Proposed changes to the Health and Human Services Guidelines for forensic urine drug testing will lower the required cannabinoid immunoassay cutoff concentration from 100 to 50 micrograms/L. We investigated the effect of this change on the sensitivity, specificity, and efficiency of eight cannabinoid immunoassays: Syva Emit d.a.u. 100; Syva Emit II 100; Syva Emit d.a.u. 50; Syva Emit II 50; Roche Abuscreen Online; Roche Abuscreen radioimmunoassay; Diagnostic Reagents; and Abbott ADx. All specimens also were assayed by gas chromatography/mass spectrometry. Lowering the cutoff concentration from 100 to 50 micrograms/L increased efficiencies and sensitivities for all immunoassays, with minor decreases in specificity (1.0-2.6%). There was a 23.2-53.6% increase in the number of true-positive specimens identified. Thus, lowering the cannabinoid immunoassay cutoff concentration from 100 to 50 micrograms/L resulted in detection of a substantial number of additional true-positive specimens, with an accompanying small increase in unconfirmed positive results.