Hundreds of DUI cases from the first half of 2016 may be adversely affected because Breathalyzer machines used by Philadelphia police were improperly calibrated.

The department was notified Wednesday by a private attorney that the police were using calibrations that had legally expired, police spokesman Lt. John Stanford said in a statement Thursday evening.

"As a result of this discovery, all instruments were immediately removed from service," properly calibrated, and returned to service the same day, Stanford said.

The department has asked the District Attorney's Office to conduct a review of cases involving Breathalyzer tests from Jan. 17 to June 29 "to determine the future of those cases," Stanford said.

Joseph Kelly, the Port Richmond attorney who notified the department, said he estimated that the improper calibrations could affect between 500 and 1,000 cases.

The department has to calibrate the machines on an annual basis using bottles of solution provided by outside companies.

The solutions establish that blood alcohol readings are accurate. The department must used updated solutions for calibrations.

In January, "due to human error," the department used an expired solution, Stanford said.

The machines were still reading accurately, but the use of the expired solution will allow defense attorneys to argue that the tests are inadmissable in court, said Kelly, who specializes in DUI cases.

Stanford said the department will put additional measures in place "to increase efficiency and reduce oversight."

He added, "While no organization is proud of errors, we do appreciate the attorney bringing this to our attention. Optimum service to our citizens is not an option; therefore, we have quickly rectified this matter and we remain committed to exceeding minimum standards set by policing guidelines."

A spokesman for District Attorney Seth Williams could not be reached for comment Thursday night.

Pending cases can still be tried without the tests. A prosecutor can have the arresting officer testify about a driver's "general impairment," including slurred speech, unsteady gait, and strong odor of alcohol.

The police and prosecutors faced a similar sitution in 2011, when Kelly also alerted the department to improper calibrations. In those cases, the machines were actually giving bad readings.

As a result, the District Attorney's Office mailed letters to affected defendants and a courtroom was set aside one day a week for hearings on petitions for new trials.

Editor's Note: This story was revised to correct the spelling of attorney Joseph Kelly's last name.