Quantifying comorbidity in peritoneal dialysis patients and its relationship to other predictors of survival

Comorbidity is the single most important determinant of outcome in patients on renal replacement therapy. The aims of this study were to evaluate a semi-quantitative approach to comorbidity scoring in predicting survival of patients commencing peritoneal dialysis (PD), and to establish the interaction between this and other known predictors of patient outcome, in particular membrane function, residual renal function (RRF) and plasma albumin.

Comorbidity was recorded in a prospective, single centre cohort study of 303 patients commencing on PD. Using seven disease domains, chosen to reflect the dominance of cardiovascular morbidity in the end-stage renal failure population, comorbidity was graded as '0' when absent, '1' when one or two, and '2' when three or more conditions were present. The Wright comorbidity index, which includes age within the scoring method, was also evaluated. RRF, plasma albumin and peritoneal solute transport were measured every 6 months. Patients were censored at death.

Median survival according to grade of comorbidity was 105, 42 and 29 months, respectively (P<0.0001), with good separation of the actuarial survival curves. Using Cox regression, the addition of age and the grade of comorbidity to Kt/V(urea), solute transport and plasma albumin increased the predictive power of the model. All were independent predictors of outcome with the exception of albumin. The Wright comorbidity index also enhanced the Cox model, although was not as powerful as when age and comorbidity were considered independently. At baseline, RRF was not different according to comorbidity unless diabetes was considered separately. Diabetics started with higher RRF, but after 6 months on PD this was the same as non-diabetic patients. Otherwise, initial rate of decline of RRF was similar across the comorbid grades, although the impact of higher drop-out due to earlier loss in patients with more comorbidity may have disguised earlier loss in these patients. Peritoneal solute transport tended to be higher in patients with increased comorbidity at baseline, chi(2) 13.8, P=0.032, and this was sustained with time on treatment.

Comorbidity has a quantitative effect on survival that is independent of age, RRF and membrane function in PD patients. Comorbidity also appears to be associated with increased solute transport at the start of treatment, which is sustained. With the exception of diabetes, grade of comorbidity does not have a profound effect on loss of RRF.