Acetoacetate

The normal reference range for quantitative acetoacetate assays measured by gas chromatography is:

• Normal range (plasma/serum): 5-30 µg/mL (conventional units)

Semiquantitative methods, although nonspecific for acetoacetate, are frequently used to determine the presence of ketone bodies (acetone and acetoacetate) in urine and blood and are discussed in Ketones.

Ketone bodies are constantly produced in insignificant amounts by the liver. When the rate of synthesis of acetoacetate and other ketone bodies exceeds the rate of utilization, their concentration in blood increases, a phenomenon known as ketonemia.

Extreme accumulation of acetoacetate and other ketones can lead to metabolic acidosis defined as ketoacidosis. ^[1] High levels of acetoacetate in blood may result from the following:

• Decreased availability of carbohydrates (eg, starvation, alcoholism)

• Abnormal use of carbohydrates storage (eg, uncontrolled diabetes, glycogen storage diseases)

Diabetic ketoacidosis and alcoholic ketoacidosis are by far the most common forms of elevated acetoacetate level and ketosis. 

Either for semiquantitative or quantitative assessments, the collection details are as follows:

• Collection method: Venipuncture

• Specimen type: Plasma or serum

• Specimen volume: 3 mL (ideal volume)/1.2 mL (minimum volume)

• Container: For plasma, draw blood in a lavender (EDTA) or pink-top tube ^[2] and centrifugate to yield 3 mL of plasma; for serum, draw blood in a red-top tube ^[2] and centrifugate to yield 3 mL of serum

• Keep refrigerated or frozen (preferred) up to 4 days

• Reject heparinized or warm specimens

• Hemolyzed, lipemic, or icteric specimens might be accepted

The underlying metabolic mechanism behind elevated acetoacetate levels might be further assessed by the following related tests:

• Blood glucose value

• Metabolic panel

• Renal function studies

• Arterial blood gas testing

• Urine ketones value

• Serum β-hydroxybutyrate value

• Serum osmolality

• Serum bicarbonate value

Description

There are three ketone bodies produced by the body: acetone, acetoacetate, and β-hydroxybutyrate. Acetoacetate, also termed acetoacetic acid, is a weak β-keto acid produced from acetyl-CoA in the mitochondrial matrix of hepatocytes. Acetyl-CoA is also a precursor of cholesterol and fatty acid synthesis, or it can enter the citric acid cycle to be oxidized to CO₂ and H₂O and generate energy. 

When there is decreased availability of glucose or impaired glucose metabolism, such as in diabetes mellitus, the acetyl-CoA is diverted to produce more acetoacetate, from which acetone and β-hydroxybutyrate are formed. Since acetoacetate and β-hydroxybutyrate are water soluble, they can circulate from the liver to other tissues, where both compounds are reconverted to acetyl-CoA to produce energy through the citric acid cycle.

Under physiologic conditions, only the myocardium uses fatty acids preferentially for energy supply, and most other tissues metabolize glucose to meet their energy demands. Under ketotic conditions, when the availability of carbohydrates is reduced, the heart and the brain can successfully use ketone bodies as an energy source. ^[3]

Three days after an individual undergoes a ketogenic insult (eg, starvation, diabetic ketoacidosis, alcoholic ketoacidosis), the brain receives up to 25% of its energy from ketone bodies. This increases to 70% after 4-5 days, ^[3] which represents the main metabolic contribution of acetoacetate.

Indications/applications

Testing of serum acetoacetate is indicated in the diagnosis of ketoacidosis, mainly in suspected diabetic ketoacidosis and alcohol-induced ketoacidosis.

The triad of hyperglycemia, anion gap metabolic acidosis, and ketonemia characterizes diabetic ketoacidosis. ^[4] Although metabolic acidosis is often the major finding in diabetic ketoacidosis, ketonemia may be challenging to prove. For β-hydroxybutyrate, the ketone body predominately elevated in these cases, levels are laborious and expensive to determine; hence, acetoacetate levels in blood may be used as an indirect reflection of the severity of acidosis, especially in cases of weakly positive semiquantitative determinations.

Alcoholic ketoacidosis is characterized by metabolic acidosis with an elevated anion gap, elevated serum ketone levels, and a normal or low glucose concentration ^[5] in the setting of excessive alcohol intake. Serum acetoacetate constitutes the most widely available and easily measured ketone body in most hospitals.

Acetoacetate level also plays a role in differentiating insulin-induced hypoglycemia from hypoglycemia secondary to limited carbohydrate intake. Only in the latter, the counterregulatory catabolic pathways resulting from starvation lead to ketogenesis and high acetoacetate levels.

Considerations

Pregnancy and fasting represent two physiologic conditions associated with elevated levels of acetoacetate without acidosis and do not necessarily represent evidence of disease.

While not always excreted in proportion to blood ketone concentrations, urine ketones can be used for monitoring of control in patients with type 1 diabetes mellitus during acute illness, stress, pregnancy, or persistent hyperglycemia. ^[6]

Serial measurements of acetoacetate are not clinically useful to assess adequacy of diabetic ketoacidosis treatment because as diabetic ketoacidosis is treated, β-hydroxybutyrate is converted to acetoacetate and ketoacidosis seems to temporarily worsen. ^[6]

A study by Inaba et al suggested that in patients with diabetes mellitus who are on hemodialysis, a link may exist between a low arterial acetoacetate/β-hydroxybutyrate ratio and increased mortality. According to the investigators, the low ratio appears to be associated with reduced serum albumin and uric acid levels, which are themselves indicative of poor nutritional status. ^[7]