Ketones in urine levels: what trace to large means, the diet, fasting and pregnancy causes, and when ketones signal ketoacidosis

When glucose is scarce or cannot get into cells, the liver converts fatty acids into three ketone bodies — beta-hydroxybutyrate, acetoacetate and acetone — that the brain, heart and muscles can burn instead. The urine dipstick detects acetoacetate (and acetone weakly) with a nitroprusside reaction and grades it negative, trace (about 5 mg/dL), small (15), moderate (40) and large (80–160 mg/dL).

It does not detect beta-hydroxybutyrate, which is the dominant ketone early in diabetic ketoacidosis. That creates two traps: the urine can under-read at the start of ketoacidosis, and it reads higher during recovery as beta-hydroxybutyrate is converted back to acetoacetate, so urine ketones lag behind the true picture in both directions. Blood ketone meters measure beta-hydroxybutyrate directly and are what diabetes guidelines now recommend for sick-day monitoring.

The urine value also depends on how concentrated the urine is and on timing — it reflects the hours since the bladder last emptied rather than the moment of the test — so it is a screening result that is confirmed, when it matters, with a blood glucose, a blood ketone and a metabolic panel.

People with type 1 diabetes, and anyone with diabetes taking an SGLT2 inhibitor (empagliflozin, dapagliflozin, canagliflozin, ertugliflozin), should check ketones whenever they are ill, vomiting, unable to eat, or have a blood glucose persistently above 250 mg/dL — and on SGLT2 inhibitors even when glucose is normal, because those drugs can cause euglycemic ketoacidosis. Children with diabetes are checked at any illness.

Pregnant women with severe nausea or hyperemesis are checked for ketones as a marker of starvation and dehydration; people on ketogenic diets sometimes test to confirm nutritional ketosis, although urine strips are an unreliable guide once the body adapts; and ketones appear on routine urinalyses after an overnight fast or a hard workout, where they usually need no action at all.

Urine ketones are part of a complete urinalysis, which can be ordered online through TestWell and collected as a walk-in urine sample at a Labcorp patient service center (the urinalysis with reflex to culture is also offered at Quest). Over-the-counter ketone strips give the same dipstick reading at home, and a blood ketone meter gives the beta-hydroxybutyrate level that diabetes care relies on.

For a routine check, collect a clean-catch sample when you have eaten normally in the previous 12 hours; a fasting or post-workout sample will show ketones for ordinary reasons. Use a fresh sample — acetoacetate breaks down and acetone evaporates, so a sample left standing reads falsely low — and tell the lab about captopril, mesna, N-acetylcysteine, levodopa or valproate, which can produce false positives, and about any low-carbohydrate diet.

Negative. Trace to small ketones after an overnight fast, in the morning in pregnancy, after prolonged exercise or on a carbohydrate-restricted diet are expected and are not a disease. Moderate or large ketones should be explained: a long fast or vomiting illness in someone without diabetes, or ketoacidosis until proven otherwise in someone with diabetes.

The difference between nutritional ketosis and ketoacidosis is acid: blood beta-hydroxybutyrate runs 0.5–3 mmol/L in a well person on a ketogenic diet with a normal bicarbonate and pH, whereas diabetic ketoacidosis is defined by ketones with a bicarbonate below 18 mmol/L, a pH below 7.3 and a raised anion gap, usually with glucose above 250 mg/dL. Insulin keeps ketone production in check, so without diabetes the body rarely makes enough ketones to turn the blood acidic — alcoholic ketoacidosis after a binge with vomiting and starvation is the exception.

The everyday causes: fasting for more than 12–18 hours, a ketogenic or very-low-carbohydrate diet, prolonged or intense exercise, pregnancy (where fasting ketosis develops faster), vomiting or diarrhea with poor intake, crash dieting and eating disorders, acute illness with fever, and heavy alcohol use. In children, any febrile illness with reduced eating produces ketones readily.

The dangerous cause is diabetic ketoacidosis — insulin deficiency in type 1 diabetes (a missed dose, a pump failure, a new diagnosis), severe stress or infection in type 2 diabetes, and the euglycemic form on SGLT2 inhibitors where the glucose can be near normal. Alcoholic ketoacidosis and, rarely, isopropyl alcohol ingestion (which produces acetone) and some inborn errors of metabolism complete the list. Drugs can mimic ketones on the strip without any ketones being present — captopril, mesna, N-acetylcysteine, levodopa, valproate metabolites and strongly pigmented urine.

Start with the glucose. In someone with diabetes, moderate or large ketones with a glucose above 250 mg/dL — or any ketones with nausea, vomiting, abdominal pain, rapid breathing or drowsiness — means a blood ketone (beta-hydroxybutyrate), a basic metabolic panel for bicarbonate and the anion gap, and same-day or emergency care; follow your sick-day plan with extra insulin and fluids as instructed in the meantime. On an SGLT2 inhibitor, ketones in an unwell person are an emergency regardless of glucose. Without diabetes, eat and hydrate and repeat: small ketones from fasting, exercise or diet clear within hours of a carbohydrate-containing meal. Pregnant women with persistent ketones who cannot keep fluids down need assessment for hyperemesis and intravenous fluids. If ketones persist in someone eating normally, check glucose, hemoglobin A1c and a metabolic panel.