A new report out today shows spending on privately insured Americans with employer-based coverage grew just under 4% in 2013, thanks to falling usage of medical care services.

The news from the Health Care Cost Institute's 2013 Health Care Cost and Utilization Report shows the spending increase of 3.9% from employer health plans last year "continuing the moderate growth trend that began in 2010."

The study is important to the health insurance industry as insurers prepared for broader coverage this year under the Affordable Care Act, with millions of Americans purchasing coverage on government marketplaces known as exchanges. Because of financial penalties built into the law for not having coverage, employers last year began to see more workers taking advantage of coverage, setting the stage for an unknown impact to private insurance costs.

It could also be good news to investors in health insurance stocks, because the HCCI report indicates that employer-based plans seem to be doing well at mitigating cost increases thanks to falling usage of medical care services, which some analysts attribute to cost-shifting strategies of employers, offsetting higher health care prices.

"Health spending grew moderately, but that was only because consumers used fewer services," David Neman, HCCI executive director, said in a statement accompanying the institute's analysis.

The report looks at health care usage of enrollees in employer-sponsored health plans who are under 65. More than 40 million claims from four of the largest health insurers -- Aetna, Humana, UnitedHealth Group, and Kaiser Permanente -- were examined for the HCCI analysis, the Institute said.

Outpatient services continued to be the "fastest-growing medical service category in terms of spending," but growth slowed to 5.2% from 6.3% between 2012 and 2013, HCCI said.

"Driving the decline in the number of outpatient visits were declines in outpatient surgery and emergency room visits," the HCCI study said.

The Affordable Care Act is a key driver of outpatient visits, which is also good news to insurers that administer self-insured plans, though examining 2013 data is still a small amount of the benefit package under the law.

"For 2013, HCCI found that utilization rose for some services and populations affected by the ACA, including preventive visits and contraception, but these services generally contributed little to overall spending," the HCCI report said.

Other highlights of the report, which you can see here, include the following: