Evaluation of a new homogeneous enzyme inhibitor immunoassay of serum thyroxine with use of a bichromatic analyzer.

We evaluated a new homogeneous enzyme immunoassay of thyroxine with use of a discrete analyzer (the ABA-100 Bichromatic Analyzer), modified with an auxiliary dispenser assembly. The assay is based on inhibition of hydrolysis of the substrate, acetyl-beta-(methylthio)choline iodide, by acetylcholinesterase (acetylcholine hydrolase; EC 3.1.1.7). Thyroxine covalently linked to a cholinesterase inhibitor irreversibly inhibits acetylcholinesterase, but if this thyroxine conjugate is bound to antibody it is not inhibitory. Seventy-five patients' samples may be analyzed in 1 h of instrument time. Precision and accuracy are excellent, results correlate well with those by radioimmunoassay, and there were no instances of confused clinical interpretation resulting from use of the proposed assay.