Could mirtazapine cause a false positive for amphetamine?

Not to my knowledge..by any chance are you taking Prozac also? That definitely will

Hello! Mirtazapine (Remeron) was not listed anywhere of causing false positives for amphetamines or any other substances in urine drug tests, although these tests are notoriously inaccurate. If in doubt specialized testing would be performed.
Best regards, Wild Cat