Narrow insurance networks are gaining traction across the United States, as insurers have limited options for offering plans at different price points, according to a report by the Leonard Davis Institute of Health Economics and the Robert Wood Johnson Foundation.
However, some states are offering more of these networks than others.
The growth comes as mandated changes in the Affordable Care Act, such as community ratings, standardized benefits and removing limits on annual or lifetime benefits, means insurers must find other strategies for offering lower-cost plans.
There is also a strong correlation between states that offer Health Maintenance Organization plans and states that have more narrow networks, the report states.
States with a 60 percent or more narrow networks are dominated by HMOs, where states that have the lowest prevalence -- 20 percent or less -- are dominated by Preferred Provider Organizations, the authors said.
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Insurers can use narrow networks to lower premiums by excluding high-cost providers or by offering a fixed lower reimbursement level to all providers, which can result in providers opting out of the insurer's network, according to the report.
Insurers can segment their network into tiers, with higher cost-sharing for the higher tiers, resulting in a narrowing of the network for price conscious consumers.
However, the report also found consumers have difficulty accessing network size, and even learning if a provider is in the network.
New federal rules for 2016 will require plans to publish up-to-date and complete provider directories.
[Also: 5 things to know about narrow network plans: Less choice, lower costs]
Similarly, it is difficult for regulators to judge the adequacy of the provider network, something the ACA requires. Networks are required to have sufficient numbers and types of providers to deliver services without "unreasonable delay," though "unreasonable" is left to the states to define.
In sizing provider networks in gold, silver and bronze tiers, the report found that in a 2014 national analysis of silver plans, 41 percent were extra-small or small, meaning that they included 25 percent or less of the physicians in a rating area.
Network size varied across type of plans, with HMOs more narrow than PPOs.
The report also found that the distribution of networks that were large or extra large could be found in Delaware, Kansas and North Dakota and those smaller are in Georgia, Florida and Oklahoma.
Others that are well balanced across sizes include Minnesota, New York and Washington.
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Although the concept of narrow networks has gained national attention, the report found 12 states had none.
The prevalence of narrow networks in states range from 83 percent in Georgia to 13 percent in Idaho and North Carolina.
The report found significant variability within plan types by state; for example, none of Idaho's three HMO networks are narrow, compared to half of its PPO networks; 91 percent of California's 11 HMO networks are narrow, compared to 33 percent of its PPOs.
The question of why one state has more narrow networks than other states awaits future research.
The report conclude that well-functioning narrow networks would survive only if they are made more transparent to consumers and are regulated to ensure sufficient network adequacy.
Twitter: @SusanMorseHFN