The GOOD News - most of the drug store drug tests are ZERO-TOLERANCE, meaning any nanogram registers.
Actually, and as Ive pointed out previously, home drug tests are a "pass/fail" endeavor and have no ability to yield a quantitative value. If both the "Control" (C) region and the "Test" or "THC" region (T) show a line, its a negative - sample is below the cutoff threshold. If only the "Control" region shows a line, its a non-negative.

Most home tests carry a 50 ng/ml cutoff threshold, which is the same threshold as the DOT initial screen.