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The statement cited by Dr Fody requires some clarification. While it was meant to be associated primarily with the EMIT single-test assay, this was perhaps not clear.Unconfirmed positive results can occur due to technician errors or cross-reacting substances, and, in the case of marijuana, they can occur when the confirmation method is less sensitive than the screening method. This is frequently the case when immunoassay screens are used at their lower limits to assay for the many metabolites of THC in the aggregate and the confirmation method is assaying only for free 9-carboxy-THC, one of the metabolites that make up the mixture. This is probably the case for most of those "false-positives" reported in the cited literature above. This issue is discussed in the second paragraph of the "Comment" section of the article. This is not to imply that errors do not occur; where good quality control
Hawks RL, Schwartz RH. Laboratory Detection of Marijuana Use-Reply. JAMA. 1985;254(24):3425. doi:10.1001/jama.1985.03360240037027