Season change bias in animal studies

To the Editor: In research, bias includes any systematic error in the design, conduct, or analysis of the study. Bias can occur at all stages of research, from the selection of the population, how treatment is provided, to how and when outcome measurements are made.¹ One report reviewed more than 50 possible source of bias in analytic research.² Season change is a possible source of bias in animal studies that was not reported previously. We recently experienced this bias in evaluation of the biocompatibility of a new biomaterial with subcutaneous implantation in Sprague-Dawley white albino rat (male, mean weight, 200±25 g). During a longitudinal study, we obtained controversial results between summer and winter. All other variables were under control and we could not find any other source of bias. This source of bias could be attributed to hibernation in rodents,³ altering the immunological, pathological, and behavioral responses. By changing the environment temperature and daylight duration (combination of natural and artificial light, not artificial light only) it can be controlled, relatively. Nevertheless, in subtle studies both investigators and readers of the medical literature should consider this source of bias.