Interleukin-6 is a stronger predictor of total and cardiovascular mortality than C-reactive protein in haemodialysis patients

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Abstract
Background. Despite the well known association between interleukin-6 (IL-6) and cardiovascular mortality, no study has so far verified whether IL-6 adds prognostic information to that provided by C-reactive protein (CRP). Methods. A cohort of 218 haemodialysis patients from four different dialytic centres was followed-up retrospectively. Plasma IL-6 and CRP concentrations were determined. Full information on co-morbidities was available in 162 patients. Results. With respect to the lowest quartile (< 3.6 pg/ml for IL-6, and < 2.2 mg/l for CRP), the crude relative risk (RR) of death from all causes of the upper quartile (> 13.9 pg/ml for IL-6, and > 12.8 mg/l for CRP) was 5.20 (95% confidence interval 2.06–13.011) for IL-6 and 3.16 (1.41–7.12) for CRP. When both variables were included, the estimates were 4.10 (1.30–12.96) for IL-6 and 1.29 (0.47–3.57) for CRP. As to continuous variables, the relationship between both variables and mortality tended to level off for the highest values, but became fairly linear after log transformation of the variables. For one unit SD of the log (variable), the RR was 2.09 (1.52–2.88) for IL-6 and 1.66 (1.23–2.24) for CRP. When they were included in the same model, the estimates were 1.90 (1.18–2.82) for IL-6 and 1.16 (0.81–1.66) for CRP. Conclusions. IL-6 has a stronger predictive value than CRP for cardiovascular mortality and provides independent prognostic information, while conveying most of that provided by CRP.