Fructosamine

A raised fructosamine means glucose has been persistently high over the preceding weeks — the signature of diabetes mellitus. Unlike a single high glucose, a high fructosamine cannot be explained away by the stress of the sampling moment, because it reflects a sustained average. That makes it the key confirmatory test: a stressed cat with a high glucose but a normal fructosamine very likely does not have diabetes, whereas a high fructosamine supports the diagnosis.

A normal fructosamine indicates good average glucose control (or no diabetes). In a diabetic patient it is a reassuring sign that treatment is holding. A low fructosamine can indicate periods of low glucose — in a treated diabetic, it may be a clue that insulin is being over-dosed and glucose is dropping too far between doses. Certain other factors (rapid protein turnover, low blood proteins, thyroid disease) can shift fructosamine independently of glucose, so it is read in context rather than as an absolute.

Fructosamine's main home is monitoring the diabetic patient. Measured every few weeks, it summarises how well glucose has been controlled across that whole window in a single, spike-proof number — a much steadier guide to whether the insulin dose is right than an isolated in-clinic glucose. A falling fructosamine as treatment takes hold, or a stable one in the target zone, is the trajectory that signals good control; a rising one signals that control is slipping. It complements, rather than replaces, glucose curves and continuous monitoring.

Fructosamine is especially prized in cats precisely because feline glucose is so easily raised by stress — it is often the deciding test between true diabetes and a one-off stress spike. In cats aiming for diabetic remission, it also helps track whether control is good enough for the pancreas to recover. In dogs, where stress hyperglycaemia is much less of an issue, fructosamine is still a valuable averaging tool for confirming diabetes and monitoring control.

Fructosamine is measured from a blood sample, reported in µmol/L. It does not require fasting (its whole point is that it reflects an average, not the current moment). Because low blood protein or altered protein turnover can affect it, results are interpreted alongside total protein and albumin where relevant. Read the value against your own report's range.

If diabetes is suspected — particularly in a cat with a high glucose — fructosamine is the test that helps confirm or refute it. In a known diabetic, discuss using fructosamine alongside glucose curves to judge control over time, and remember that a low fructosamine can be as important a message (over-treatment) as a high one.