Intra‐individual variability and longitudinal changes in glycaemic control in patients with Type 1 diabetes mellitus

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Abstract
Aims Patients with Type 1 diabetes mellitus (DM) appear to have remarkably stable HbA1c levels, regardless of the need for improvement. The purpose of the present study was therefore to study predictors of intra‐individual variability of the HbA1c level together with changes in HbA1c over time. Methods Hospital records of patients with Type 1 DM seen at our diabetes clinic from February 1992 to May 1997 were reviewed for HbA1c measurements and clinical data. In the main study, 214 patients who had been on insulin for more than 1 year, and in a sub‐study, 14 patients newly started on insulin, were included. Results The coefficient of variation (CV) of the intra‐individual HbA1c measurements, after at least 1 year of insulin, was 8.8 ± 3.7% (mean ± sd). There was a positive association between the CV and the HbA1c measurement at inclusion in the study (P < 0.05), and also a negative association between the CV and age (P < 0.05). Fifty per cent of the patients had a difference between first and last HbA1c below 1%, and 83.6% had a difference below 2%. In the sub‐study, there was a positive association between the mean HbA1c value the first year on insulin (excluding the first 3 months) and the last HbA1c measurement (P < 0.01). Conclusions The HbA1c levels in individual patients remain remarkably stable over time. Furthermore, the HbA1c level shortly after starting insulin is a predictor of future glycaemic control.