Correspondence

Prognostic impact of hyponatraemia in patients with colorectal cancer doi:10.1111/codi.12939

Dear Sir, We read with interest the article by Chol et al. [1] presenting the negative impact on long-term overall survival in patients with various degrees of hyponatraemia during hospitalization following surgery for colorectal cancer. The authors are to be congratulated on the original concept and robust statistical analysis to provide important data relevant to a large proportion of general surgical/ colorectal practice. Importantly the authors provide considerable discussion into the confounding factors and limitations of this retrospective single centre analysis. We wish to add to this discussion and underline the importance of distinguishing correlation and causation. In this study, 2944 out of 3466 patients undergoing surgery for colorectal cancer in a large teaching hospital were divided into groups: (i) normonatraemia (135 147 mEq/L), n = 2130; (ii) mild hyponatraemia (130 134 mEq/L), n = 643; (iii) moderate hyponatraemia (125 129 mEq/L), n = 145; (iv) severe hyponatraemia (