Assessing glycaemic control in diabetes: relationships between fructosamine and HbA1C

Glycated haemoglobin (HbA1C) has become the internationally established method of assessing long term glycaemic control in people with diabetes. In New Zealand the measurement of glycated albumin (fructosamine), which is substantially cheaper than HbA1C has been widely adopted. In this study we have sought to determine if the value of HbA1C can be reliably estimated from knowledge of plasma fructosamine.

Fifty subjects with diabetes and stable glycaemic control as assessed by 3-5 simultaneous measurements of HbA1C and fructosamine made sequentially over a median of 6 months, were studied. The relationship between the two measures was assessed by determining 95% prediction intervals for HbA1C from the regression equation relating mean HbA1C and fructosamine. A further 8 subjects with significantly changing glycaemic control were also studied.

Mean stable plasma fructosamine and HbA1C measurements were closely correlated (r = 0.661, p < 0.0001) with HbA1C increasing on average 1% for every 56 mumol L-1 increase in fructosamine. The prediction intervals for HbA1C were however wide. Thus at a plasma fructosamine of 350 mumol L-1 the 95% prediction intervals for HbA1C ranged from 6.6 to 11.2% (3 to 11 standard deviations above the mean of the normal reference range). This variability could not be accounted for by the presence of albuminuria or by the exclusion of those subjects with the greatest variability in fructosamine. In the subjects showing changes in glycaemic control, a change in HbA1C of 1% was associated with a change in fructosamine of between 29 and 63 mumol L-1.

Fructosamine levels generally correlate well with HbA1C within a population but the value of HbA1C in an individual cannot be inferred with any reliability from the level of fructosamine, nor can the change in HbA1C be inferred from the change in fructosamine. We suggest that if fructosamine is to be used as an index of glycaemic control in diabetes, it is supplemented by a measurement of HbA1C when fructosamine measurements are stable, in order to determine whether the given value of fructosamine is consistent with the glycaemic control targets for that individual.