Doctors call them frequent fliers. They are the patients who leave the hospital, only to boomerang back days or weeks later. They have become a front-burner challenge not only for hospitals and doctors but also for those trying to rein in rising costs.

Typically elderly and suffering from the chronic diseases that account for 75 percent of health-care spending, their experiences of being readmitted time and again reflect many of the deficiencies in a fragmented, poorly coordinated health system geared toward acute care.

There are many reasons for readmissions, including high rates of medical errors and hospital-acquired infections; lack of communication between doctors who care for patients in the hospital and their regular physicians; trouble getting a prompt doctor’s appointment after discharge; missed referrals for home health care; and poor coordination and medication management during transitions from hospital to home or nursing home.

Experts don’t agree on how many readmissions are avoidable. Dozens of promising initiatives designed to cut down on them are underway. But many experts say sweeping changes are needed in how health care is delivered and how hospitals and doctors are paid – sensitive issues that confront Congress and the medical industry in the debate on overhauling the health system.

One idea is to bundle the payments to hospitals, doctors and perhaps nursing homes or rehabilitation centers, to cover both the hospitalization and those first critical weeks after discharge. Another proposal is to have Medicare penalize hospitals with high readmission rates for eight common chronic diseases.

Both doctors groups and the American Hospital Association have agreed that it’s time to address readmissions. The association, however, prefers to start with pilot programs to test new payment systems rather than implementing an across-the-board new approach. The AHA also says hospitals should not be held responsible for problems that patients encounter when they’re outside the hospitals’ control.

Readmission costs are staggering. One of five Medicare hospital patients returns to the hospital within 30 days – at a cost to Medicare of $12 billion to $15 billion a year – and by 90 days the rate rises to one of three.

Some doctors are skeptical of this new stress on avoidable hospitalizations. At an American Medical Association meeting in Washington, some questioned whether they could do much to reduce hospitalizations. Cases can be very complicated, they said; patients don’t always follow directions.