Summer/winter changes in serum S100B protein concentration as a source of research variance.

S100B is a calcium binding protein that can be measured in cerebral and extra cerebral biological tissues and fluids. Circadian and seasonal variations have been described in several biological molecules such as melatonin, cortisol and testosterone. Healthy subjects do not have a circadian rhythm of S100B. There is no information on seasonal variations of S100B levels. The aim of this research is to study whether healthy subjects present summer/winter changes in serum S100B protein concentrations.Ninety-eight subjects were studied in summer, of those, 64 participated in the winter evaluation. Blood was drawn by venipuncture at 09:00 h, 12:00 h and 00:00 h in summer and winter. Serum was separated from blood by centrifugation and stored at -70° until analysis. Serum S100B concentrations were measured by ELISA.Serum S100B concentrations were significantly higher in summer than winter (09:00 h: 43.4 ± 24.6 ng/ml vs. 29.3 ± 22.7 ng/ml, p < 0.001; 12:00 h: 42.8 ± 25.0 ng/ml vs. 23.0 ± 22.1 ng/ml, p < 0.001; 00:00 h: 44.5 ± 23.2 ng/ml vs. 28.5 ± 24.6 ng/ml, p < 0.001). Age, gender, body mass index and time points when blood was extracted did not affect serum S100B concentrations neither in summer nor in winter.Our results point to the fact that there is an important difference in serum S100B concentrations between summer and winter. It is strongly advisable to consider this summer/winter difference in serum S100B concentrations when researching into this area.