Testing Negative On Adderall

Christy Says:

My son (17) was recently put on 10 mg Adderall 1-2 pills daily. 30 days later they drug tested him and it was negative. He swears he has been taking it and had not taken one 2 days before the drug test because he was out. Can you test negative for this drug if you have only been on it 30 days?