I ran these tests back when I was a lab technician. A qualitative test just looks for the presence of THC. If that comes back pos, then it is sent out for quantitative test. Qualitative asks "is it there" and quantitative asks "how much is there."

This substance is detectable for a month. There will be a faint pos 30 days after even smoking 1 joint. Most lab techs will throw out a faint pos or just record a negative, but some of them are real asswipe Christian anti-drug sorts, basically drunk on their own power. They'll take a faint pos result and send it out for quantitave analysis in a heartbeat.

If you smoked longer than a month ago, you're fine.