Supplementary Material for: Serum Potassium Levels and Mortality in Hemodialysis Patients: A Retrospective Cohort Study

Background: Hyperkalemia is common in patients receiving maintenance hemodialysis. However, few studies have examined the association between serum potassium level and mortality. Methods: This study used annual cohorts of hemodialysis patients during 2007-2010. To determine hyperkalemia prevalence, monthly hyperkalemia was defined as serum potassium level ≥5.5 mEq/l; prevalence was calculated as a ratio of hyperkalemia episodes to follow-up time, reported separately by long and short interdialytic interval. To determine the impact of hyperkalemia on mortality, patients in the 2010 cohort were followed from first potassium measurement until death or a censoring event; hyperkalemia was defined, sequentially, by potassium levels 5.5-6.0 mEq/l at 0.1 mEq/l intervals. Time-dependent Cox proportional hazards modeling was used to estimate the association between hyperkalemia and mortality. Results: The 4 annual cohorts ranged from 28,774 to 36,888 patients. Mean age was approximately 63 years, about 56% were men, 51% were white and 44% had end-stage renal disease caused by diabetes. Hyperkalemia prevalence was consistently estimated at 16.3-16.8 events per 100 patient-months. Prevalence on the day after the long interdialytic interval was 2.0-2.4 times as high as on the day after the short interval. Hyperkalemia, when defined as serum potassium ≥5.7 mEq/l, was associated with all-cause mortality (adjusted hazards ratio (AHR) 1.13, 95% CI 1.01-1.28, p = 0.037, vs. <5.7 mEq/l) after adjustment. AHRs increased progressively as the hyperkalemia threshold increased, reaching 1.37 (95% CI 1.16-1.62, p < 0.0001) for ≥6.0 mEq/l. Conclusions: The long interdialytic interval was associated with increased likelihood of hyperkalemia. Hyperkalemia was associated with all-cause mortality beginning at serum potassium ≥5.7 mEq/l; mortality risk estimates increased ordinally through ≥6.0 mEq/l, suggesting a threshold at which serum potassium becomes substantially more dangerous.