Study co-author Dr. Mark Williams of Northwestern's Feinberg School and Northwestern Memorial Hospital said the study also found that more than half the patients rehospitalized within 30 days appear not to have seen a physician as an outpatient since they were released from the hospital.
"We were surprised that more than half of these patients weren't being seen by their primary care doctors before they went back into the hospital," Williams said in a statement. "This represents a major disconnect between care in the hospital and outside it. We've got to do better."
The readmission rates rise with time: 19.6 percent of patients were readmitted within 30 days of discharge, 34 percent within 90 days and 56.1 percent within a year.
The study, published in the New England Journal of Medicine, also said that the unplanned readmissions cost more than $17 billion in 2004, a sizable portion of the $102.6 billion Medicare paid to hospitals.
When patients are readmitted after surgery, 70 percent of them suffer from a medical problem such as a urinary tract infection or pneumonia.
"There is also the emotional toll on patients," Williams said. "They suffer when they must be rehospitalized after the initial difficult experience of hospitalization."