Endocrine Abstracts

The aim of this study was to investigate seasonal variation in haemoglobin A1c (HbA_1c) using a retrospective population analysis linking HbA_1c values to the time of year. The study cohort included 171 442 NHS patients from Scotland, UK. Patients were divided into three groups. The ‘stable group’ were defined as having a HbA1c that varied less than the tertile either side of the mean HbA_1c. The ‘high summer’ was comprised of patients with a HbA_1c that varied by the second and third tertile above the mean HbA_1c in summer. A ‘high winter’ group was defined similarly but HbA_1c being higher during the winter months. There was a significant temporal trend in the mean HbA_1c levels for all patients in the study with a maximal variation of 2.4 mmol/mol (P<0.01) between May and August. In high summer patients, there was a variation of 7.5 mmol/mol (P<0.01) between January and October. In high winter patients, there was a variation of 8.7 mmol/mol (P<0.01) between March and September. Overall 45.3% of all patients (n=77 720) were in the high winter HbA_1c group, 31.3% (n=53 656) were in the high summer HbA1c group, and 23.4% (n=40 066) were in the stable HbA_1c group. Associations with seasonal variation of HbA_1c included social deprivation, male gender, T1DM, increasing BMI and increasing diastolic blood pressure (DBP). The most likely explanation for this is treatment adherence, however this could not be measured in this retrospective population analysis. More than half the patients have a clinically relevant annual variation in HbA_1c, which could impact on clinical decision making. Both clinicians and researchers need to be aware of the influence of time of year on the HbA_1c value, especially in the context of meeting treatment target goals for HbA_1c.