Test Question Related to Metabolism

I would call myself a light user (2-3 per week with occasionally more) and only smoke loud. Became concerned about a drug test that I had coming up for employment so invested in some urine drug tests that tested for a threshold of 50ng/ml (standard). I bought a few to monitor myself until I was clean, and insured to follow all the proper procedures (waiting for sample to reach room temp., etc). The surprising thing I found was I was already clean according to the test. Tried again the morning after smoking at night a couple more times and even tried another brand that tested for a much lower threshold of 20ng/ml. Since then, I have been testing each week to be safe. I found this strange since even light users tend to keep THC in there system for at least a few days. I am in my early 20s and have always lived a healthy lifestyle and am fairly active (workout about 3-4 times a week, not overweight, etc.), is this why I am always clean? I purchased certified and good quality tests, so I know that's not it. Something to do with metabolism maybe? Should I be concerned about the THC randomly starting to show up when I don't expect it?

If youre repeatedly testing negative using the first urination of the day, then youre good to
go. At that point, its pointless to worry as to why youre testing negative.
A negative is a negative. Plain, sweet, & simple. :cool:

Okay thanks :) , I was mostly just curious about this. Anyone else experience the same thing?