Misdemeanor DWI cases are so common in Houston's criminal courthouse that they seem to merit attention only when they involve a pro athlete, a law enforcement official or the daughter of a mayor.

One scheduled to continue on Friday may dwarf any others, with significant implications for an embattled Harris County District Attorney's Office.

A day after the revelation that her office is the target of a grand jury investigation, District Attorney Pat Lykos was called to testify Thursday in an ongoing DWI trial because she made public comments regarding the accuracy of results from the Houston Police Department's troubled mobile breath-testing vehicles.

"The reliability of the BAT vans has come into serious question by the chief prosecutor of Harris County herself," declared Jackie Carpenter, a defense attorney in the DWI trial. "If they're so unreliable, then why are you prosecuting someone with evidence you deem unreliable?"

Reversing an earlier decision to compel Lykos to testify, County Court at Law judge John Clinton ruled Thursday that the top prosecutor's opinions on the breath-alcohol testing vehicles - known as BAT vans - are relevant in the DWI trial.

An assistant district attorney, one of more than 240 under Lykos' authority, is handling the misdemeanor case. If Lykos appears, she could be cross-examined by one of her own prosecutors.

The testimony could affect dozens of past and future DWI cases that relied on evidence handled by the testing equipment in the vans.

Even more serious is the possibility that Lykos and other prosecutors had doubts about the tests' accuracy while prosecuting past DWI cases but did not alert defense attorneys.

Jim Leitner, the first assistant district attorney, declined to comment because the trial is ongoing.

Video subpoenaed

Lykos, who is out of town, is unlikely to appear in the trial. Her words, however, are expected to be heard by jurors Friday.

Clinton said television station KTRK (Channel 13) would be subpoenaed to provide an uncut version of a recent Lykos interview.

If the tape is not provided, an audio tape of the interview, recorded by the District Attorney's Office, will be played, Clinton said in open court.

Lykos was interviewed by the television station and the Houston Chronicle this week after defense lawyers first raised the possibility that her office was the target of the investigation.

During an interview with the Chronicle, Lykos said that she has concerns with HPD's crime lab.

"You see what a quandary we are in, yet we have had no communication from the director of the (HPD) crime lab," Lykos said on Monday.

She noted that the Texas Department of Public Safety, which oversees the state's breath-alcohol testing program, has temporarily taken over responsibility for HPD's mobile DWI testing.

"I am perfectly happy to take breath tests because they are being supervised on a temporary basis by DPS," the district attorney said. "And I'd like to make it permanent."

Lykos said her office continues to investigate allegations concerning the vehicles from a whistle-blower named Amanda Culbertson.

Culbertson, a former HPD forensic lab supervisor, testified in July that she had doubts about lab results from the vans because of overheating. Those results have continued to be used to convict DWI defendants.

Court records show that a Harris County grand jury is investigating Lykos' office in regard to allegations about the BAT vans.

Jury's term extended

The grand jurors last week threw three senior prosecutors out of their courtroom as Brent Mayr, a defense attorney and critic of the BAT vans, testified.

Mayr is a former prosecutor who specialized in vehicular crimes.

Because grand jury proceedings are secret, Mayr has not commented on his testimony.

The grand jury was empaneled three months ago by state District Judge Susan Brown. Eight grand juries are created every year to hand down indictments in Harris County's thousands of criminal cases.

Just weeks before its term was scheduled to end, the grand jury began investigating the District Attorney's Office and calling witnesses familiar with the controversial BAT vans.

Brown on Wednesday appointed two special prosecutors to help the grand jurors, and extended their term to Feb. 1.

Anita Hassan contributed to this report.

brian.rogers@chron.com