i really don't trust those "test kits"(labs are far more inclusive when they test)...im sure they could be used for monitering levels...until you dropped off the test kits scale...but labs test ppm(parts per million) with mass spectrometers...(not available in kit form(sorry)
Labs use assays for the initial screening which function on the same principle as the home test kits (being assays themselves). Therefore, neither are more accurate or reliable than the other.
The Gas Chromotography/Mass Spectrometry (GC/MS) is used for confirmation and only used if the sample tests "non-negative" on the initial assay screen. It would be way too costly for both the lab and the client to use the GC/MS as the primary analysis.

Assays are a pass/fail endeavor and lack the ability to yield a quantitative value (unlike the GC/MS). Therefore, they are far from being the methodology of choice for monitoring THC metabolite levels.

Regardless if its an assay or the GC/MS, THC metabolite concentrations are measured in nanograms per milliliter (ng/ml), rather than parts per million.