As is stated here multiple times, if you pass your at home panel you will pass the lab. All the lab does is screen a 5 (or however many they wish) panel on your urine. If any of those is positive, then it will be sent over to the GC/MS to confirm the precise level of toxin(s). It is not in their best interest to run a confirmation on negative screens.

After being testing negative for 3 weeks, I smoked ~3g over the weekend of the 27th. I'm 6'1" and 290lbs and just tested negative on a 50ng panel today the 3rd. Hope this eases your mind.