A well-controlled diabetic cat should have blood glucose levels that stay between 80 and 300 mg/dL throughout the day. The lowest point, called the nadir, ideally sits around 80 to 100 mg/dL, while pre-insulin readings at the high end of the range are expected and normal. These numbers fluctuate significantly over a 12-hour insulin cycle, so a single reading rarely tells the full story.
Target Range Throughout the Day
Blood glucose in a diabetic cat on insulin isn’t meant to stay flat. It rises and falls in a predictable wave between injections. The 2025 iCatCare consensus guidelines describe the goal as blood glucose ranging from a low of 80 to 100 mg/dL at its lowest point up to 250 to 350 mg/dL at its peak, which typically occurs just before the next insulin dose. Readings in the broader range of 90 to 360 mg/dL are generally associated with good clinical control.
The AAHA diabetes management guidelines define acceptable control for cats as glucose staying between 80 and 300 mg/dL, with a nadir of around 120 mg/dL being ideal. In practice, this means your cat’s glucose will spend some hours in the low-to-mid hundreds and climb back toward the upper end before the next injection. That’s expected. The goal isn’t to eliminate every high reading. It’s to keep glucose within a range where your cat feels good, drinks normal amounts of water, and maintains a healthy weight.
Why the Nadir Matters Most
The nadir is the lowest glucose reading between two insulin doses, and it’s one of the most important numbers your vet looks at. In cats, this low point typically occurs 3 to 6 hours after an insulin injection, with an average around 4 hours. Blood glucose then climbs back to pre-injection levels roughly 8 to 10 hours after the shot. Because many insulin types used in cats have this relatively short duration, there’s often a 2 to 3 hour window of higher glucose before each injection.
The nadir tells your vet two things: whether the insulin dose is strong enough to bring glucose down, and whether it’s bringing it down too far. A nadir around 80 to 120 mg/dL is the sweet spot. If it drops below 80, the dose may be too high. If it stays above 200 to 250, the dose may need adjusting or a different insulin type might work better.
When Glucose Drops Too Low
Hypoglycemia in cats is defined as blood glucose below 65 mg/dL, and it’s the most dangerous short-term risk of insulin therapy. The signs that owners most commonly notice are inability to walk, lethargy, weakness, failure to respond to touch or sound, and muscle twitching. Less commonly, cats may vocalize, have seizures, vomit, pant, or drool.
One important pattern from clinical data: most cat owners didn’t recognize the early warning signs like tremors or anxiety. Instead, they brought their cats in only after more severe neurological symptoms appeared, such as mental dullness, stupor, or collapse. If your cat seems unusually quiet, wobbly, or unresponsive, checking blood glucose right away (or offering food and contacting your vet immediately) is critical. Keeping corn syrup or honey on hand to rub on your cat’s gums during a hypoglycemic episode can buy time.
Glucose Curves vs. Spot Checks
A single glucose reading has limited value in a diabetic cat. What vets really want is a glucose curve: a series of readings taken every 1 to 2 hours over a full 12-hour insulin cycle. This shows how high glucose peaks, how low it dips, how quickly insulin takes effect, and how long it lasts. Many owners learn to do these at home using a pet glucometer and ear-prick blood samples, which produces more reliable results than in-clinic curves because cats are less stressed in their own environment.
For longer-term monitoring, your vet may check fructosamine levels through a blood test. Fructosamine reflects average blood glucose over the previous 1 to 2 weeks. In cats, the normal reference range is 146 to 271 µmol/L. A diabetic cat with satisfactory control will have fructosamine within or near that range, while levels above 400 µmol/L indicate poor control. This test is especially useful because it isn’t affected by the stress of a vet visit.
The Stress Factor in Cats
Cats are uniquely prone to stress hyperglycemia, and this complicates both diagnosis and monitoring. A stressed cat can have blood glucose well above 250 to 300 mg/dL, complete with glucose spilling into the urine, without actually being diabetic. Some cats maintain this stress-driven elevation for several days. What makes it trickier is that there’s no reliable way to predict which cats will spike: some visibly anxious cats have normal glucose, while apparently calm cats sometimes develop severe hyperglycemia.
This is why diabetes in cats requires persistent hyperglycemia to diagnose, not just a single high reading. If your cat gets a high glucose result at the vet, a fructosamine test can confirm whether glucose has been elevated for weeks or if it was a one-time stress response. For already-diagnosed cats, this also means that glucose readings taken at the vet’s office may run higher than what you’d see at home, which is another reason home monitoring tends to give more accurate data.
What Good Control Looks Like Day to Day
Numbers on a glucometer matter, but they’re a means to an end. The real markers of good glucose control are clinical: your cat is drinking a normal amount of water, urinating normally, maintaining or gaining weight, and staying active. Excessive thirst and urination are usually the first signs that glucose is running too high, and their resolution is one of the clearest signals that treatment is working.
Some diabetic cats eventually achieve remission, meaning they maintain normal glucose without insulin. This is more common in cats than in dogs, particularly when treatment starts early, the cat transitions to a low-carbohydrate diet, and glucose comes under tight control quickly. If your cat’s insulin needs start dropping and nadir readings consistently fall below 80, your vet may begin reducing the dose to test whether the cat can manage on its own. Remission doesn’t always last, so continued monitoring of glucose and clinical signs remains important even after insulin is stopped.