Methadone radioimmunoassay: two simple methods.

Two simple and economical radioimmunoassays for methadone in blood or urine are described. Haemolysis, decomposition, common anticoagulants and sodium fluoride do not affect the results. One assay uses commercially-available 1-3H-methadone hydrobromide as the label, while the other uses a radioiodinated conjugate of 4-dimethylamino-2,2-diphenylpentanoic acid and L-tyrosine methyl ester. A commercially-available antiserum is used in both assays. Normethadone and alpha-methadol cross-react to a small extent with the antiserum while methadone metabolites, dextropropoxyphene, dipipanone and phenadoxone have negligible cross-reactivities. The 'cut-offs' of the two assays as described are 30 and 33 ng ml-1 for blood, and 24 and 21 ng ml-1 for urine. The assay using the radioiodinated conjugate can be made more sensitive if required by increasing the specific activity of the label.