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Abstract

Introduction: Hospital readmissions contribute significantly to the cost of inpatient care and are targeted as a marker for quality of care. Little is known about risk factors associated with hospital readmission in survivors of critical illness. Methods: We studied 62,587 patients, age ≥ 18 years, who received critical care between 1997 and 2012 and survived hospitalization. The exposure of interest was acute kidney injury (AKI) defined as meeting RIFLE Risk, Injury or Failure criteria occurring 3 days prior to 7 days after critical care initiation. Patients with end stage renal disease prior to critical care initiation were identified by submitting the administrative data to the United States Renal Data System and excluded. In patients whom developed end stage renal disease following an AKI episode, the date chronic dialysis began was recorded. The primary outcome was unplanned hospital readmission in the 30 days following hospital discharge. The first hospitalization associated with critical care is identified as the index critical care exposure, and a 30-day unplanned readmission was defined as a subsequent unplanned admission to the hospitals under study within 30 days of discharge following the hospitalization associated with the index critical care exposure. Readmissions with DRG codes commonly associated with planned readmissions were excluded. The secondary outcome was mortality in the 30 days following hospital discharge. Information on vital status for the study cohort was obtained from the Social Security Administration Death Master File. Adjusted odds ratios were estimated by multivariable logistic regression models with inclusion of covariate terms thought to plausibly interact with both AKI and readmission status. Adjustment included age, race (white versus non-white), gender, Deyo-Charlson Index, patient type (medical versus surgical) and sepsis. Additionally, long-term progression to End Stage Renal Disease in patients with AKI was analyzed with a risk-adjusted Cox proportional hazards regression model. Results: Most cohort patients were men (60%) and white (78%). 48% had medically related DRGs. The mean age at hospital admission was 57 (SD 18) years. 30-day post-discharge hospital readmission rate was 13% and the 30-day post-discharge mortality rate was 3%. In patients who received critical care and survived hospitalization, AKI was a robust predictor of unplanned hospital readmission and post-discharge mortality and remained so following multivariable adjustment. The odds of 30-day post-discharge hospital readmission in patients with RIFLE class Risk, Injury or Failure fully adjusted were 1.44 (95%CI, 1.25, 1.66), 1.98 (95%CI, 1.66, 2.36), and 1.55 (95%CI, 1.26, 1.91) respectively, relative to patients without AKI. Further, the odds of 30-day post-discharge mortality in patients with RIFLE class Risk, Injury or Failure fully adjusted per our primary analysis were 1.39 (95%CI, 1.28, 1.51), 1.46 (95%CI, 1.30, 1.64), and 1.42 (95%CI, 1.26, 1.61) respectively, relative to patients without AKI. Finally, taking into account age, gender, race, Deyo-Charlson Index, and patient type (medical versus surgical), we observed a relationship between AKI and development of End Stage Renal Disease. Patients with RIFLE class Risk, Injury, Failure experienced a significantly higher risk of End Stage Renal Disease during follow-up than patients without acute kidney injury (HR 2.03, 95% CI, 1.56, 2.65; HR 3.99, 95%CI, 3.04, 5.23; HR 10.40, 95%CI, 8.54, 12.69 respectively). Conclusions: Patients who suffer acute kidney injury during critical illness are among a high-risk group of ICU survivors for adverse outcomes. In patients treated with critical care who survive hospitalization, acute kidney injury is a robust predictor of subsequent unplanned hospital readmission. In critical illness survivors, acute kidney injury is also associated with the odds of 30-day post-discharge mortality and the risk of subsequent End Stage Renal Disease.