title: Seasonal variation of haemoglobin A1c in a Portuguese adult population.
abstract: 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 hospital-based adult population over a period of 5 years.
  
  
  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 August-October (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.
authors.affiliation: Division of Endocrinology, Diabetes and Metabolism, Hospital de Santo Antóni, Centro Hospitalar do Porto, Porto, Portugal.
authors.forename: Maria Teresa Rocha e Pinho
authors.initials: MT
authors.lastname: Pereira
authors.affiliation: Division of Endocrinology, Diabetes and Metabolism, Hospital de Santo Antóni, Centro Hospitalar do Porto, Porto, Portugal.
authors.forename: Domingos
authors.initials: D
authors.lastname: Lira
authors.affiliation: Division of Endocrinology, Diabetes and Metabolism, Hospital de Santo Antóni, Centro Hospitalar do Porto, Porto, Portugal.
authors.forename: Conceição
authors.initials: C
authors.lastname: Bacelar
authors.affiliation: Department of Clinical Pathology, Hospital de Santo António, Centro Hospitalar do Porto, Porto, Portugal.
authors.forename: José Carlos
authors.initials: JC
authors.lastname: Oliveira
authors.affiliation: Division of Endocrinology, Diabetes and Metabolism, Hospital de Santo Antóni, Centro Hospitalar do Porto, Porto, Portugal.
authors.forename: André Couto
authors.initials: AC
authors.lastname: de Carvalho
mesh_terms: D000328:Adult; D000367:Age Factors; D000368:Aged; D000369:Aged, 80 and over; D001786:Blood Glucose / Q000032:analysis; D003920:Diabetes Mellitus / Q000097:blood; D006442:Glycated Hemoglobin / Q000032:analysis* / Q000378:metabolism; D006801:Humans; D008875:Middle Aged; D011174:Portugal; D012016:Reference Values; D012189:Retrospective Studies; D012621:Seasons; D012737:Sex Factors; D018709:Statistics, Nonparametric; D013997:Time Factors; D055815:Young Adult
chemical_list: D001786:Blood Glucose; D006442:Glycated Hemoglobin