ApoA-I (Apolipoprotein A-I)

Apolipoprotein A-I is a 28-kDa hepatic and intestinal apolipoprotein that constitutes about 70% of total HDL protein and activates lecithin-cholesterol acyltransferase (LCAT), driving reverse cholesterol transport from peripheral tissues back to the liver. It is measured by immunonephelometric or immunoturbidimetric assay, calibrated against the WHO/IFCC SP1-01 reference material; typical reference ranges are 110-180 mg/dL in men and 120-200 mg/dL in women. In the INTERHEART case-control study of 12,461 cases and 14,637 controls across 52 countries, the ApoB/ApoA-I ratio was the strongest single lipid predictor of incident myocardial infarction (odds ratio approximately 3.25 for top vs bottom quintile), outperforming the conventional LDL-C/HDL-C ratio. Major confounders include acute infection and inflammation (acute-phase decreases of 20-30%), pregnancy, oral estrogens (raise it), severe hepatic disease, and rare ApoA-I mutations (Tangier disease, ApoA-I Milano).