Blood ketones

Most laboratories and finger-prick meters print an upper limit of 0.6 mmol/L; levels are higher than this in the general population after prolonged fasting or on a very low carbohydrate diet without any illness.

Ketones are chemicals the liver makes from fat when the body cannot use glucose for energy, either because there is very little glucose available or because there is not enough insulin to let glucose into the cells. The test measures the main ketone in blood, beta-hydroxybutyrate. Small amounts are normal after a long gap without food.

Doctors request it, usually by finger-prick, in people with diabetes who are unwell or whose blood sugar is high, to check whether the body has switched to burning fat because of a lack of insulin. It is also used to follow people on very low carbohydrate diets.

• Fasting for many hours, particularly overnight and in pregnancy
• A very low carbohydrate diet
• A shortage of insulin in people with diabetes, especially during illness
• Heavy alcohol intake with little food
• Prolonged vomiting
• Hard, prolonged exercise
• Some newer diabetes tablets, the SGLT2 inhibitors

• Eating carbohydrate
• Insulin
• Recovery from the illness that caused them to rise

• A long gap since eating, or a deliberate low carbohydrate diet, raises it without any illness
• Finger-prick meters and laboratory methods give slightly different figures

Usually a finger-prick sample read on a meter, so the time and whether the person had eaten matter. A single value is read alongside the glucose taken at the same time.

Written by a UK consultant physician. Typical UK adult reference ranges; your laboratory's own printed range takes precedence.