The glycosylated proteins include fructosamine and glycosylated hemoglobin (A1C). Fructosamine, the glycosylated protein used in veterinary medicine, is formed by nonenzymatic, irreversible binding of glucose to serum proteins, mainly albumin.⁴⁷ Rate of formation is proportional to the average BG level, so the higher the mean BG concentration is over time, the greater the fructosamine concentration should be. Because fructosamine concentration is also affected by the half-life of albumin, it reflects glycemic control over the previous 1–2 wk. Unfortunately, well-controlled diabetics can have elevated fructosamine concentrations. Conversely, uncontrolled diabetic pets can have normal levels.⁴⁸ Fructosamine may be elevated in sick, hyperglycemic, but nondiabetic cats.⁴⁸ For these reasons, fructosamine trends are more useful than isolated values. Because fructosamine is typically not affected by stress, it can help to differentiate stress hyperglycemia from diabetes.

One of the best uses of fructosamine is to evaluate trends in glycemic control if measured at each recheck. Declining fructosamine values indicate a lowering in BG overall, whereas increasing values indicate the opposite. A fructosamine concentration below the reference range is highly suggestive of chronic hypoglycemia, in which case a BGC should be performed. Additionally, this scenario may be an indicator that a feline patient may be nearing diabetic remission. Cats with hyperthyroidism or conditions that cause hypoalbuminemia, increased protein turnover rates, or hypoglobulinemia may have decreased fructosamine concentrations. Corrections can be performed by the laboratory performing the analysis.

Commercial testing of canine and feline A1C is available. This glycated hemoglobin is commonly used to monitor diabetes in humans. More studies are needed to assess clinical use in pets.