As the expansion of health care coverage becomes mandatory nationwide, people are looking to Massachusetts, which had already expanded health insurance coverage to nearly everyone in the state, for implications.

The answer may be a source of dread for states like California, where high taxes and an onerous business climate have caused most of the middle class to disappear: Emergency Room visits went up, even as uninsured visits went down, and that means higher cost.

The new report in the Annals of Emergency Medicine can't determine why they went up; perhaps people did not go before because they did not have coverage or perhaps people went more afterward because fewer private doctors accept their plans and the waiting period was too long.

"This obviously has implications about what we can expect to see nationally as the roll-out of the Affordable Care Act continues," said Peter Smulowitz, MD, FACEP, of Beth Israel Deaconess Medical Center in Boston. "Other states should be prepared for equal or greater influxes of patients into the ER after reform is fully implemented."

Smulowitz and colleagues found emergency department visits increased by as much as 1.2 percent between October 1, 2006 and September 30, 2007 after Massachusetts implemented its first-in-the-nation law to increase health care insurance coverage. That rose to a 2.2 percent increase in ER visits for the period that ended Sept. 30, 2009.

The data included all emergency department visits in Massachusetts submitted annually from 69 acute care hospitals, accounting for approximately 2 million outpatient emergency visits, 850,000 inpatient admissions and 150,000 observation stays.

Overall emergency visits by the uninsured under the age of 65 decreased from 9.5 percent of overall visits before health reform to 5.7 percent of the visits after health reform implementation. Emergency department visits from those 65 and older remained steady at about 1 percent.

"Our study should further weaken the long-held notion that high use of the emergency department is driven mainly by the uninsured," said Dr. Smulowitz. "Barriers to primary care are serious and persistent across the country. It appears that when people have health insurance, they will seek medical care wherever they can get it, which is sometimes only the ER."