Ketone levels measure how many ketone bodies are circulating in your blood, appearing in your urine, or present in your breath. A normal blood ketone level is below 0.6 mmol/L. Levels above that threshold indicate your body has shifted from burning primarily glucose to burning fat for fuel, a metabolic state that can be intentional (as with a ketogenic diet or fasting) or a warning sign of a serious complication in people with diabetes.
What Ketones Are and Why Your Body Makes Them
Ketones are molecules your liver produces from fat when glucose is scarce. This happens during fasting, prolonged exercise, very low-carb diets, or when insulin levels are too low to move glucose into cells. Your liver takes fatty acid fragments and combines them in a series of steps that ultimately produces two main ketone bodies: acetoacetate and beta-hydroxybutyrate. A third substance, acetone, forms when acetoacetate breaks down spontaneously. Acetone is a volatile waste product that your lungs exhale, which is why people in deep ketosis sometimes have a fruity or nail-polish-remover smell on their breath.
Once released into the bloodstream, beta-hydroxybutyrate and acetoacetate travel to your brain, muscles, and other organs, where cells convert them back into usable energy. This is a normal survival mechanism. Your brain, which normally runs almost entirely on glucose, can get a significant share of its energy from ketones when carbohydrate intake drops low enough. Beta-hydroxybutyrate is the most abundant of the three ketone bodies in the blood and is the one most blood meters measure.
Normal and Elevated Blood Ketone Ranges
Blood ketone testing gives the most precise snapshot of what’s happening in your body right now. The standard unit is millimoles per liter (mmol/L), and the ranges break down like this:
- Below 0.6 mmol/L: Normal. This is what most people on a standard diet will see. Your body is running primarily on glucose.
- 0.6 to 1.5 mmol/L: Slightly elevated. For someone on a ketogenic diet, this range indicates early or light ketosis. For someone with diabetes, the NHS recommends retesting in two hours.
- 1.6 to 3.0 mmol/L: Moderate to high. People intentionally following a strict ketogenic or therapeutic protocol may target this range. For people with diabetes, this level signals a real risk of diabetic ketoacidosis (DKA) and warrants contacting a care team immediately.
- Above 3.0 mmol/L: High. In a person with diabetes, this is a medical emergency that may indicate DKA, a condition where dangerously acidic blood can become life-threatening within hours.
The critical distinction is context. A blood ketone reading of 1.5 mmol/L in someone deliberately fasting or restricting carbs is a predictable metabolic response. The same reading in someone with type 1 diabetes who missed an insulin dose is a red flag. The number alone doesn’t tell the whole story.
How Long It Takes to Reach Ketosis
If you restrict carbohydrates to roughly 20 to 50 grams per day, it typically takes two to four days for blood ketone levels to cross the 0.5 mmol/L threshold that marks the beginning of nutritional ketosis. Some people take a week or longer, depending on how much glycogen (stored glucose) their muscles and liver are holding, their activity level, and individual metabolic differences. Fasting accelerates the timeline because it depletes glycogen stores faster than carb restriction alone.
Three Ways to Test Ketone Levels
Blood Meters
A finger-prick blood meter measures beta-hydroxybutyrate, the most abundant ketone body in circulation. This is the most accurate method and gives a real-time reading in mmol/L. The downside is cost: test strips typically run $1 to $2 each, and you need a new one every time. Blood testing is the standard used in clinical settings and is the method behind the mmol/L ranges listed above.
Urine Strips
Urine test strips detect acetoacetate, not beta-hydroxybutyrate. They’re inexpensive and easy to use: you dip a strip in urine and compare the color change to a chart. Results are reported as small (under 20 mg/dL), moderate (30 to 40 mg/dL), or large (above 80 mg/dL). The main limitation is that urine strips reflect what your body excreted over the past few hours rather than what’s in your blood right now. As your body becomes more efficient at using ketones, fewer spill into your urine, so the strips can actually show lower readings even as your blood ketone levels remain steady. This makes them less reliable for people who have been in ketosis for several weeks.
Breath Analyzers
Breath meters measure acetone in parts per million (ppm). Normal breath acetone sits between 0.3 and 1 ppm. Readings above roughly 4 ppm correlate with blood ketone levels at or above the 0.6 mmol/L threshold, though the correlation is imperfect. One study found that a breath analyzer cutoff of 3.9 ppm had 94.7% sensitivity for detecting ketosis but only 54.2% specificity, meaning it catches most cases of true ketosis but also produces a fair number of false positives. Breath meters have the advantage of no ongoing strip costs after the initial purchase, but they’re less precise than blood testing.
Why Your Readings Fluctuate Throughout the Day
Ketone levels are not static. They shift in response to what you eat, when you eat, how much you move, and even how hydrated you are. Morning readings tend to be higher because your body has been fasting overnight and relying on fat for energy while you sleep. A meal heavy in carbohydrates will suppress ketone production within a few hours, while a high-fat meal can push levels up.
Exercise adds another variable. Intense or prolonged workouts increase ketone production as your muscles burn through glucose stores and your liver ramps up fat metabolism to compensate. Harder workouts generally produce a bigger spike than easier ones, and endurance activities like running or cycling can keep levels elevated for longer than a short weightlifting session. If you want to see your peak post-exercise reading, testing right after a workout will capture it.
Hydration matters too, especially for urine testing. Drinking large amounts of water dilutes urine and can make strip readings appear lower than they would otherwise. Conversely, dehydration concentrates urine and can inflate the result. Blood meters are less affected by hydration status, which is another reason they’re considered the gold standard.
Ketosis vs. Diabetic Ketoacidosis
These two states involve the same molecules but operate on entirely different scales. Nutritional ketosis typically produces blood ketone levels between 0.5 and 3.0 mmol/L, with blood pH remaining normal. Your body regulates the process through insulin: even a small amount of circulating insulin keeps ketone production from spiraling out of control.
Diabetic ketoacidosis occurs when insulin is absent or severely insufficient, most commonly in type 1 diabetes but occasionally in type 2. Without insulin acting as a brake, the liver floods the bloodstream with ketones far faster than the body can use them. Blood becomes acidic, and ketone levels can climb well above 3.0 mmol/L. Symptoms include nausea, vomiting, abdominal pain, rapid breathing, fruity-smelling breath, and confusion. Blood ketone levels above 3.0 mmol/L in someone with diabetes, or urine strips reading 2+ or higher, are grounds for emergency medical care.
For people without diabetes, the risk of DKA is extremely low. The body’s normal insulin response prevents ketone production from reaching dangerous levels, even during extended fasting or strict carbohydrate restriction.