A Sensitive Manual Enzyme Immunoassay for Thyroxine

We describe an immunoassay for thyroxine in serum. In the assay specific antibody covalently bonded to latex particles is used,along with horseradish peroxidase as the label, and o-phenylenediamine as the chromogen. The flexible protocol is designed for manual execution. Per-formance is similar to that of the highest-sensitivity thy-roxine radioimmunoassays. Results correlate well with radioimmunoassay (r = 0.99, slope = 0.93, y-intercept = 2.4 pg/liter for 201 samples) and an automated enzyme immunoassay (r = 0.97, slope = 0.99, y-intercept = 4.7 zg/liter for 105 samples). Between-assay and within-assay coefficients of variation are <7.2 % over the entire useful range of the assay (20-240 tg/liter). The limit of detection is <94 pg/tube at 20 jg/Iiter. Only 0-thyroxine is known to interfere with serum assays. This assay has no dis