Question 1

How do you calculate LDL from a lipid panel?

Accepted Answer

The standard method is the Friedewald equation: LDL = total cholesterol − HDL − triglycerides ÷ 5 (all in mg/dL). For a panel of TC 200, HDL 50 and TG 150, LDL = 200 − 50 − 30 = 120 mg/dL. This tool also computes the newer Sampson-NIH equation, which is more accurate when triglycerides are high.

Question 2

What is the Friedewald equation and when is it inaccurate?

Accepted Answer

Friedewald (1972) estimates LDL as TC − HDL − TG/5, assuming VLDL equals one-fifth of triglycerides. That assumption breaks down when triglycerides rise above 400 mg/dL (about 4.5 mmol/L) or when LDL is very low, so the equation under-estimates LDL there. Above 400 mg/dL this calculator suppresses the Friedewald result and points you to the Sampson-NIH value.

Question 3

What is a healthy LDL cholesterol level?

Accepted Answer

Using NCEP ATP III bands, an LDL below 100 mg/dL (2.6 mmol/L) is optimal, 100–129 is near optimal, 130–159 is borderline high, 160–189 is high and 190 or above is very high. People with diabetes or existing heart disease are usually given a lower personal target by their doctor, often below 70 mg/dL.

Question 4

What is non-HDL cholesterol and why does it matter?

Accepted Answer

Non-HDL cholesterol is simply total cholesterol minus HDL, so it captures LDL plus every other atherogenic particle (VLDL, IDL, remnants). The 2018 AHA/ACC guideline treats it as a secondary target because, unlike Friedewald LDL, it stays reliable even when triglycerides are high and does not require fasting. The non-HDL target is roughly 30 mg/dL above the matching LDL target.

Question 5

Why can't I use Friedewald when triglycerides are over 400 mg/dL?

Accepted Answer

Friedewald approximates VLDL cholesterol as triglycerides ÷ 5. At triglycerides above 400 mg/dL that ratio no longer holds, so the formula increasingly under-estimates LDL — sometimes by a large margin. In that range labs switch to a directly-measured LDL or to equations like Sampson-NIH (validated up to 800 mg/dL), which this tool calculates for you.

Question 6

My Sri Lankan lab report is in mmol/L but my doctor quoted mg/dL — how do I convert?

Accepted Answer

Multiply a cholesterol value in mmol/L by 38.67 to get mg/dL, and a triglyceride value by 88.57. For example LDL 3.0 mmol/L × 38.67 = 116 mg/dL. Use the mg/dL ↔ mmol/L toggle at the top of the calculator and it re-expresses every value for you, so a mmol/L report can be read against mg/dL targets.

Question 7

Do I need to fast before the blood test?

Accepted Answer

Triglycerides and therefore Friedewald LDL are affected by recent meals, so most Sri Lankan labs ask for a 9–12 hour fast before a lipid profile. Non-HDL cholesterol and the Sampson-NIH estimate are less sensitive to fasting. Follow the instructions your lab or doctor gives; if your sample was non-fasting, mention it when reading the result.

Question 8

Is this the same LDL my lab prints on the report?

Accepted Answer

Usually yes — most labs print a calculated LDL using Friedewald, which matches this tool to the rupee of rounding. Some labs measure LDL directly with an assay, which can differ by a few mg/dL. If your report omits LDL entirely (common when only TC, HDL and triglycerides are listed), this calculator fills it in.

Marker	Your value	Category
LDL cholesterol	120 mg/dL	Near optimal
Total cholesterol	200 mg/dL	Borderline high
HDL cholesterol	50 mg/dL	Acceptable
Triglycerides	150 mg/dL	Borderline high
Non-HDL cholesterol	150 mg/dL	Near optimal

LDL Cholesterol Calculator — Friedewald & Sampson-NIH

Classification (NCEP ATP III)

How it works

Worked examples

Frequently asked questions

Sources & references

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