It depends.  When an employee files a claim under the American with Disabilities Act ("ADA"), two of the key issues for determination are whether the employee is "qualified" for the position and whether the employee can perform the "essential functions" of the job with or without a reasonable accommodation.   In determining these issues, courts focus on employers' job descriptions and performance reviews.

Accordingly, job descriptions that identify the "essential functions" of the position may be helpful to prove that a person is not "qualified" under the ADA.  For example, in Galloway v. Aletheia House, 509 Fed. Appx. 912, 2013 BL 41809 (11th Cir. 2013), the court reviewed the employer's job description for the case manager position at issue and found that because driving was essential for the position, the applicant, who was blind, was not qualified.  In Wehrley v. American Family Mutual Insurance Company, 2013 WL 1092856 (10th Cir. 2013), the court found that the written job description, stating that the job required the ability to work in high, precarious places between 1 and 33% of the time, and to stoop, kneel, crouch or crawl between 1 and 33% of the time, and further stating that "The information in this job description is intended to describe the essential job functions" was evidence that climbing ladders and working at exposed heights were essential functions of the job.  The court in Richardson v. Friendly Ice Cream Corp., 594 F.3d 69 (1st Cir. 2010) also relied on the employer's job description to conclude that the plaintiff's restaurant job as an assistant manager had a "substantial physical component" that was essential to the position.

Contrarily, job descriptions that are vague or general are not likely to be interpreted to create an "essential function".  This was the case in Szarawara v. County of Montgomery, No. 12-5714 (E.D. Penn. June 27, 2013) where the court rejected the employer's argument that the language in the job description, requiring employees to be able to work "various shifts", "rotating schedules", weekends and holidays, made working the night shift an essential function of the job.   

A performance review after the disability occurs may be evidence as to whether or not the person is "qualified" under the ADA.   Poor performance ratings may be helpful in showing the employee was not qualified, as was the case in Jones v. Walgreen Co., 679 F.3d 9, 26 AD Cases 261 (1st Cir. 2012) where the employee received bad reviews.  Contrarily, satisfactory to exceptional ratings may be evidence that the employee is "qualified".  In Colan-Fontanez v. Municipality of San Juan, 660 F.3d 17, 25 AD Cases 423, 2011 BL 262540 (1st Cir. 2011), the employee received good to superior ratings, which the court found to be strong evidence of the employee's qualifications and skills.  In EEOC v. Chevron Phillips Chemical Co., 570 F.3d 606, 21 AD Cases 1729 (5th Cir. 2009), the court found that the employee was "qualified" because the employee received satisfactory job reviews.     

These cases highlight the importance of assuring that job descriptions spell out the "essential functions" with enough detail to support the argument that the functions are truly "essential" to the job, and that employee evaluations should include sections directly related to the performance of those "essential functions" and should accurately state the employees' job performance.