Glucose

A high glucose (hyperglycaemia) has one headline cause and several confounders:

• Diabetes mellitus — a persistent, marked elevation, usually with increased thirst, urination, appetite and weight loss. This is the finding that matters.
• Stress — this is the great confounder, above all in cats. The adrenaline of a vet visit or a struggle for the sample can push a cat's glucose high enough to mimic diabetes, entirely transiently.
• A recent meal, certain medicines (steroids), and some hormonal conditions.

Because stress hyperglycaemia is so common, a single high glucose in a cat is not a diagnosis of diabetes. The way through is to look at the trend and to measure fructosamine, which reflects the average glucose over weeks and cuts straight through momentary spikes. Persistent hyperglycaemia with sugar spilling into the urine and typical clinical signs is what builds the diabetic picture.

A low glucose (hypoglycaemia) can be an emergency and is taken seriously. Causes include an insulin-producing tumour (insulinoma), severe illness or sepsis, liver failure, certain toxins, and, in small or young animals, simply going too long without food. A genuinely low glucose with symptoms (weakness, tremors, collapse) needs prompt attention. One common false low, though, is a delay in processing the sample — glucose keeps being consumed by blood cells in the tube, so a value that looks low on a sample that sat too long may be an artefact.

In a diagnosed diabetic, glucose is central to monitoring, but its moment-to-moment swings make single in-clinic readings a blunt tool. Trends — a glucose curve over a day, or continuous glucose monitoring, plus fructosamine every few weeks — give a much truer read of control than any one figure. Watching these over time is how insulin doses are tuned and how remission (especially achievable in cats) is recognised.

The species difference is central: cats are far more prone to stress hyperglycaemia than dogs, so a high glucose in a cat is interpreted with real caution and confirmed with fructosamine and the clinical picture. Feline diabetes also behaves differently — it can go into remission with good early control — which makes accurate interpretation, rather than over-reaction to a single stressed reading, genuinely important. In dogs, stress raises glucose far less, so a clearly high value is more directly meaningful.

Glucose is measured on a routine chemistry panel, in mg/dL (US) or mmol/L (UK/SI). Prompt sample handling avoids the falsely low readings from glucose being consumed in the tube. Because the aim is often to judge diabetes, the result is read together with clinical signs, the urine (for glucose spilling over), and fructosamine.

A high glucose — particularly in a cat — is a prompt to confirm rather than to conclude: your vet will typically check fructosamine, look for glucose in the urine, and weigh the clinical signs before diagnosing diabetes. In a known diabetic, the useful work is in the trend — curves, continuous monitoring and fructosamine — not any single spot value.