How to Measure Ketones: Blood vs Breath vs Urine
There are three ways to check ketosis. They do not measure the same molecule, and that explains most confusing readings.
Short answer: A blood ketone meter measuring beta-hydroxybutyrate is the most accurate home method.
The three methods
| Method | Measures | Accuracy | Running cost | Best for |
|---|---|---|---|---|
| Blood meter | Beta-hydroxybutyrate | Highest | A strip per test | Confirming ketosis, tracking GKI |
| Breath meter | Acetone | Moderate, varies by device | Low after purchase | Frequent checks, trends |
| Urine strips | Acetoacetate | Low after adaptation | Very low | First week only |
Blood meters
A finger-prick blood meter measures BHB, the main circulating ketone, and gives a number in mmol/L. Many meters also measure glucose, which lets you calculate the glucose ketone index. The main downside is the cost of ketone strips.
Breath meters
Breath devices estimate acetone. They are reusable and painless, so they suit frequent checks. Readings vary between brands, so compare a device with itself over time rather than with charts made for blood.
Urine strips
Strips turn colour with acetoacetate. They are cheap and useful in the first days, but as the body adapts, less acetoacetate reaches the urine, and hydration changes the concentration. A negative strip after a month does not mean you left ketosis.
Testing routine that gives useful data
- Test at the same time each day — morning, before food, is the most comparable.
- Avoid testing right after exercise, which can raise readings temporarily.
- Log meals and sleep with readings so you can spot patterns.
- Look at a weekly average rather than reacting to a single number.
Frequently asked questions
What is the most accurate way to test for ketosis?
A blood ketone meter measuring beta-hydroxybutyrate is the most accurate home method.
When is the best time to check ketones?
In the morning before eating gives the most consistent baseline. Readings often rise through the afternoon and evening.
This guide is general information for healthy adults, not medical advice. See our medical disclaimer.