RESUMEN
Background: On an initial urine screening test for illegal substances, if the concentration of a substance is at or above the determined legislative threshold, it is reported as positive. Repeating testing with the same sample to verify it before reporting is a common practice in clinical laboratories. This study aimed to determine whether measurement uncertainty (MU) results can be used to detect a grey zone to reduce repeat testing. Methods: A retrospective study was conducted using data from the laboratory information system between January 1, 2020, and July 1, 2022. Samples studied twice within one hour before reporting for the same urine sample were analyzed. The MU values for urinary amphetamine, cannabinoid, cocaine, and opioid parameters were calculated using ADVIA Chemistry reagents on a Siemens ADVIA 1800 chemical analyzer. The grey zone was defined as the cut-off value ± MU.