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

Aims Patients with Type 1 diabetes mellitus (DM) appear to have remarkably stable HbA(1c) levels, regardless of the need for improvement. The purpose of the present study was therefore to study predictors of intra-individual variability of the HbA(1c) level together with changes in HbA(1c) 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 HbA(1c) 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 HbA(1c) 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 HbA(1c) 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 HbA(1c) below 1%, and 83.6% had a difference below 2%. In the sub-study, there was a positive association between the mean HbA(1c) value the first year on insulin (excluding the first 3 months) and the last HbA(1c) measurement (P < 0.01). Conclusions The HbA(1c) levels in individual patients remain remarkably stable over time. Furthermore, the HbA(1c) level shortly after starting insulin is a predictor of future glycaemic control.