Laboratory Detection of Marijuana Use

To the Editor.— Schwartz and Hawks have provided an excellent discussion of the laboratory's role in the detection of marijuana use. 1 However, their statement that false-positive results of the homogeneous enzyme immunoassay technique (EMIT) and radioimmunoassay are rare when the tests are performed under optimal conditions requires clarification. Frederick et al 2 were unable to confirm by gas chromatography— mass spectrometry (GC-MS) the positive results obtained using the EMIT-d.a.u. (drugs of abuse in urine) on three of 42 urine specimens. Another study, using high-performance liquid chromatography and high-efficiency thin-layer chromatography, confirmed only 72% of the EMIT results that were positive at a lower cutoff of 20 ng/mL of 11-nor-Δ 9 -tetrahydrocannabinol-9-carboxylic acid (9-carboxy-THC). 3 The confirmation rate rose to 91% when 75 ng/mL was used as the lower detection limit. In a multicenter study, it was found that 2% of the results obtained from the EMIT single-test and EMIT-d.a.u.