The US Army is shipping portable lie detectors to Afghanistan to help test the loyalty of local staff.

The Preliminary Credibility Assessment Screening System (PCASS) is a handheld lie detector that measures the electrical conductivity of skin and heart rate. The computer then gives an estimate of whether the person is lying, telling the truth or if it is impossible to tell.

But the accuracy of the detectors has been questioned by many academics and even the military admits they will provide false positive results in around one in five cases. In a presentation the accuracy results are given as 80-92 per cent.

But a memo uncovered by the news station MSNBC finds that this figure was reached by ignoring results where no conclusion of the subject’s honesty was reached. Once these were included, accuracy fell to between 63 to 79 per cent.

"I don't understand how anybody could think that this is ready for deployment," statistics professor Stephen E Fienberg told MSNBC.

"Sending these instruments into the field in Iraq and Afghanistan without serious scientific assessment, and for use by untrained personnel, is a mockery of what we advocated in our report."

Professor Fienberg headed a 2003 study by the National Academy of Sciences which said that polygraphs were too inaccurate to be a useful tool for identifying terrorists. This was true of professional polygraph systems that take many biological readings, rather than just two like PCASS.

The US Army has bought 94 of the $7,500 machines and is shipping them overseas, where users will have a 13-week training course before beginning testing. Professional polygraph operators traditionally do a four-year degree and have some experience in law enforcement.

The PCASS system is being used only on native personnel and will not be used on American military staff as part of its deployment. The results of tests are not being accepted as the only evidence necessary to make a decision and their use has been defended by the Pentagon.

"Let's take a worst-case scenario here, and let's say PCASS really is 60 per cent accurate," said Donald Krapohl, who heads the project for the Defense Academy for Credibility Assessment.

"So let's get rid of the PCASS because it makes errors, and go back to the approach we're currently using, which has less accuracy? As you can see, that's really quite untenable if we're interested in saving American lives and serving the interests of our commanders overseas.”