Radioimmunoassay of drugs of abuse in hair. Part 1: Methadone in human hair, method adaptation and the evaluation of decontamination procedures.

A method suitable for the determination of methadone in human hair is presented. Adaptation and evaluation of a solid-phase I125 radioimmunoassay, designed for the quantitative measurement of methadone in urine, and development of a pre-analytical wash procedure has enabled a specific, sensitive and accurate analytical procedure to be developed. The specificity of the antiserum towards other drugs or biologically active compounds is evaluated up to a concentration of 100,000 ng ml-1 and accuracy covering a range of 0-450 ng ml-1 is found to be within 6% of expected methadone concentrations prepared in both drug free hair extract and urine. Inter-assay relative standard deviation (RSD) at concentrations of 5.1, 76.0 and 247 ng ml-1 methadone are 5.5, 2.5 and 3.6% respectively (n = 10) and intra-assay RSD at concentrations 2.3, 25.2 and 217 ng ml-1 are 5.3, 3.6 and 6.8% (n = 5). The limit of detection is 0.5 ng ml-1. Extraction of control drug free hair samples spiked with methadone at concentrations of 100, 250 and 400 ng ml-1 achieved recoveries of 86, 80 and 89%, respectively. Control hair samples contaminated with methadone are examined under differing wash procedures to assess their effectiveness in the removal of methadone contaminant. A suitable pre-analytical wash regime is proposed for removal of contaminant derived from external or environmental sources. The mechanics of the wash action and contaminant application to the hair is discussed. It is concluded that the adapted radioimmunoassay and developed pre-analytical decontamination procedure is a suitable technique to employ for the measurement of methadone in human hair, be it prescribed or abused, with concentrations expressed as ng methadone per mg hair.