What is the nitroprusside test? Why should I care?

Ketone bodies come in three flavors:

1. acetoacetic acid

2. acetone

3. B-hydroxybutyric acid

They are formed from the metabolism of fatty acids when there is a defect in carbohydrate metabolism, absorption, or an inadequate amount of carbohydrate in the diet. Ketone bodies are initially noted in the serum then urine.

The nitroprusside test - commonly used to detect ketones in urine and serum - detects acetoacetic acid and acetone, not B-hydroxybutyric acid. Because all three ketone bodies are generally present in the serum and urine under ketotic conditions, a test that indicates the presence of any one of these three ketone bodies is generally satisfactory.

However, in alcoholic ketoacidosis this assumption is not true. Under these circumstances, a large part of the acidosis is caused by B-hydroxybutyric acid. So although it's called alcoholic KETOacidosis, ketones may not be detected by the nitroprusside test.

Furthermore, as a patient is being treated for DKA, B-hydroxybutyric acid is converted to acetoacetic acid. So while the nitroprusside test will detect an increasing concentration of ketones, the total body ketosis is actually decreasing.

Source

McPherson, R. and Pincus, M. "Components of Basic Urinalysis." McPherson & Pincus: Henry's Clinical Diagnosis and Management by Laboratory Methods, 21st ed

Mattu A. et al. Avoiding Common Errors in the Emergency Department. 2010.