This is one reason why prohormones and designer steroids were so popular several years ago. In most cases, you must know the exact chemical you are testing for; many PHs and DSs were unknown to the anti-doping community, thus they could not be detected.It tests for testosterone... and they are not testosterone. Tren even shows as e2 on some tests.
Meanwhile, the test your doctor might order is not a AAS screening. He is only looking for testosterone, because that is what your testicles make or what he prescribed to you. He has no reason to look for any other AAS, so the test is testosterone specific.