only think you can try and do is tell them is was a false positive THC - Substances or Conditions which can cause false positives
Dronabinol (Marinol)
Ibuprofen; (Advil, Nuprin, Motrin, Excedrin IB etc)
Ketoprofen (Orudis KT)
Kidney infection (Kidney disease, diabetes) Liver Disease
Naproxen (Aleve)
Promethazine (Phenergan, Promethegan)
Riboflavin (B2, Hempseed Oil)