BOSTON (AP) — State officials say there’s no evidence breath test machines used to screen for drunken driving are working improperly.

But a review of 39,000 breath tests released Tuesday by the Massachusetts Department of Public Safety did reveal a small number — fewer than 150 — to be invalid because the instruments weren’t calibrated according to state regulations.

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The department blamed the invalid results on operator error, not a malfunction with the machines.

Public Safety Secretary Daniel Bennett said his office is working with local district attorneys to identify cases that might have been affected.

“Motor vehicle operators and the law enforcement community must have faith that breath tests are accurate and reliable, and we have taken the steps necessary to make sure that is the case,” Bennett said.

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Several prosecutors had suspended the use of breath test evidence pending the state review, and the Massachusetts Bar Association had previously called for an independent review into the reliability of the tests.

Massachusetts regulations require instrument operators to validate the tests with a calibration range between 0.074 and 0.086. That’s more stringent than the setting assigned by the manufacturer, Bennett said.

Bennett said the manufacturer has agreed to update the instruments with a software patch so that the margin of error coded into the instrument is identical to that required under Massachusetts regulations. He said that would reduce the potential for operator error.

In the meantime, all breath test operators will be instructed again of proper breath test calibration standards until the software changes take effect.

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