If your sample tests positive for THC on the initial assay screen, the sample would then be sent to the GC/MS for the confirmation test which will give a quantitative value. The lab would still run a validity test to check the creatinine and specific gravity, and if the creatinine and S/G is below acceptable levels, the sample is ruled "diluted" and will be reported as such to your P/O.
You'd better hope your P/O doesnt take the result to mean that youve intentionally diluted in an effort to bring your ng/ml level below the cutoff thresholds -- but dont be surprised if the P/O does.