My Son 10 Today Had A Urine Drug Screen On

my son 10 today had a urine drug screen on Wednesdat the first one came back positive for amphetamines cocaine and tricyclics. The second only came back positive for tricyclics and amphetamines. He does take Quetiapine, sertraline, oxcarbanzapine and Dextroamphetamine. I understand the positive for amphetamines and now understand the tricyclic is because of the quetiapine. But why the cocaine and only on the first test??