How Ketones Are Measured

Two different measurements

Ketones are recorded two ways, and they are not interchangeable.

Breath acetone

A handheld breath ketone meter reads acetone in exhaled air, in parts per million. Acetone is a by-product of ketone metabolism, so breath readings track ketosis indirectly and with a lag. Taken after a slow full exhale, several hours since any food or drink other than water, and never straight after brushing teeth or drinking alcohol.

Blood beta-hydroxybutyrate

A finger-prick meter reads blood BHB directly, in millimoles per litre. This is the reference measurement: it is what research uses and what the body is actually running on. It is used less often because it costs a strip and a lancet each time.

Timing

Readings are taken at roughly the same point in a fasting day where possible — usually late afternoon or evening, when a full day without food has had time to show. A morning reading and an evening reading on the same day can differ substantially, so the time is noted with the value.

What can distort a reading

• Recent exercise, which can raise ketones sharply
• Hydration state, which affects breath readings more than blood
• Alcohol, which interferes with breath acetone sensors
• Meter and strip variation — a repeat reading on the same drop of blood is not always identical

What this measurement cannot tell you

A single reading is close to meaningless on its own. What matters is the shape of the series across a year, read alongside the fasting schedule, sleep, training and the notes for that week. Nothing on this site is presented as evidence that fasting caused a change: this is one person, with no control condition, and many things change over a year besides the fasting days.