False negative GC/MS assay for carboxy THC due to ibuprofen interference.
A case of a false negative THC metabolite confirmation by GC/MS is presented. A urine specimen testing positive by the EMIT d.a.u. Cannabinoid 100-ng assay was subjected to confirmation by a GC/MS procedure designed to detect the methyl derivative of 11-nor-delta 9-tetrahydrocannabinol-9-carboxylic acid (THC-COOH). No methylated THC-COOH was found. Analysis of the specimen by the TOXI-LAB Cannabinoids TLC procedure yielded a positive confirmation. Subsequent work-up of the specimen revealed a high concentration of ibuprofen, which is shown to interfere with the methylation of THC-COOH. Reanalysis of the specimen by GC/MS with an increased amount of methylating reagent yielded a positive result.
Brunk
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SD
Toxicology Department, Baptist Regional Laboratories, Memphis, Tennessee 38105.
D013759:Dronabinol / Q000031:analogs & derivatives* / Q000652:urine; D004347:Drug Interactions; D005188:False Negative Reactions; D008401:Gas Chromatography-Mass Spectrometry / Q000379:methods; D006801:Humans; D007052:Ibuprofen / Q000097:blood*
D013759:Dronabinol; C025351:delta(9)-tetrahydrocannabinolic acid; D007052:Ibuprofen