As a former mass spectrometrist with extensive experience working in an analytical lab, I will chime in with my two cents...
The important thing to realize is that gas chromatography (GC) and liquid chromatography (LC) are similar chromatographic techniques used to separate analytes. In this particular instance, the analytes are active pharmaceutical molecules and their respective metabolites. Directly following the separation, the volatalized eluent (GC) or liquid eluent (LC) passes directly into the mass spectrometer. This is where the actual identification of the molecules occur. The MS determines the mass to charge ratio of all analytes conatined within a patients specimen. The patients results are then comapered to a standard injection which is used to calibrate the instrument every morning. This reference standard depends upon the test ordered and will contain all the compounds from a certain class of drugs - i.e. benzodiazepienes, opioids, etc. This is how a positive result is generated. From an analytical chemistry standpoint, this is about as specific and sensitive as it gets....
To make a long story short, both of these techniques are comparable and appropriate for the intended purpose. The take-home message is that post-separation, the actual identification of the drug or metabolite is being performed in the same manner - via MS. In fact, with a little sample preparation, you could eliminate the GC or LC separation, and infuse directly into the MS.
While the turn-around times for the tests are different, both methods are likely performed in under an hour...
Please dont hesitate to let me know if you have any more questions...
