Anything less than 100% should not be admissible in court. When the consequences are so high, the restriction of an individual’s civil liberties, there can be no room for error. Even if lie detectors are 95% accurate, as some practitioners claim, in the best-case scenario, 5% of suspects would have their names sullied over a crime they did not commit. 
In polygraph testing, false positives are also more common than false negatives. In one study, an accused person who was innocent had a 50% chance of displaying a guilty reading, reducing the fate of an innocent suspect to little more than a coin toss. 
In addition, almost no studies on the effectiveness of polygraph tests have tested their accuracy on children, those with mental disabilities, people speaking their second language, or even those of below-average intelligence (test subjects are usually university students of above-average intelligence). There simply isn’t enough evidence to say they can detect lies to a high level of accuracy.