Aims We aim to develop smoothed continuous 2.5th and 97.5th percentile values for labile glycated haemoglobin A_1c to glycated haemoglobin A_1c (LHbA_1c:HbA_1c) ratio against HbA_1c, and apply them on our patient population for identification of potentially spurious HbA_1c measurements.

Methods The LHbA_1c and HbA_1c were measured using Bio-rad Variant II high-performance liquid chromatography system. We recorded the LHbA_1c and HbA_1c values of 1555 patients who had normal chromatograms. Using these results, the 2.5th and 97.5th percentile reference limits of the LHbA_1c:HbA_1c ratio were described by LHbA_1c:HbA_1c=−0.0072×HbA_1c +0.2925 and LHbA_1c:HbA_1c=−0.0132×HbA_1c +0.5327, respectively.

Results When the reference intervals were applied on a separate 1000 patients, 34 and 29 of them had abnormally high and low LHbA_1c:HbA_1c ratios, respectively. Most of the observed high ratios were associated concurrently with elevated plasma glucose, anaemia, chronic liver and kidney diseases. A suppressed ratio was mostly associated with haemoglobin variants. Patients with heterozygous HbE or HbS variants tend to have lower LHbA_1c:HbA_1c ratios while the converse is true for heterozygous HbJ.

Conclusions The continuous LHbA_1c:HbA_1c ratio may be used to detect confounding factors or spurious HbA_1c results, but its performance is confounded and reduced by the ambient plasma glucose.