Dont call the MRO. Instead, give your employer a call and ask for your "start date". The employers response there should tell you whether youre good to go, or if the employer had moved on to the next candidate for hire.
Never inquire about the drug test results. That will only serve to raise suspicious eyebrows. A person who doesnt use drugs would never do this because it is a non-issue.
If you already have a start date secured, just show up for work and carry on your duties with the "I dont use drugs, therefore my drug test is a non-issue" mindset.