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Association of hospital volume with readmission rates: a retrospective cross-sectional study

BMJ 2015; 350 doi: (Published 09 February 2015) Cite this as: BMJ 2015;350:h447

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  1. Leora I Horwitz, director, Center for Healthcare Innovation and Delivery Science12,
  2. Zhenqiu Lin, statistician3,
  3. Jeph Herrin, statistician45,
  4. Susannah Bernheim, director of quality measurement programs36,
  5. Elizabeth E Drye, director of quality measurement programs37,
  6. Harlan M Krumholz, Harold J Hines Jr professor of medicine, Section of General Internal Medicine348,
  7. Joseph S Ross, associate professor of medicine and of public health368
  1. 1Division of Healthcare Delivery Science, Department of Population Health, New York University Langone Medical Center, New York, NY, USA
  2. 2Center for Healthcare Innovation and Delivery Science, New York University School of Medicine, New York
  3. 3Center for Outcomes Research and Evaluation, Yale-New Haven Hospital, New Haven, CT, USA
  4. 4Section of Cardiovascular Medicine, Department of Medicine, Yale School of Medicine, New Haven
  5. 5Health Research and Educational Trust, Chicago IL, USA
  6. 6Section of General Internal Medicine, Department of Medicine, Yale School of Medicine, New Haven
  7. 7Department of Pediatrics, Yale School of Medicine, New Haven
  8. 8Section of Health Policy and Administration, Yale School of Epidemiology and Public Health, New Haven
  1. Correspondence to: L Horwitz, Division of Healthcare Delivery Science, Department of Population Health, New York University Langone Medical Center, New York, NY, USA leora.horwitz{at}
  • Accepted 30 December 2014


Objective To examine the association of hospital volume (a marker of quality of care) with hospital readmission rates.

Design Retrospective cross-sectional study.

Setting 4651US acute care hospitals.

Study data 6 916 644 adult discharges, excluding patients receiving psychiatric or medical cancer treatment.

Main outcome measures We used Medicare fee-for-service data from 1 July 2011 to 30 June 2012 to calculate observed-to-expected, unplanned, 30 day, standardized readmission rates for hospitals and for specialty cohorts medicine, surgery/gynecology, cardiorespiratory, cardiovascular, and neurology. We assessed the association of hospital volume by quintiles with 30 day, standardized readmission rates, with and without adjustment for hospital characteristics (safety net status, teaching status, geographic region, urban/rural status, nurse to bed ratio, ownership, and cardiac procedure capability. We also examined associations with the composite outcome of 30 day, standardized readmission or mortality rates.

Results Mean 30 day, standardized readmission rate among the fifth of hospitals with the lowest volume was 14.7 (standard deviation 5.3) compared with 15.9 (1.7) among the fifth of hospitals with the highest volume (P<0.001). We observed the same pattern of lower readmission rates in the lowest versus highest volume hospitals in the specialty cohorts for medicine (16.6 v 17.4, P<0.001), cardiorespiratory (18.5 v 20.5, P<0.001), and neurology (13.2 v 14.0, p=0.01) cohorts; the cardiovascular cohort, however, had an inverse association (14.6 v 13.7, P<0.001). These associations remained after adjustment for hospital characteristics except in the cardiovascular cohort, which became non-significant, and the surgery/gynecology cohort, in which the lowest volume fifth of hospitals had significantly higher standardized readmission rates than the highest volume fifth (difference 0.63 percentage points (95% confidence interval 0.10 to 1.17), P=0.02). Mean 30 day, standardized mortality or readmission rate was not significantly different between highest and lowest volume fifths (20.4 v 20.2, P=0.19) and was highest in the middle fifth of hospitals (range 20.6–20.8).

Conclusions Standardized readmission rates are lowest in the lowest volume hospitals—opposite from the typical association of greater hospital volume with better outcomes. This association was independent of hospital characteristics and was only partially attenuated by examining mortality and readmission together. Our findings suggest that readmissions are associated with different aspects of care than mortality or complications.


  • An earlier version of this work was presented at the Academy Health 2012 Annual Research Meeting in Orlando, FL, on 25 June 2012.

  • Contributors: Study concept and design: all authors. Acquisition of data: HMK. Analysis and interpretation of data: all authors. Drafting of the manuscript: LIH. Critical revision of the manuscript for important intellectual content: all authors. Approval of final manuscript for submission: all authors. Statistical analysis: ZL, JH. Obtained funding: HMK. Study supervision: LIH, EED.

  • Financial support: This work was performed under contract HHSM-500-2013-13018I, Task Order HHSM-500-T0001, funded by the Center for Medicare & Medicaid Services, an agency of the US Department of Health and Human Services. This work was also supported by the Agency for Healthcare Research and Quality (R01 HS022882). LIH and JSR are supported by the National Institute on Aging (K08 AG038336 and K08 AG032886, respectively) and by the American Federation for Aging Research through the Paul B Beeson Career Development Award Program. HMK is supported by grant U01 HL105270-03 (Center for Cardiovascular Outcomes Research at Yale University) from the National Heart, Lung, and Blood Institute. No funding source had any role in the study design; in the collection, analysis, and interpretation of data; or in the writing of the report. The Center for Medicare & Medicaid Services reviewed and approved the use of its data for this work and approved submission of the manuscript.

  • Competing interests: All authors have completed the Unified Competing Interest form at (available on request from the corresponding author) and declare that all authors have support from the Centers for Medicare & Medicaid Services for the submitted work. In addition, JSR is a member of a scientific advisory board for FAIR Health, and HMK chairs a cardiac scientific advisory board for UnitedHealth and is the recipient of research grants from Medtronic and Johnson & Johnson through Yale University.

  • Transparency: The lead author (LIH) affirms that the manuscript is an honest, accurate, and transparent account of the study being reported; that no important aspects of the study have been omitted; and that any discrepancies from the study as planned (and, if relevant, registered) have been explained. ZL had full access to all of the data in the study and takes responsibility for the integrity of the data and the accuracy of the data analysis.

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