Breath and Blood Ketone Testing

Urgent safety note first

Ketones are normal in small amounts, but very high levels can make the blood too acidic. This is called ketoacidosis. The most common type is diabetic ketoacidosis (DKA), which is a medical emergency that often develops quickly and can be life-threatening. [1] If you have diabetes and suspect DKA, or you feel very unwell with vomiting, belly pain, confusion or trouble breathing, get emergency care (in the US, call 911). [1] Ketone testing for diabetes belongs with your diabetes care team, not with a consumer wellness plan. See Diabetes Medicines and Consumer Glucose Data.

What ketones are and what each test measures

When the body relies more on fat for fuel, it makes ketone bodies: beta-hydroxybutyrate (BHB), acetoacetate and acetone. [2] Three kinds of test are in use, and they do not measure the same molecule.

1. Blood meter

   Used in clinical care to identify or resolve DKA. [2]

   What it measures directly: BHB in a fingertip blood sample

2. Urine strip

   A standard clinical test. [2]

   What it measures directly: Acetoacetate in urine

3. Breath device

   An indirect, less invasive signal that is compared against blood BHB in studies. [3]

   What it measures directly: Acetone in exhaled breath

The word "ketone meter" is used for all of these, which makes comparisons confusing. A breath number is not a blood number, and the two are not interchangeable.

How well do breath and blood readings agree?

The evidence labels used on this page are explained at the end of this section. Overall, the evidence for breath acetone devices as a stand-in for blood ketones is promising but limited.

• A review of breath acetone sensing technology concluded that data on current devices is lacking, and that where human testing exists, sample sizes are very small. [3]
• In a 14-day study of 21 people following either a low-carbohydrate/ketogenic or a standard diet, one portable breath acetone meter predicted whether blood BHB was above several cut-offs fairly well. Single paired readings were only moderately correlated (R squared 0.57). [4] This is a result for that one device in that group, not a general rule for breath devices.
• The same study found breath and blood levels each varied by about 44% and 46% across a day, and the authors concluded that single readings are often not enough to judge daily ketone exposure. [4]
• In 19 adults and children with type 1 diabetes, a breath analyzer agreed with a blood meter in non-fasting adults but not in children. At the best cut-off it caught most ketosis but called many readings positive that were not (sensitivity 94.7%, specificity 54.2%). [5]
• A review of breath acetone as a marker notes that personal factors such as age, weight, food, medication and other things have to be taken into account when reading concentrations. [6]

Blood meters have been compared with each other, too. In a lab study of 13 young adults, two blood meters agreed well on ketone readings, with the second meter identifying a nutritional-ketosis threshold well (area under the curve 0.913). [7] That tells us the two meters agree; it does not tell us a meter is useful for health decisions.

Evidence labels used here: established means supported by consistent, good-quality human research; promising but limited means encouraging findings from small or early human studies; insufficient evidence means too little to judge.

A ketone reading is not a health score

There is no single "good" ketone number for everyone. A higher reading shows the body is using more fat for fuel. It does not show that a person is healthier, and a biomarker change is not the same as a health outcome. Ketone levels on a ketogenic diet are higher but are usually not high enough to be ketoacidosis, yet ketoacidosis is still possible on such a diet, so MedlinePlus advises talking with a provider about whether it is safe. [1]

This library does not recommend or discourage any eating pattern, and it gives no advice on aiming for ketosis. It only explains what the tests can and cannot show.

How to judge a ketone device claim

1. Ask what is measured: BHB, acetoacetate or acetone.
2. Ask which model was tested, in whom, against what reference, and what the error was. Accuracy in one model or group does not carry over to another.
3. Separate accuracy from benefit. Even a perfectly accurate meter has not been shown to improve health.
4. Be cautious of marketing that treats a high reading as a goal. See How to Evaluate a Health Device and Nutrition Tracking and Continuous Glucose Monitors.