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 al2 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-Δ9-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.
Fody EP. Laboratory Detection of Marijuana Use. JAMA. 1985;254(24):3425. doi:10.1001/jama.1985.03360240037026