Am I screwed because of a hair test?

I have only smoked weed once before in my life and that was at least eight years ago. About 25 days ago I only took a couple of hits from a joint, didn't even feel anything, and I haven't touched it since. I had to take a hair test today, and I'm a bit worried. Is any detection at all going to be a positive result or do they only test for a certain amount of THC, or what?