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Discordance Between HbA_1c and Fructosamine

Evidence for a glycosylation gap and its relation to diabetic nephropathy

¹ Division of Endocrinology, Department of Medicine, University of Cincinnati Medical Center, Cincinnati, Ohio
² Institute for Health Care Policy and Research, University of Cincinnati Medical Center, Cincinnati, Ohio
³ Department of Pediatrics, University of Cincinnati Medical Center, Cincinnati, Ohio
⁴ Medical Service, Cincinnati Veterans Affairs Medical Center, Cincinnati, Ohio
⁵ Sickle Cell Center, Children’s Hospital Research Foundation, Cincinnati, Ohio

OBJECTIVE—Discordances between HbA_1c and other measures of glycemic control are common in clinical practice and remain unexplained. We developed a measure of discordance between HbA_1c and fructosamine (FA) (glycosylated serum proteins) to conduct a systematic evaluation. We termed this the glycosylation gap (GG) and sought to determine its relationship to diabetic nephropathy.

RESEARCH DESIGN AND METHODS—Measurements of HbA_1c and FA on the same sample in 153 people were used to calculate GG, defined as the difference between measured HbA_1c and HbA_1c predicted from FA based on the population regression of HbA_1c on FA.

RESULTS—GG had a broad distribution (range, -3.2% to 5.5%); 40% of samples had values indicating major differences in prediction of complications risk by the measured versus predicted HbA_1c. GG was highly correlated (r = 0.81) between measurements repeated in 65 patients 23 ± 2 weeks apart, indicating that the discordances are reliable and not explained by differences in turnover of underlying proteins. In 40 patients with type 1 diabetes of 15 years’ duration, an increase in GG by 1% was associated with a 2.9-fold greater frequency of increasing nephropathy stage (P = 0.0014). GG was -0.8 ± 0.2% in subjects with no nephropathy, -0.3 ± 0.2% with microalbuminuria/hypertension, and 0.7 ± 0.3% in subjects with proteinuria or renal dysfunction (P < 0.05). GG correlated better with nephropathy than did either HbA_1c or FA alone in this population.

CONCLUSIONS—The glycosylation gap may be a useful clinical research tool for evaluating physiologic sources of variation in diabetic complications beyond glycemic control.

Abbreviations: FA, fructosamine • GG, glycosylation gap

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