Hi. I Have Gone To The Same Pain Clinic Since

Hi. I have gone to the same pain clinic since 2000 for CRPS in my R UE. I currently take Dilaudid 8mg tablets, 4-6every 24 hours, and use Fentanyl patches, 100 mcg and 50 mcg, every 72 hrs. In the past 2 years, 2 occasions arose when I was unable to provide enough urine for the standard Utox screen, and a mouth swab was performed. Both times a mouth swab was performed, the results for opiates were negative. I use bot narcotics religiously, never missing a dose of Dilaudid (generic) and promptly changing my patches every 3 days. Is there any reason why a mouth swab would NOT show these opiates? Thank you. KJM