Depends on the level of the test. Some companies use the lower and cheaper tests. But, the more advanced ones also identify higher levels of water in the system...which will trigger a failed/questionable result.
We have always used the higher tests when we hire executive candidates.
More specifically it depends on the company policy in regard to what is termed a "negative dilute". 99% of testers use the DOT standards which state that two negative dilutes equal a negative result. I would imagine that high level executive jobs and people on probation would fall into that other 1%. Sure your higher level tests might have a threshold of only 15 nanograms instead of the usual 50ng, but most diluted samples have only trace amounts well below that threshold.
As someone who was in a committed relationship from the ages of 17-22 my advice would be to seriously evaluate if you want to be married at your young age or not. If not you're not doing yourself or your girlfriend any favors.
I still look back on that time of my life filled with regret for missed opportunities. She was a great girl but that time of your life is about becoming who you want to be and that's damn near impossible in a relationship.
If you two are meant to be together it will happen regardless of what you do.