May 25 (Reuters) - A worker suing her employer under the Americans with Disabilities Act does not have to prove that she was fired solely because of her disability, a federal appeals court ruled on Friday.

For the past 17 years, the Cincinnati-based U.S. Court of Appeals for the 6th Circuit has required district courts to instruct juries that an employee's disability must be sole reason for his or her firing.

But in considering the case of Susan Lewis, a former nurse suing her employer, the court abandoned that standard in a splintered 9-7 ruling, making it somewhat easier for workers to sue for disability discrimination.

"The longer we have stood by this standard, the more out of touch it has become with the standards used by our sister circuits. At this point, no other circuit imports the 'solely' test into the ADA," Judge Jeffrey Sutton wrote for the nine-judge majority.

Lewis sued retirement-home operator Humboldt Acquisition Corp in 2007, claiming she was fired for a medical condition that made it difficult to walk and required use of a wheelchair. Humboldt claimed that Lewis was fired because of a profanity-laced outburst at her supervisors.

Humboldt had asked the court to instruct the jury that Lewis could only prevail on her ADA claim if her disability was the sole reason for her firing. The district court obliged, following the 6th Circuit's longstanding rule, and the jury ruled in Humboldt's favor.

On appeal, Lewis argued that the district court should have instructed the jury that her disability only had to be one "motivating factor" -- the same standard used for Title VII claims alleging discrimination based on race, religion and sex. The ADA was enacted to expand protections beyond Title VII to cover disability, and so the same test should apply to both laws, Lewis argued.

Most other federal circuits apply the more lenient "motivating factor" standard to ADA claims, requiring plaintiff employees to prove that their disability was one of several contributing reasons for the firing.

While the 6th Circuit majority retreated from the sole-cause test, it refused to embrace the "motivating factor" jury instruction. Rather, the court settled on an intermediate test, requiring employees to prove that they would not have been fired if they had not been disabled.

The current ADA bars discrimination against an employee "because of" a disability. For guidance on what that means, the 6th Circuit majority turned to a 2009 Supreme Court case, Gross v. FBL Financial Services, which set the standard for age discrimination claims. Federal age discrimination law bars discrimination "because of" age. In Gross, the Supreme Court said that meant employees suing for age discrimination had to show that they would have kept their jobs "but for" their age. The same test should apply to disability claims under the ADA, the 6th Circuit majority concluded, sending the case back to the district court for a new trial.

Seven judges disagreed with the majority's interpretation in three separate partial dissents. The majority failed to accomplish the court's original goal of lining up with the prevailing legal opinion across the country, Judge Eric Clay wrote.

Eric Schnapper of the University of Washington School of Law, a lawyer for Lewis, said the question of whether the Gross ruling applied to ADA claims was just beginning to surface and was likely to be litigated in lower courts across the country for years to come.

James Simms of Cornelius & Collins, who represented Humboldt, was not immediately available for comment.

Business groups including the U.S. Chamber of Commerce and the Equal Employment Advisory Council weighed in on the appeal as amici in support of Humboldt. Rae Vann, a lawyer for the groups, said the 6th Circuit decision bodes well for employers.

"Post-Gross, courts aren't applying the motivating factor test as a matter of course," she said. "The Gross ruling does stand as a huge obstacle in going down that path now."

The case is Lewis v. Humboldt Acquisition Corp, U.S. Court of Appeals for the 6th Circuit, No. 09-6381.

For Lewis: Eric Schnapper of the University of Washington School of Law.

For Humboldt: James Simms of Cornelius & Collins.

(Reporting By Terry Baynes)

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