At this time I would like to direct you to: Drug Testing - A Comprehensive Guide
Now assuming that you have already read threw that, a big factor in the meth
ingested is the cut. How pure was it? We may never know, especially considering it a first time user event.
Most local meth is very impure, lab grade meth (be it underground, or Mexico) can have some very pure Meth.
Home tests are knowingly unreliable. They can give false positives, and bring on un-needed punishment. This is one way to explain a positive drug
test then requesting another. Lab analysis are pretty spot on, so if it goes that route they will be able to see ALL metabolites in ones system.