Objective: Haemoglobin A1c (Hb A1c) is routinely used for monitoring glycemic control in patients with diabetes. Hb A1c seasonal fluctuations can be directly related to different biological, geographical and cultural influences. Our purpose was to evaluate seasonal variation of Hb A1c in a hospitalbased adult population over a period of 5 years. Materials and methods: We analyzed retrospectively monthly Hb A1c mean values (DCCT, %) based on all the assays performed to adult patients at a tertiary care university Portuguese hospital between 2008-2012. Results: We obtained 62,384 Hb A1c valid measurements, with a peak level found in January-February (7.1%), a trough in AugustOctober (6.8%) and an average peak-to-trough amplitude value of 0.3%. This trend was observed in both genders and age subgroups evaluated. Conclusions: There is a Hb A1c circannual seasonal pattern with peak levels occurring in winter months in this Portuguese population. This finding should be recognized in daily clinical practice to warrant better clinical and epidemiological interpretation of Hb A1c values. Arch Endocrinol Metab. 2015;59(3):231-5

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