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Fentanyl as a potential false positive with color tests commonly used for presumptive cocaine identification

Chemical color tests are widely utilized as part of the analytical scheme approved to identify drugs in forensic laboratories and in the field by law enforcement officers. Although these test results are considered preliminary indications of the presence of a drug, forensic scientists sometimes use these test results to direct their confirmatory testing and law enforcement officers use these test results when making arrest decisions and decisions on how to impound evidence. The color tests commonly used to identify cocaine are aqueous cobalt thiocyanate, the Young's test, the Scott's test, and the modified Scott's test. Field testing of a white powder was reported by a law enforcement officer to be positive for cocaine hydrochloride using a commercially available test kit based on the modified Scott's test. The forensic laboratory determined that the powder contained fentanyl and mannitol; cocaine was not detected. Subsequently, the case material, fentanyl and cocaine reference materials, and cocaine cut with mannitol were tested using aqueous cobalt thiocyanate, the Young's test, the Scott's test, and the modified Scott's test. The fentanyl standard and case material produced the colors that would be interpreted as cocaine using the aqueous cobalt thiocyanate and Young's tests. The misidentification of fentanyl as cocaine with these tests could create a potentially hazardous situation. The cocaine containing samples were distinguishable from the fentanyl containing samples with the Scott's and modified Scott's test when 1 mg of cocaine material was tested, whereas a 3‐mg cocaine sample produced the same color sequence as fentanyl.