In more detail
A lipid panel reports total cholesterol, HDL, LDL and triglycerides. The total-to-HDL ratio combines two of them, so a higher HDL lowers the ratio; non-HDL cholesterol is total minus HDL. LDL is often calculated with the Friedewald formula, total − HDL − triglycerides / 5 in mg/dL (/ 2.2 in mmol/L), which is valid only when triglycerides are below 400 mg/dL; many labs now use newer equations or measure LDL directly. To convert cholesterol from mg/dL to mmol/L divide by 38.67, and triglycerides by 88.57.
Why it matters
The ratio feeds into risk calculators together with age, blood pressure, smoking and diabetes; on its own it does not decide treatment. Many references describe a ratio below 5 as desirable and about 3.5 as ideal, but targets depend on overall risk. Units differ by country, so a total of 5.2 mmol/L and 200 mg/dL are essentially the same result. Interpretation and targets are for a clinician.
Example
Total 200 mg/dL, HDL 50 mg/dL, triglycerides 150 mg/dL
Ratio = 200 / 50 = 4.0
Non-HDL = 200 − 50 = 150 mg/dL
LDL (Friedewald) = 200 − 50 − 150 / 5 = 120 mg/dL
In mmol/L: 5.17 − 1.29 − 1.69 / 2.2 = 3.11 mmol/L