Studies of the pervasive problem of medical errors have focused on the estimated 3 percent to 4 percent of hospitalized patients who received the wrong drug, dose or treatment, mistakes that experts say are implicated in the death of one in 10 of these victims.

Now a new study examining the largely overlooked problem of errors that don't appear until patients get home has concluded that the transition from hospital to home may be a source of errors that are more numerous, though less deadly, than mistakes made in hospitals.

Researchers from Harvard Medical School and the University of Ottawa report in the Feb. 4 issue of the Annals of Internal Medicine that nearly 20 percent of 400 patients discharged from a large, unnamed teaching hospital were victims of an "adverse event" that occurred after discharge and resulted from the care they received rather than an underlying disease or condition.

Although none of the 76 patients who suffered from a treatment-related problem when they got home died, 30 percent were temporarily disabled either by severe dehydration, serious diarrhea caused by an antibiotic, or a fall that resulted in rib fractures. Two patients suffered permanent disability, one from a life-threatening infection that followed a procedure and was not recognized while the patient was in the hospital.

The team concluded that two-thirds of these incidents, many of which were related to antibiotics and other drugs, could have been prevented or minimized by better communication among doctors or between doctors and patients.

"There's just a big voltage drop when patients go home from the hospital," observed researcher David W. Bates, an associate professor of medicine at Harvard and an expert in the field of medication errors.

Aftercare, Bates noted, tends to be spotty, poorly coordinated or nonexistent. "I think this has been an issue for a long time," said Bates, who added that he was surprised by the frequency of errors that surfaced once patients left the hospital. Post-discharge errors, Bates said, have received scant attention in part because they are much more difficult to study than the care of hospitalized patients.

To assess the frequency of such problems, researchers contacted nearly 600 patients who had been discharged three weeks earlier from a general medical service.

Patients who could not be reached by telephone after 20 attempts were not included. Of the 400 who did respond, nearly two-thirds were women whose average age was 57; most had been hospitalized for lung or heart ailments.

Two board-certified internists independently reviewed the patients' charts and the results of telephone interviews with patients to determine whether they had suffered a treatment-related injury.

The reviewers also determined whether the post-release event could have been prevented altogether (by, for example, monitoring a patient on a blood-thinning drug), whether it could have been ameliorated (by treating insomnia that resulted from a steroid) or whether it was neither (as in the case of a patient who suffered a reaction at home to a drug not taken previously).

Half of the 76 patients did not seek additional medical attention. Among the half who did, 11 percent went to an emergency room and 24 percent were readmitted to the hospital.

Researchers concluded that all of the outright errors or remediable events were attributable to poor communication, a problem that experts say is epidemic in medicine. This problem appears to be getting worse as care becomes technically more complicated and the health care system becomes increasingly fragmented.

"I think it's cultural," said Bates. "Doctors think, 'I'll do a really good job with this piece of it and then the patient's not my responsibility' " once they are no longer under his or her care. Researchers noted that the hospital where the study was performed takes more pains than most: Doctors there send an e-mail detailing the medication prescribed to the patient's primary physician when a patient is discharged.

Experts say that patients are being discharged "quicker and sicker" and that it is rare for a doctor or nurse to call a patient to see how he or she is faring after discharge. Budget cuts also have made it increasingly difficult to arrange for a visiting nurse to see a patient shortly after discharge, a time when many problems surface.

There's another element, noted Debra Roter, a professor at Johns Hopkins School of Public Health and an expert in medical communication.

"Patients leave the hospital not knowing very much about what they need to do," Roter said, noting that written discharge instructions tend to be ambiguous or are written at a level too advanced for many patients.

And previous studies of oral communication have found that "physicians think they're doing a much more complete job than they are, and patients tend to hear less than is being given" because they are sick and stressed, she added.

"Once somebody gets home they may have a sense that something isn't right, but they don't know whom to call," she observed. Many patients, particularly the elderly, are reluctant to "bother" the doctor with a question, while others say they fear they will be seen as hypochondriacs.

Bates said that patients and their families should err on the side of calling and that hospitals need to redesign systems to improve communication during the transition from hospital to home.

While the rate of problems may seem high, Bates and his co-authors wrote, the true figure is probably greater than the 20 percent they detected. "The sicker patients who were disgruntled with hospital care, were too ill to speak on the phone for 20 minutes [or] were readmitted to the hospital" were omitted from the study.

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A new study suggests that 20 percent of patients are victims of medial errors that occur after discharge from the hospital.