More than two decades after the Americans with Disabilities Act became law, a new survey indicates that more than 20 percent of doctors’ offices remain inaccessible to those with special needs.

The finding comes from a “secret shopper” style poll of 256 randomly selected medical practices in Boston, Dallas, Houston and Portland, Ore. Researchers called to schedule an appointment for a fictitious patient, indicating that the individual used a wheelchair and was unable to transfer independently from the chair to an exam table.

Under the ADA, doctors are required to provide “full and equal access to their health care services and facilities.” Nonetheless, of the medical and surgical subspecialists phoned, 22 percent indicated that they could not accommodate the patient, researchers reported Tuesday in the Annals of Internal Medicine.

The reasons for turning away the fictitious patient varied, but most doctors cited an inability to transfer the patient from a wheelchair to an exam table. Meanwhile, nine practices indicated that their building was inaccessible.

Similarly, researchers found that some types of doctors were more likely to accommodate a patient with special needs than others. Gynecologists proved the least likely of the physicians polled, with 44 percent of those practices declining the patient. By comparison, just 4 percent of psychiatrists were inaccessible.

Researchers said the survey results highlight the need for physicians to be more aware of their obligations under the ADA.