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Isolab's T4 Test Kit is intended for use in determining the total thyroxine (T4) in blood specimens dried on filter paper.

The Isolab's T4 Test Kit is a sequential, competitive enzyme immunossay with fluorometric detection in which T4 is eluted from dried blood disks and simultaneously forms a complex with the biotin-labeled monoclonal antibody which binds to the streptavidin coated solid phase. After sufficient incubation with biotinylated antibody, a conjugate of T4 with horseradish peroxidase is added to the wells and incubated with the reaction mixture. This conjugate competes with the analyte for the free on the bound monoclonal antibody. Excess of unbound reagent is washed away after a sequential incubation with biotinylated antibody and conjugate, and the enzymatic reaction with a fluorogenic substrate 3-phydroxyphenylpropionic acid (HPPA) [9] takes place. The reaction is stopped by addition of glycine buffer and the fluorescence in each well is measured with Fluoroscan II Neonate at 405 nm (excitation being 320 nm).