Quantitative nephelometric assay for determining alpha-foetoprotein evaluated

A recently introduced automated nephelometric immunoassay involving shell/core particles for determination of alpha-foetoprotein in serum and amniotic fluid was evaluated with the Behring Nephelometer analyser II. Method stability was good: reconstituted reagents and calibration curve were stable for at least one week. The intra-assay CV varied between 2.3% and 4.0%. The inter-assay CV varied between 3.5% and 4.6%. Samples with alpha-foetoprotein concentrations up to 273000 micrograms/l were analysed without high-dose "hook" effect after automatic dilution. No significant interference from haemoglobin, bilirubin, rheumatoid factors, or human anti-mouse antibodies was detected up to concentrations of 0.15 mmol/l haemoglobin, 268 mumol/l bilirubin, 470 int, units/l rheumatoid factor and a titre of 1/1000 human anti-mouse antibodies. Interference due to triacylglycerols depended on the size of triacylglycerol containing particles: for VLDL, interference did not occur up to triacylglycerol levels of 6.0 mmol/l, for chylomicrons interference was already noted at triacylglycerol levels of 1.0 mmol/l. Correlation with a commercial RIA (Kabi Pharmacia) was excellent both for serum (n = 65) and amniotic fluid (n = 100). The effect of the molecular variation of the carbohydrate moiety of alpha-foetoprotein on the test results was studied using concanavalin A affinity chromatography. The detection of both concanavalin A-reactive and concanavalin A-non-reactive alpha-foetoprotein was equivalent by both methods. Multimeric forms of alpha-foetoprotein were prepared by gel permeation chromatography. The effect of autopolymerization of alpha-foetoprotein on the nephelometric determination of alpha-foetoprotein was negligible. We conclude that latex-enhanced immunonephelometry is a rapid, practical, and reliable method for measuring alpha-foetoprotein in serum and amniotic fluid.