Hmm, Explanation? Anyone?
Here's the facts: As stated, the test was eCup/eScreen. The subject took 70mg of Lisdexamfetamine. It took approximately one week for the employer to contact the subject with the test results. Lisdexamfetamine is metabolized to d-amphetamine:
Clin Drug Investig. 2008;28(12):745-55. doi: 10.2165/0044011-200828120-00002.
Metabolism, distribution and elimination of lisdexamfetamine dimesylate: open-label, single-centre, phase I study in healthy adult volunteers.
Krishnan SM, Pennick M, Stark JG.
"Relative to dose administered, 41.5% was recovered in urine as d-amphetamine, 24.8% as hippuric acid and 2.2% as intact lisdexamfetamine dimesylate. Less than 0.3% of the administered dose was recovered in the faeces."
"CONCLUSIONS: Following a single 70 mg oral dose, lisdexamfetamine dimesylate was quickly absorbed, extensively metabolized to d-amphetamine and its derivatives, and rapidly eliminated."
Let's assume the test performed as it should and did not return a false negative. It was approximately a week before the results were returned, is this enough time for a confirmation test to be conducted? Possibly, but I think it's unlikely that the testing facility had a GC/MS or HPLC and the sample would have had to be shipped out.
Assuming approximately 42% of the lisdexamfetamine was converted into the detectable metabolite d-amphetamine, that means the dose was approximately 30mg of d-amphetamine.
So what happened? Given that we are talking about a pro-drug, the detectable metabolite could have been low enough in the urine to not trigger a positive.