The Illinois Supreme Court issued a ruling in September that questioned the reliability of one of the key field sobriety tests used to convict motorists of driving under the influence (DUI) of alcohol. The high court found that the results of a test known as horizontal gaze nystagmus should be open to challenge in court as an unsettled matter. The state court of appeals had earlier ruled that the principles underlying the test could not be questioned.

Nystagmus is the abnormal and involuntary jerking of the eyeballs as they track an object to the extreme range of their motion. The jerking movement tends to occur earlier for those who are either intoxicated or suffer from a number of different ailments.

Joanne McKown failed this eye test when it was administered to her in a hospital bed on June 8, 2002. She was then charged with multiple counts of DUI and reckless driving. At around 11:30am, McKown 's car crossed the center line and struck three motorcycles causing severe injuries. McKown admitted she drank two beers before leaving her home, had one beer while driving and was just about to start a fourth when the crash happened.

A blood test taken at 6pm did not reveal the presence of alcohol and McKown's broken toe precluded the usual one-leg stand and walk-and-turn tests. McKown's conviction for DUI rested substantially on the nystagmus test which she took while under the influence of medication administered at the hospital.

The Illinois Supreme Court in overturning the appeals court's blanket approval of nystagmus noted that courts around the country do not agree whether the test is accepted as reliable.

"The NHTSA has admitted that the 45-degree angle test is wrong 22 percent of the time," the high court wrote in its treatment of prior case law.

The supreme court also recognized that research has found a number of substances, including caffeine, nicotine or aspirin, can cause reactions similar to alcohol that a highly trained medical expert would have difficulty distinguishing from the effects of intoxication. Embarrassingly, this argument came from the prosecutions own material.

"At least one of the sources the state provides in the appendix to its brief actually denounces the use of HGN testing for roadside sobriety tests," the court wrote.

"The technical writings above reveal a dichotomy in the scientific community, rather than the unequivocal or undisputed viewpoint necessary for us to take judicial notice," the court concluded. "As such, we cannot take judicial notice of the general acceptance of the HGN test as a reliable indicator of alcohol impairment based on these technical writings.... we find that this issue cannot be resolved in Illinois on judicial notice alone. "

California defense attorney Lawrence Taylor, who runs the DUIBlog website suggests this may not be the only field sobriety test that produces questionable results.

"These tests are highly unreliable and largely used to justify the officer's suspicions," Taylor wrote. "Although they are in reality no more valid at determining intoxication than the other tests, their adoption by the federal government has given them an aura of credibility."

The supreme court did not find McKown innocent. Rather, it ordered the trial court to conduct an extensive evidentiary hearing to establish the reliability of the nystagmus test. The full court opinion is available in a 180k PDF file at the source link below.