Sensitivity of pre-employment drug tests?

Hi guys,

I'm new to the site and haven't yet gotten a handle on how to fine-tune my searches.. so I extend my apologies in advance if my exact same question has been posted before.

Here's my predicament: I have been doing a lot of research on pre-employment testing in the last few days as I recently was offered a lab job in a research institute. Completion of the hiring process is contingent upon passing a background check and drug test.

The position in question is funded by federal grants at a research institute, so I am wondering what kind of drug test(s) I should expect. I don't have anything to worry about aside from marijuana. I know that most pre-employment testing entails immunoassay type tests with fairly high cut-offs (50ng/ml). I understand also that gas chromatography methods are as sensitive as 15 ng/ml.

Since the job is funded my federal money, I am kind of assuming that hiring processes are fairly stringent and the drug test will be stringent as well.

I took two at-home tests (from Walgreens) and passed both, but I also have a friend who abstained for nearly 2 months, passed an at-home test, and then failed a drug test with 85 ng/ml.

I am an infrequent user (maybe once or twice a week) and my last time was just about a week ago. I am female, about 30% body fat, and I'd say I have average metabolism.

I am wondering how it all works. Do urine tests only go in for confirmatory testing (the more highly sensitive gas chromatography testing) if the initial 50 ng/ml test comes up as positive? Or are there instances wherein samples are put through the CG test as a standard.

Again, sorry if it's been asked already. I appreciate any insight anyone can offer. Thanks!