Fasting and Ketones in Urine

Yes, longer hours of fasting can cause ketones to appear in urine on lab work. 1 This is a normal physiological response to fasting conditions.

How Fasting Leads to Ketones in Urine

Ketone bodies are normally present in urine and blood but usually at concentrations below detection limits of routine testing methods 1
During fasting, the body shifts from using glucose as its primary energy source to utilizing stored fat, which produces ketone bodies 1
Positive ketone readings can be found in normal individuals during fasting, representing a normal physiological response 1
The longer the fast, the higher the ketone levels may become, with ketone levels typically rising markedly during the early phase of fasting and reaching a plateau after about 5 days 2

Ketone levels follow a diurnal pattern, with the lowest blood β-hydroxybutyrate (BHB) and urine acetoacetate concentrations typically measured in the morning (around 10:00) 3
The highest ketone concentrations are often noted during the night (around 03:00) 3
In subjects with stable ketosis, ketonuria is highest and most reliably detected in early morning and post-dinner urine samples 3

1+ ketones in urine indicates mild ketosis, which is generally not concerning in the context of fasting in non-diabetic individuals 1
Prolonged fasting (beyond several days) can lead to more significant ketosis and potentially starvation ketoacidosis in extreme cases 4
In diabetic patients, especially those with type 1 diabetes, elevated ketones may indicate insufficient insulin and potential diabetic ketoacidosis (DKA), which requires medical attention 1, 5

The presence of ketones in urine due to fasting differs from diabetic ketoacidosis, which is a medical emergency 1
Urinary tract infections do not typically cause false positive ketone readings in urinalysis 6
False-positive urine ketone results can occur with:
- Highly colored urine 1
- Several sulfhydryl drugs, including the antihypertensive drug captopril 1
False-negative readings can occur when:
- Test strips have been exposed to air for an extended period 1
- Urine specimens are highly acidic 1

Blood ketone testing is preferred over urine ketone testing as it measures β-hydroxybutyrate directly, which is the predominant ketone body in metabolic ketosis 1, 7
Urine ketone tests using nitroprusside-containing reagents only detect acetoacetate and not β-hydroxybutyrate, which may underestimate total ketone body concentration 5, 7
As ketosis resolves, β-hydroxybutyrate is oxidized to acetoacetate, which is the predominant ketone in urine, creating a potential lag in measurement accuracy 7

For non-diabetic individuals, finding ketones in urine after fasting is generally not concerning and represents a normal physiological response 1
For diabetic patients, even mild ketosis requires monitoring and potential intervention 1, 5
Patients with type 1 diabetes should test for ketones during acute illness, stress, or fasting 1
Individuals treated with SGLT2 inhibitors are at increased risk for DKA, even with relatively normal blood glucose levels (euglycemic DKA) 5