Ahh I think I'm starting to get it, So 50ng is pretty much similar to 15ng for the GC-MS, So passing at 50ng would make you a pass at 15ng? And they'd only use it if you have a positive on the first strip test.

Thanks for this, Only think worrying me is if they test straight for 20ng rather 50ng as I'm genuinely worried sick to if I'll pass at a 20ng.