Cholesterol Calculator (Friedewald LDL, non-HDL, TG/HDL, AIP)

Understanding the cholesterol ratio calculator — concept and principle

This calculator turns a standard lipid panel (total cholesterol, HDL and triglycerides) into five numbers doctors use to judge cardiovascular risk: Friedewald LDL cholesterol, non-HDL cholesterol, the TG/HDL ratio, the atherogenic index of plasma (AIP) and the total/HDL ratio.

How it is calculated

• LDL (Friedewald) = TC − HDL − TG ÷ 5 (mg/dL), valid only when TG < 400 mg/dL
• Non-HDL = TC − HDL
• TG/HDL = TG ÷ HDL (mg/dL)
• AIP = log₁₀(TG ÷ HDL) with both converted to mmol/L (TG ÷ 88.57, HDL ÷ 38.67)
• Total/HDL = TC ÷ HDL

The principle

Total cholesterol is carried by three main particle groups: LDL, HDL and VLDL. Friedewald and colleagues (1972) noticed that in fasting blood VLDL cholesterol is about one-fifth of triglycerides in mg/dL, so LDL can be found by subtraction without a separate test. That assumption breaks down when triglycerides are high or the sample is not fasting. Non-HDL counts the cholesterol in all particles that can enter the artery wall. The TG/HDL ratio and AIP reflect the balance between triglyceride-rich particles and protective HDL, and a high value is linked with small, dense LDL particles.

Worked example

1. Inputs: TC 210, HDL 50, TG 150 mg/dL.
2. LDL = 210 − 50 − 150 ÷ 5 = 210 − 50 − 30 = 130 mg/dL.
3. Non-HDL = 210 − 50 = 160.
4. TG/HDL = 150 ÷ 50 = 3.
5. AIP: TG = 150 ÷ 88.57 = 1.694 mmol/L, HDL = 50 ÷ 38.67 = 1.293 mmol/L; log₁₀(1.694 ÷ 1.293) = 0.12.
6. Total/HDL = 210 ÷ 50 = 4.2.

How to read the result

Under the long-used US NCEP ATP III categories, LDL below 100 mg/dL is optimal, 130–159 borderline high and 160 or more high; HDL below 40 mg/dL is low; triglycerides below 150 are normal. Non-HDL targets are usually set 30 mg/dL above the LDL target. A total/HDL ratio under about 5 is often considered favourable, and AIP is commonly grouped as below 0.11 low, 0.11–0.21 intermediate and above 0.21 high. Modern guidelines, however, set LDL goals by your overall risk, so the same number can be fine for one person and too high for another.

Common mistakes

• Using a non-fasting sample with high triglycerides, which makes Friedewald LDL too low.
• Entering mmol/L values into a mg/dL calculator.

Frequently asked questions

Why does my lab show a different LDL?

Many labs measure LDL directly or use newer equations such as Martin-Hopkins or Sampson, which handle high triglycerides better.

How do I convert cholesterol from mmol/L to mg/dL?

Multiply cholesterol (TC, LDL, HDL) by 38.67 and triglycerides by 88.57.

Is high HDL always good?

Higher HDL is generally linked with lower risk, but HDL-raising drugs have not clearly reduced heart disease.

Sources: Friedewald WT, Levy RI, Fredrickson DS. Estimation of the concentration of low-density lipoprotein cholesterol in plasma. Clin Chem 1972;18(6):499–502. · Dobiášová M. Atherogenic index of plasma [log(triglycerides/HDL-cholesterol)]. Clin Chem 2004;50(7):1113–1115. · NCEP Adult Treatment Panel III Final Report, Circulation 2002;106:3143–3421.