Managed care companies are requiring increasing levels of cost sharing to control costs -- including reducing the use of hospital emergency services for nonemergency care. Cost sharing shifts a portion of health care expenses to patients through copayments, deductibles and coinsurance.

• Studies have shown that patients in a fee-for-service medical care system who had a coinsurance requirement for emergency services used them 20 percent to 40 percent less often than those with no coinsurance requirement.

• Similarly, Kaiser Permanente members in northern California had nearly 15 percent fewer visits to the emergency department during the first year after a copayment of $25 to $35 for emergency services was introduced.

• There was no reduction in visits for the most serious conditions, but visits for the least severe problems, which accounted for almost one-third of all emergency room visits, were reduced by nearly 30 percent.

Insured patients respond to remarkably small degrees of cost sharing, and at low levels, middle-income patients appear able to distinguish necessary from inappropriate medical care and to reduce their use of less appropriate services selectively. For example,

• In the mid-1980s, the introduction of a $5 copayment for an office visit at the Group Health Cooperative of Puget Sound, in western Washington, reduced visits by 11 percent and physical examinations by 14 percent.

• However, the copayment had no effect on the immunization of children, screening for cancer in women or the use of cardiovascular medications.

• In response to a required $1.50 copayment for prescriptions, patients selectively decreased use of discretionary drugs more than use of drugs deemed essential.

However, larger cost sharing burdens lead to further decreases in the use of services but begin to affect the patient's ability to make selective reductions. For example, a $20 copayment for visits for mental health care at the Puget Sound cooperative led to reductions in visits regardless of the severity of disease.