When to Check Ketones, and What the Levels Actually Mean

You're 287 and it won't come down. The correction you took two hours ago did nothing, you feel vaguely awful, and somewhere in your memory a nurse from diagnosis week says "check your ketones when you're high." Then the memory ends, because nobody ever explained what that means, what to check with, or what the result should make you do. This guide is the missing explanation. It's education, and the numbers here are the standard published ones; the plan that governs your actions is the one your care team wrote for you.

What are ketones, and why do they matter in diabetes?

Ketones are what your body produces when it burns fat for fuel. That happens normally in small amounts, overnight or when you haven't eaten. The problem in diabetes is scale and cause: when there isn't enough insulin in your body, your cells can't use the glucose in your blood, so your body starts burning fat hard, and ketones pour out faster than you can clear them. At scale they turn your blood acidic, which is diabetic ketoacidosis. So a ketone check answers a specific question a glucose meter can't: is there enough insulin working in my body right now? A high glucose with negative ketones and a high glucose with large ketones are two very different situations.

This is a different thing from the keto diet. Nutritional ketosis in a person without diabetes is mild, capped, and controlled, because their pancreas still makes insulin, which keeps ketone production in check. DKA is what happens when that brake is missing. If you have type 1 and you've read that ketones are trendy, the trend does not apply to you; carbohydrate restriction for anyone on insulin is a conversation to have with your care team, never a solo experiment.

When should you check ketones?

• Blood sugar persistently high. The ADA's common trigger is above 240 mg/dL, especially when it stays there through a correction or shows up in more than one reading. Your care team may have given you a different number; theirs wins.
• Any illness or vomiting, if you have type 1. Check even if your glucose looks normal. Illness raises stress hormones and can drive ketones on its own, which is why every sick-day plan includes ketone checks every few hours while you're sick.
• Symptoms. Nausea, stomach pain, unusual thirst, fruity-smelling breath, deep or fast breathing, or feeling foggy and exhausted for no reason. Check first, explain later.
• On a pump, after any unexplained high. A kinked cannula, a leaking site, or a failed pod cuts off your entire insulin supply, and with no long-acting insulin on board, ketones can build within hours. An unexplained high on a pump is a check-ketones-now event, per standard pump training.

Urine strips or a blood ketone meter?

• Urine strips are cheap (often under $15 for 50 to 100), sold over the counter at any pharmacy, and simple: dip, wait the labeled seconds, compare the pad's color to the chart on the vial. The catch is timing: urine ketones reflect what your body was doing hours ago, because the urine collected in your bladder over that time.
• Blood ketone meters work like a glucose meter with special strips and measure ketones in your blood at that moment, which is why they're the tool care teams prefer during illness, when things change fast. The meters are inexpensive; the strips cost more per test than urine strips.
• Check the expiration. Urine strips expire, and most brands also expire a few months after the vial is first opened. Expired strips read falsely low or negative, which is exactly the lie you cannot afford. Write the open date on the vial.
• Either beats nothing. If the choice tonight is a urine strip from the pharmacy down the street or no data, take the strip.

What do the results mean?

• Negative or trace. Insulin is doing its job on this front. If glucose is still high, that's its own problem to manage per your plan, but DKA is not the immediate story.
• Small. Generally the fluids-and-recheck zone: water, insulin as your care team's existing plan directs, and another check in a few hours. Your plan's specifics govern; if you don't have written specifics, that's a call to ask for them.
• Moderate or large. Act on your care team's sick-day plan now and call them. If you're also vomiting or can't keep fluids down, this is the call-or-ER decision from the callout above; DKA develops fast and is treated in a hospital. (If it ends in an ER visit, the DKA bill guide is for the aftermath.)
• Blood meters report a number in mmol/L instead of a color. The cutoffs your meter's manual and your care plan give you map onto the same idea: low numbers mean recheck, higher numbers mean act and call.

The SGLT2 caveat: DKA with a normal-looking glucose

One drug class breaks the "only check when high" rule. SGLT2 inhibitors (empagliflozin/Jardiance, dapagliflozin/Farxiga, canagliflozin/Invokana) lower glucose by sending it out in urine, and the FDA has warned that they're associated with DKA at near-normal blood sugar, so the usual early-warning high never shows up. If you take one and feel the symptoms (nausea, vomiting, stomach pain, unusual fatigue), check ketones even with a fine-looking glucose reading, and call your care team. Your prescriber will have given specific guidance, often including when to pause the drug around illness, fasting, or surgery; that guidance is theirs to give, so follow the version they gave you. While you're thinking about it, do the two-minute prep: unexpired strips or a blood meter in the cabinet, and your sick-day plan on paper where you'd actually find it at 2am.