Medicare Advantage plan networks included 46 percent of all physicians in a county, on average, according to a new analysis from the Kaiser Family Foundation. The study, using 2015 data, is the first to examine the size and composition of physician networks among increasingly popular Medicare Advantage private plans, which now cover 19 million people, or one-third of the Medicare population.

The study, which analyzed data from 391 plans offered by 55 insurers in a geographically diverse sample of 20 counties, found wide variation in the size and scope of provider networks, both across and within counties. Access to physicians tended to vary by specialty as well.

With Medicare’s open enrollment period beginning Oct. 15, the analysis documents one of the biggest trade-offs for consumers choosing between Medicare Advantage plans and traditional Medicare. Medicare Advantage plans have a more limited network of doctors and other providers than traditional Medicare, but typically have lower cost-sharing, limits on out-of-pocket spending, and some extra benefits. In contrast, traditional Medicare allows enrollees to see the overwhelming majority of providers without referrals or prior authorization but has no annual out-of-pocket spending limit. Provider networks are important for Medicare Advantage enrollees because seeing an out-of-network provider can result in significant out-of-pocket expenses.

Access to psychiatrists was typically more restricted than for any other specialty, according to the study. Medicare Advantage plans included 23 percent of the psychiatrists in a county, on average; 36 percent of plans had less than 10 percent of psychiatrists in their county. Some plans provided relatively little choice for other specialties as well; 20 percent of plans included less than five cardiothoracic surgeons, 18 percent of plans included less than five neurosurgeons, 16 percent of plans included less than five plastic surgeons, and 16 percent of plans included less than five radiation oncologists.

The study also found that both HMO and PPO Medicare Advantage plans with narrow provider networks (defined as providing access to less than 30 percent of physicians in a county) typically have lower premiums than broad-network plans (defined as including at least 70 percent of physicians). Premiums were $4 per month for narrow-network plans versus $64 per month for broad-network plans, on average. This finding suggests that in some counties Medicare Advantage enrollees may be able to use premiums as a rough proxy for physician network size.

The study found that more than one in three Medicare Advantage enrollees were in a plan with a narrow network. Enrollees may choose narrow network plans because they are comfortable with the way in which the plan delivers care, or based on other factors, such as low premiums. In large counties, like Los Angeles, some narrow network plans nevertheless have many doctors, so for consumers the question is whether the network includes the specific doctors they want or need when someone gets sick.

The analysis adds to the Foundation’s prior work that documents variations across Medicare Advantage plans in premiums, cost-sharing, extra benefits, quality, and in the composition and size of their hospital networks.