As it does in the general population, mortality shows seasonal variation in patients on hemodialysis, reported a recent study published in the Clinical Journal of the American Society of Nephrology (2012;7:108-115).
All-cause mortality was highest in winter (14.2 deaths/100 patient-years), followed by spring (13.1 deaths/100 patient-years) and autumn (12.3 deaths/100 patient-years), with the lowest mortality rate observed in summer (11.9 deaths/100 patient-years), found the researchers, who were led by Len A. Usvyat, MCP, of the Renal Research Institute (RRI). Peter Kotanko, MD, also of the Renal Research Institute, was the senior author.
“Mortality differences between seasons are no longer significant when adjusting for seasonally variable clinical and laboratory parameters,” the study authors wrote. “This finding suggests that the relationship between season and mortality is either mediated by the variation in risk factors or by an underlying factor (e.g., intercurrent disease), which is seasonally variable and affects both risk factors as well as mortality.”
Seasonal variations also were seen across different climates.
The data came from a retrospective record review of chronic in-center hemodialysis patients treated in 51 RRI and New York Dialysis Services clinics between April 1, 2004, and March 31, 2009.
For the mortality analysis, an open cohort of the 15,056 chronic hemodialysis patients who were treated in an RRI clinic during the time period was included. For the analysis of physiologic and laboratory parameters, 10,303 patients from the RRI database who had at least one treatment in each of the four calendar seasons were studied.
Of the participants, 55% were male, 49% were black, 42% were white, and 49% had diabetes. Mean age was 61.7, and mean vintage was 3.2 years. For the mortality analysis, median follow-up was 13.8 months, with a range of one day to 4.9 years.
Compared with all other seasons, mortality during winter was significantly higher in males and in patients younger than 75. Winter mortality was also higher than summer or autumn mortality in whites and in patients with diabetes.
The main cause of death in the study population was cardiovascular disease, which was significantly higher in winter (8.74 deaths/100 patient-years) compared with all other seasons. The clinical and laboratory parameters of predialysis systolic blood pressure, intradialytic blood pressure drop, neutrophil-to-lymphocyte ratio, serum potassium, and platelet count also were highest in winter.
In terms of study limitations, several relevant laboratory parameters were not included in the database, and patients in the different climate regions in the United States were unevenly distributed in the data, the authors noted.
“Seasonal variations should be taken into account when designing and interpreting longitudinal studies in dialysis patients,” they wrote.