If your cat has diabetes, the short answer is: test before their morning meal and insulin injection, then at regular intervals throughout the day depending on what your vet needs to see. But the full picture depends on where your cat is in treatment, whether you’re doing a one-time spot check or a full glucose curve, and what symptoms you’re watching for.
The Pre-Insulin Check
The single most important time to test is right before you give insulin. This reading tells you your cat’s baseline blood sugar before the medication takes effect, and it helps you confirm the dose is appropriate. Normal fasting blood glucose in cats is 75 to 120 mg/dL. A diabetic cat on insulin will often read higher than that before their dose, but how much higher gives your vet critical information about whether the current treatment plan is working.
One useful detail: cats don’t experience the post-meal blood sugar spikes that humans do. Research in the Journal of Feline Medicine and Surgery found no significant post-meal hyperglycemia in diabetic cats, even when eating typical cat foods. An overnight fast also didn’t change morning glucose readings. This means you don’t need to worry about timing tests around meals the way a human diabetic would. Your cat can eat freely, and the readings will still be reliable.
How to Do a Full Glucose Curve
A glucose curve maps your cat’s blood sugar across an entire dosing cycle, usually 12 hours. It’s the gold standard for seeing how insulin is performing: how low it brings glucose, when it peaks, and how long it lasts.
The standard approach is to take a reading before the morning meal and insulin, then test every two hours for 12 hours. If any reading drops below 150 mg/dL, switch to hourly testing, since that means your cat’s glucose is in a range where it could potentially dip too low. The goal is to find the “nadir,” the lowest point blood sugar reaches after an insulin injection.
When that nadir happens depends on the type of insulin your cat uses. With glargine, the most common long-acting insulin for cats, blood sugar typically hits its lowest point around 5 to 6 hours after injection, though there’s wide individual variation (anywhere from about 2 to 9 hours). With detemir, another long-acting option, the nadir tends to come a bit later, around 7 hours post-injection. If you can’t test every two hours for a full 12-hour stretch, checking at the pre-insulin baseline and then every 3 to 4 hours until the next injection will still give your vet a useful approximation.
How Often to Test at Each Stage
The 2025 iCatCare consensus guidelines lay out a clear schedule based on where your cat is in treatment:
- Early treatment (first weeks to months): Do a glucose curve once a week. This is when your vet is still dialing in the right insulin dose, so frequent data matters.
- Stable diabetes: Once your cat’s blood sugar is well controlled and the dose hasn’t changed, reduce to a curve every 3 to 4 weeks.
- Remission: If your cat comes off insulin entirely (which happens in a meaningful number of diabetic cats), check blood sugar at least once a week and watch for returning symptoms like excessive thirst, hunger, or urination. Diabetes can relapse.
When to Test Immediately
Some situations call for an unscheduled blood sugar check right away. The biggest concern is hypoglycemia, when blood sugar drops dangerously low. Signs include weakness, wobbliness or collapse, disorientation, trembling, and in severe cases, seizures. If your cat shows any of these, test immediately. If you can’t test or the reading is very low, rub a small amount of corn syrup on your cat’s gums and contact your vet.
On the other end, if your cat suddenly starts drinking and urinating much more than usual, or seems ravenously hungry despite eating, those are signs blood sugar may be running high and the current dose isn’t keeping up. A spot check can help you give your vet useful information before an appointment.
Stress Can Distort Results
Cats are uniquely prone to stress hyperglycemia, and this is one of the main reasons home testing is so valuable. In a study measuring the effect of stress on blood sugar, cats that were stressed (in this case by bathing) saw their glucose nearly double, jumping from a baseline of 83 mg/dL to a mean peak of 162 mg/dL. The more the cat struggled, the higher the reading climbed.
This means a blood glucose reading taken at the vet’s office, where your cat is anxious and restrained, can be artificially inflated by 50% or more. A single high reading at the clinic doesn’t necessarily mean diabetes is poorly controlled. Home testing, where your cat is calm and in familiar surroundings, gives a much more accurate picture. If your cat gets very stressed even at home during testing, give them a few minutes to settle before pricking, and try to make the process as quick and low-key as possible.
Continuous Glucose Monitors
Many cat owners now use continuous glucose monitors (CGMs) like the FreeStyle Libre, a small sensor applied to the skin that reads glucose levels in the tissue beneath it. The sensor can stay on for up to 14 days, though in practice the median wearing time in cats is about 5 to 6 days, often because of the sensor falling off or the cat removing it.
CGMs are clinically accurate enough for managing feline diabetes. In validation studies, 100% of readings fell within acceptable clinical accuracy zones when compared to traditional blood glucose meters. The correlation between sensor readings and blood readings is strong (r = 0.93). The main limitation is a roughly 30-minute lag: the sensor measures glucose in tissue fluid rather than blood directly, so during rapid changes (like a sudden spike), the sensor can be slow to catch up. For day-to-day monitoring and curve generation, this lag rarely matters.
Once your cat is stable on insulin, a CGM can be applied continuously or used periodically, such as every 2 to 3 months, or whenever symptoms suggest something has changed. The big advantage is that you get hundreds of data points without repeatedly pricking your cat’s ear, which makes it far easier to spot patterns your vet can act on.
Practical Tips for Home Testing
Most home testing uses a small lancet to prick the outer edge of your cat’s ear, where a network of tiny blood vessels sits close to the surface. A warm cloth held against the ear for 30 seconds beforehand increases blood flow and makes it easier to get a usable drop. You need very little blood, just enough to touch to the test strip.
Keep a simple log with the time, the reading, when insulin was given, and any notes about your cat’s behavior or appetite. This record is far more useful to your vet than any single number. Patterns over days and weeks reveal whether insulin is working, whether the dose needs adjustment, and whether your cat is heading toward remission or needs a change in strategy.