False-Positive Results in the Detection of Methadone in Urines of Patients Treated with Psychotropic Substances

In the context of methadone maintenance programs, immunochemical detection of methadone in urine has evolved, with rapid and sensitive techniques gaining impact in clinical laboratories because of their cost-effectiveness and suitability for automation (1). However, immunochemical methods may lack the specificity necessary for accurate measurement. We report spurious methadone results encountered in a psychiatric hospital. We have been detecting urinary methadone with the Kinetics Interaction of Microparticles in Solution (KIMS) methodology on the Roche Integra 800 since April 2003 (Roche Diagnostics). The test is used in the qualitative mode, with a methadone concentration exceeding 0.300 mg/L being reported as positive. This technology is known for its steep dose–response curve around the cutoff of the assay (1). In its first formulation, the reagent included a polyclonal antibody that remained free in solution and captured either methadone or a methadone multivalent conjugate coupled on microparticles. In September 2004, Roche Diagnostics switched from the polyclonal antibody to a monoclonal antibody and modified the original KIMS technology by coupling the antibody to the microparticles and keeping the multivalent drug conjugate …