A calibration error has affected the results of 4,001 blood alcohol tests statewide, according to Michigan State Police.
The impacted tests were done between December 14, 2015 and April 13, 2016.
The error was discovered last month with a machine called a chromatograph, which is calibrated automatically.
MSP said 402 cases involved Wayne County police departments. Wayne County Prosecutor Kym Worthy revealed 89 of those cases are being or were prosecuted by her office.
MSP said the calibration mistake was minor. The tests have been properly recalibrated and the new results will not affect how cases are handled going forward or result in any dismissals.
Former state attorney general Mike Cox said that may be wishful thinking.
"If the machine's off, if it's giving a wrong reading, literally, that can blow up the results and the trooper's testimony," Cox said.
The Wayne County prosecutor's office expressed concern about the impact the calibration error will have on cases that have been affected.
"It will depend on how the changed results affect the cases," said prosecutor Maria Miler.
Miller said calibrating chromatographs automatically, as opposed to having someone do it manually, is also a concern.
"Prosecutors around the state are having a conference this week. I am sure this will be a topic of discussion," Miller said.