome.

Do I need to fast before a lipid panel?

Usually not. Current European and American guidance accepts non-fasting lipid panels for routine assessment, with a fasting repeat reserved for cases where non-fasting triglycerides come back markedly elevated. Follow whatever instruction the ordering clinician gives.

My LDL changed between two tests. Is that meaningful?

Often not. Cholesterol varies biologically between measurements even in a stable person, and triglycerides vary considerably more. Small differences between consecutive panels are usually variation rather than a real change; the trend across several results is what carries information.

المراجع

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.
2. Martin SS 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.
3. National Heart, Lung, and Blood Institute. ATP III At-A-Glance: Quick Desk Reference.
4. MedlinePlus. LDL: The Bad Cholesterol.