Concentration of Physician Services Across Insurers and Effects on Quality: Early Evidence From Medicare Advantage

A growing proportion of Medicare beneficiaries is covered by private insurers through Medicare Advantage, yet little is known about how these plans are structured in terms of relationships with physicians and implications for quality of care.

The objective of this study was to assess whether greater physician concentration of services across insurers was associated with higher quality in Medicare Advantage (MA), overall and particularly among MA insurers serving a high proportion of vulnerable enrollees.

A retrospective cohort design with regression analysis.

The primary dataset was 2014 MA encounter records submitted by insurers to the Centers for Medicare and Medicaid Services, covering 600,329 physicians across 119 insurers. These data were merged with Centers for Medicare and Medicaid Services data on MA contract quality rating as well as physician characteristics in the Medicare Data on Provider Practice and Specialty file.

Two measures were generated to capture the concentration of physician services across insurers: the percentage of a physician's Medicare services which was through MA (MA penetration); and the percentage of a physician's MA services with a specific insurer (insurer share of MA services).

Greater MA penetration and insurer share of MA services were each associated with higher MA plan quality. The relationship between insurer share and quality was stronger in contracts with a relatively high percentage of disabled enrollees.

Greater physician concentration of services across MA insurers was associated with a higher quality of care overall and especially among vulnerable enrollees.