Confirmation Tests

As immunoassays lack absolute specificity for their target analytes, an initial immunoassay positive result should be considered as a presumptive positive only. Definitive identification of the drug must be based on confirmation testing. The standard of practice in analytical toxicology defines a confirmation test as a technique that employs a different analytical principle and has better specificity and equivalent or better sensitivity than the initial test. Therefore, re-testing by a second immunoassay is not confirmation, because the second test is not based on a different analytical principle. Nor is an high performance liquid chromatography (HPLC) result confirmed by immunoassay acceptable because the second test is less specific than the first. While confirmation using a method based on MS is mandatory in forensic testing, other analytical techniques such as thin layer chromatography, GC, or HPLC are acceptable for clinical confirmation as long as the confirmation assay has been vigorously validated for its precision and accuracy.

Confirmation testing may not be needed in clinical toxicology in certain settings (20). Physicians interpreting the initial test results have other information on the patient— history, physical examination, and other laboratory tests. Therefore, clinical drug test results are interpreted within the clinical context of the patient. In clinical settings where administrative or punitive actions maybe taken (e.g., removing a patient from drug treatment program because of repeated positive drug tests), confirmation testing may be indicated. Laboratories performing clinical testing that may have high potential for being involved in a medical legal challenge should consider instituting confirmation testing. An example would be drug testing of newborns for suspected in utero drug exposure because results, in some localities, are reported to local health authorities.

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