Quantitative determination of human plasma apolipoprotein A-I by laser immunonephelometry

A technical procedure is described for quantitation of human apolipoprotein A-I (apo A-I) in normal plasma or serum by immunonephelometry. Dilution of the plasma samples with 6 mol/L guanidine chloride ensures maximum exposure of the antigenic sites of the apoprotein and enables optimum quantitation of the apo A-I without requiring extraction with organic solvents. Similar data are obtained by this assay and with radioimmunoassay for normal subjects (1.2--1.5 g/L), and the results obtained on 31 patients are correlated with a coefficient of 0.92. The apo A-I values are correlated with values for plasma high-density lipoprotein cholesterol (r = 0.64). The interassay CV for immunonephelometry is about 7% and the standard curve is linear between 0.1 and 1.0 microgram of apo A-I per sample, corresponding to a 150-fold dilution of serum or plasma. The assay is applicable to plasma samples containing as much as 4 g of triglycerides per liter. At higher concentrations plasma delipidation is required.