Comorbidity burden at dialysis initiation and mortality: A cohort study

BACKGROUND A high level of comorbidity at dialysis initiation is associated with an increased risk of death. However, contemporary assessments of the validity and prognostic value of comorbidity indices are lacking. OBJECTIVES To assess the validity of two comorbidity indices and to determine if a high degree of comorbidity is associated with mortality among dialysis patients. DESIGN Cohort study. SETTING QEII Health Sciences Centre (Halifax, Nova Scotia, Canada). PATIENTS Incident, chronic dialysis patients between 01 Jan 2006 and 01 Jul 2013. MEASUREMENTS EXPOSURE The Charlson Comorbidity Index (CCI) and End-Stage Renal Disease Comorbidity Index (ESRD-CI) were used to classify individual comorbid conditions into an overall score. Comorbidities were classified using patient charts and electronic records. OUTCOME All-cause mortality. Confounders: Patient demographics, dialysis access, cause of ESRD and baseline laboratory data. METHODS Regression coefficients were estimated on the CCI and ESRD-CI. Discrimination for death was assessed using Harrell's c-index. Adjusted Cox proportional hazard models were used to calculate relative hazards and 95 % confidence intervals for each category of the CCI and ESRD-CI. RESULTS The cohort consisted of 771 ESRD patients from 01 Jan 2006 to 01 Jul 2013. Most were male (62 %) and Caucasian (91 %). The cohort had a high proportion of diabetes (48 %), history of previous myocardial infarction (31 %) and heart failure (22 %). Regression coefficients on the CCI and ESRD-CI were 0.55 and 0.52, respectively. The c-index, for the prediction of death, was 0.61 for the CCI and 0.63 for the ESRD-CI. ESRD-CI scores of 4, 5 and ≥6 were associated with a similar mortality risk (adjusted relative hazard of 1.95, 1.89 and 1.99, respectively). There was a small increased mortality risk for CCI scores of 4, 5 and ≥6 (adjusted relative hazard of 1.86, 2.38 and 2.71, respectively). LIMITATIONS Classification of comorbidities for each patient was determined by clinical impression. CONCLUSIONS The CCI and ESRD-CI have a limited ability to discriminate mortality risk for incident dialysis patients. Acknowledging the frequency with which they are used, this study emphasizes the need to re-examine the usefulness of previously derived comorbidity indices in contemporary dialysis cohorts.