IMPORTANCE To reduce inefficiency and waste associated with care fragmentation, many current programs target greater clinical integration among physicians. However, these programs have led to only modest Medicare spending reductions. Most programs focus on formal integration, which often bears little resemblance to actual physician interaction patterns. OBJECTIVES To examine how physician interaction patterns vary between health systems and to assess whether variation in informal integration is associated with care delivery payments. DESIGN, SETTING, AND PARTICIPANTS National Medicare data from January 1, 2008, through December 31, 2011, identified 253 545 Medicare beneficiaries (aged-66 years) from 1186 health systems where Medicare beneficiaries underwent coronary artery bypass grafting (CABG) procedures. Interactions were mapped between all physicians who treated these patients-including primary care physicians and surgical and medical specialists-within a health system during their surgical episode. The level of informal integration was measured in these networks of interacting physicians. Multivariate regression models were fitted to evaluate associations between payments for each surgical episode made on a beneficiary's behalf and the level of informal integration in the health system where the patient was treated. EXPOSURES The informal integration level of a health system. MAIN OUTCOMES AND MEASURES Price-standardized total surgical episode and component payments. RESULTS The total 253 545 study participants included 175 520 men (69.2%; mean [SD] age, 74.51 [5.75] years) and 78 024 women (34.3%; 75.67 [5.91] years). One beneficiary of the 253 545 participants did not have sex information. The low level of informal clinical integration included 84 598 patients (33.4%; mean [SD] age, 75.00 [5.93] years); medium level, 84 442 (33.30%; 74.94 [5.87] years); and high level, 84 505 (33.34%; 74.66 [5.72] years) (P < .001). Informal integration levels varied across health systems. After adjusting for patient, health-system, and community factors, higher levels of informal integration were associated with significantly lower total episode and component payments (β coefficients for informal integration were-365.87 [95%CI,-451.08 to-280.67] for total episode payments,-182.63 [-239.80 to-125.46] for index hospitalization,-43.13 [-55.53 to-30.72] for physician services,-74.48 [-103.45 to-45.51] for hospital readmissions, and-62.04 [-88.00 to-36.07] for postacute care; P < .001 for each association). When beneficiaries were treated in health systems with higher informal integration, the greatest savings of lower estimated payments were from hospital readmissions (13.0%) and postacute care services (5.8%). CONCLUSIONS AND RELEVANCE Informal integration is associated with lower spending. Although most programs that seek to promote clinical integration are focused on health systems' formal structures, policy makers may also want to address informal integration.
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