"Most of the synthetic cannabinoids added as not-listed ingredients to Spice products are very difficult to detect by commonly used drug screening procedures. Apart from the analogs of THC such as HU-210, the structure of these new synthetic cannabinoids differs from that of THC, so that they probably will not trigger a positive test for cannabinoids in immunoassays of body fluids."
Source
Fattore, Liana and Fratta, Walter, "Beyond THC: the new generation of cannabinoid designer drugs," Frontiers in Behaviorial Neuroscience (Lausanne, Switzerland: September 2011) Volume 5, Article 60, p. 4.
