Factors Associated with Patient-Sharing Networks among Physicians

Physicians tend to share patients with colleagues who have similar personal traits and practice styles, and there is substantial variation in physician network characteristics across geographic areas, according to study findings reported in the Journal of the American Medical Association [2012;308(3):265-273].

Physicians are embedded in informal networks that result from their sharing of patients, information, and behaviors. Researchers conducted a study to identify professional networks among physicians, examine how such networks vary across geographic regions, and determine factors associated with physician connections. Using novel, validated methods adopted from social network analysis, Medicare administrative data from 2006 were used to study 4,586,044 Medicare beneficiaries seen by 68,288 physicians practicing in 51 hospital referral regions (HRRs).

Distinct networks depicting connections between physicians (defined based on shared patients) were constructed for each of the 51 HRRs. The randomly sampled HRRs were distributed across all regions of the country and included urban and rural locations. The average physician age was 48.8 years and about 80% were male. Among the Medicare patients, the mean age was 70.7 years and 40% were male.

The primary outcome measures were variation in network characteristics across HRRs and factors associated with physicians being connected. The results indicated substantial variation across HRRs. The number of physicians ranged from 135 in Minot, North Dakota, (1568 ties) to 8197 in Boston, Massachusetts, (392,582 ties). The average adjusted degree (number of other physicians each physician was connected to per 100 Medicare beneficiaries) across all HRRs was 27.3 (range, 11.7-54.4).

Patterns varied by physician specialty as well. Physician ties to each other were far more likely to be based on the same hospital (69.2% of unconnected physician pairs vs 96.0% of connected physician pairs; adjusted rate ratio, 0.12; 95% confidence interval, 0.12-0.12; P<.001). Connected physician pairs were also more likely to be in close geographic proximity. The average distance for connected pairs was 13.1 miles versus 24 miles for those without connections (P<.001).

Characteristics of physicians’ patient populations were also linked with the presence of ties between physicians. Connected physicians had similar patient panels in terms of the race or illness burden than unconnected physicians. For example, connected physician pairs had an average difference of 8.8 points in the percentage of African-American patients in their 2 patients panels, compared with a difference of 14.0 percentage points for unconnected physician pairs (P<.001). Similarly, differences in mean patient age and percentage of Medicaid patients were also smaller for connected physicians versus unconnected physicians. Medical comorbidities were also similar, suggesting that connected physicians had similar patients regarding clinical complexity than unconnected physicians.

Study limitations discussed by the authors included that Medicare data was used to identify shared patients among physicians; data was limited to a single year; the analyses of network characteristics included physician ties only if they were in the top 20% of ties for each individual physician; and the adoption of electronic medical records since 2006 could lead to different relationship patterns. Although the researchers demonstrated variation in networks across geographic areas, they noted that additional research is needed to establish whether network characteristics are associated with variations in care.

Because novel methods to define social networks among physicians in geographic areas with specific parameters were used, the researchers concluded, “This approach might have useful applications for policy makers seeking to influence physician behavior [whether related to accountable care organizations or innovation adoptions] because it is likely that physicians are strongly influenced by their network of relationships with other physicians.”