LD tests are an unreliable technique if the subject has a clue of what LDs do measure, namely stress, and how to control those forms of stress measured by the device. After a little behavioral training, most anyone should be able to significantly reduce their stress response when lying, making the test results much harder to interpret effectively.

LDs don't catch spies. They catch novices. Aldrich Ames beat the test repeatedly. But he would, since his job was counterespionage.