Lipid profile (enzymatic methods)

Principle

• Total cholesterol: cholesterol esterase hydrolyses esters; cholesterol oxidase forms H2O2; peroxidase couples 4-aminoantipyrine with phenol to a red quinoneimine (Trinder), read at about 500 nm.
• Triglycerides: lipase releases glycerol; glycerol kinase and glycerol-3-phosphate oxidase generate H2O2, measured by the Trinder reaction.
• HDL cholesterol: direct homogeneous assays block or remove non-HDL lipoproteins, or precipitation methods (e.g. phosphotungstate–Mg2+, dextran sulfate–Mg2+) remove them before measuring cholesterol in the supernatant.
• LDL cholesterol: calculated (Friedewald: LDL = TC – HDL – TG/5 in mg/dL; TG/2.2 in mmol/L) or by direct assay. Newer equations (Martin–Hopkins, Sampson/NIH) perform better at high TG or low LDL.
• Non-HDL cholesterol = total cholesterol – HDL.

Specimen

• Serum or plasma. Fasting 9–12 hours traditionally required, mainly for TG; many guidelines now accept non-fasting samples for routine screening.

Results

• Typical adult targets (NCEP ATP III style): total cholesterol <200 mg/dL desirable; LDL <100 mg/dL optimal; HDL <40 mg/dL low, ≥60 mg/dL protective; TG <150 mg/dL normal. Targets vary with individual risk; local guidance applies.
• Cholesterol mg/dL ÷ 38.67 = mmol/L; TG mg/dL ÷ 88.57 = mmol/L.

Quality control and pitfalls

• Friedewald is invalid when TG >400 mg/dL (4.5 mmol/L), in non-fasting samples with high TG, and in type III hyperlipoproteinaemia.
• Free (endogenous) glycerol falsely raises TG unless a glycerol blank is used.
• Ascorbic acid and high bilirubin cause negative interference in Trinder reactions.
• Acute illness or recent myocardial infarction temporarily lowers cholesterol; posture and tourniquet time affect results.

Clinical use

• Cardiovascular risk assessment and monitoring of lipid-lowering therapy.