What is a normal level of ketones in the urine?

Normal Levels of Ketones in Urine

Normal urine ketone levels are negative to trace amounts (<0.5 mmol/L), with positive readings classified as + (0.5 mmol/L), ++ (0.7 mmol/L), and +++ (~3 mmol/L). 1

Understanding Ketone Bodies

Ketone bodies are catabolic products of free fatty acids that are normally present in urine and blood but in very low concentrations. The three main ketone bodies are:

• Acetoacetate (AcAc)
• β-hydroxybutyrate (bOHB)
• Acetone (present in smaller quantities)

Under normal physiological conditions, total serum ketones should be less than 0.5 mmol/L 2. Urine ketone tests primarily measure acetoacetate rather than β-hydroxybutyrate, which is the predominant ketone body in blood during states of ketosis 3.

Interpretation of Urine Ketone Levels

Urine ketone levels are typically reported semi-quantitatively as:

• Negative: Normal finding
• Trace: Minimal elevation
• • (~0.5 mmol/L): Mild elevation
• ++ (~0.7 mmol/L): Moderate elevation
• +++ (~3 mmol/L): Significant elevation 1, 4

When Positive Ketones May Be Normal

It's important to note that positive urine ketone readings can be found in:

• Normal individuals during fasting
• Up to 30% of morning urine samples from pregnant women 1
• People following very-low-carbohydrate (ketogenic) diets 2
• After prolonged exercise 5

Measurement Considerations

Time of Day Variations

Ketone levels in urine fluctuate throughout the day. Research shows that:

• The highest levels of ketonuria in individuals with stable ketosis are typically found in early morning and post-dinner urine samples
• The highest detection rates (>90%) for ketosis occur at 07:00,22:00, and 03:00 6

Measurement Methods

Semiquantitative test-strip methods that utilize specific reactions for ketones are recommended for urine testing 2. These strips are moistened with freshly voided urine and after a short period (typically 10 seconds), the color is compared with a chart.

Clinical Significance

Elevated ketone levels may indicate:

1. Diabetic Ketoacidosis (DKA): Particularly when accompanied by hyperglycemia and symptoms like abdominal pain or nausea 2, 1
2. Nutritional Ketosis: In individuals following ketogenic diets
3. Starvation Ketosis: During prolonged fasting

Limitations of Urine Ketone Testing

Urine ketone tests have several limitations:

• They primarily measure acetoacetate, not β-hydroxybutyrate (the predominant ketone in DKA)
• They can give false-positive results with certain medications
• False-negative results can occur with prolonged exposure to air or acidic urine samples 1
• As DKA resolves, urine ketone levels might increase even as the condition improves, due to the conversion of β-hydroxybutyrate to acetoacetate 3

Blood vs. Urine Ketone Testing

Blood ketone testing (measuring β-hydroxybutyrate) is more accurate and reliable than urine ketone testing for diagnosing and monitoring DKA 1. Blood ketone levels are classified as:

• <0.5 mmol/L: Normal
• 0.5-1.5 mmol/L: Mild elevation
• 1.5-3.0 mmol/L: Moderate elevation

• 3.0 mmol/L: Dangerous level

• 7.0-8.0 mmol/L: Severe ketoacidosis 1

However, urine ketone testing remains useful for routine screening due to its convenience and lower cost.