Beta-Hydroxybutyrate

The reference range is less than 0.4-0.5 mmol/L. ^{[1, 2, 3]} Levels of more than 1 mmol/L require further action, whereas levels of more than 3 mmol/L require immediate medical review. ^[4]

Elevated serum β-hydroxybutyrate levels can be observed in various conditions associated with metabolic substrate use disorders, insulin deficiency, and altered redox status, including the following ^{[2, 5, 6]} :

• Diabetic ketoacidosis: Ketone body production is stimulated by dehydration and insulin deficiency. Levels are usually more than 3 mmol/L.

• Alcoholic ketoacidosis: Ketone body production is stimulated by altered redox status within the liver mitochondria.

• High fat diet

• Steroid or growth hormone deficiency

• Salicylate poisoning

• Fasting and starvation: Serum β-hydroxybutyrate levels are increased after approximately 3 days, rising to a plateau after 4 weeks of food deprivation. ^[2]

• Lactation: Ketone body production is stimulated by the high-fat content of milk. ^[2]

• Ketogenic diets: These diets are popular for the control of refractory seizures and body weight in obese individuals. ^[2]

• Glycogen-storage diseases and other metabolic disorders

A prospective study by Flores-Guerrero et al indicated that high plasma levels of β-hydroxybutyrate signal an increased risk of heart failure with reduced ejection fraction (HFrEF), especially in females. In terms of incident HF (which in these results was primarily due to HFrEF), the hazard ratio per one standard deviation increase in the β-hydroxybutyrate concentration was 1.40. More specifically, in women, one standard deviation was associated with a hazard ratio for HFrEF of 1.73, compared with 1.14 in men. ^[7]

See the list below:

• Specimen type: β-hydroxybutyrate levels can be measured in serum. Point-of-care devices that can measure β-hydroxybutyrate levels in a single drop of blood within 30 seconds have been developed. Most methods for β- hydroxybutyrate estimation are rapid enzymatic assays that can be performed in the hospital or the office. ^[2]

• Container: A red-top Vacutainer is used.

• Collection method: For serum β-hydroxybutyrate, venipuncture is used. For point-of-care β-hydroxybutyrate, a drop of capillary blood is sufficient.

• Specimen volume: Obtain 0.5 mL for serum examination. The volume required for the point-of-care test is 5-10 μL.

• Determination method: The usual method for β-hydroxybutyrate determination in blood or serum involves the use of β-hydroxybutyrate dehydrogenase and a redox mediator. Serum β-hydroxybutyrate reacts with the enzyme, producing NADH from NAD+. The redox mediator transforms NADH back to NAD+. This is accompanied by a change in the electrochemical properties of the solution, with the current generated correlating with β-hydroxybutyrate levels. ^[1]

• Point-of-care versus serum β-hydroxybutyrate: In numerous studies, point of care devices have provided accurate results compared with the criterion-standard laboratory method, within the range determined by the manufacturer (usually 2-6 mmol/L). ^{[1, 2, 8, 9, 10, 11, 12]} They tend to lose their precision near the edge of their range (>5 mmol/L), but this usually occurs at concentrations higher than those used to guide clinical judgment. ^[9] In one study, point-of-care β-hydroxybutyrate meters tended to slightly overestimate the concentration, without resulting in any clinically significant consequences. ^[8] Concern has been expressed about the effects of serum acetoacetate on serum β-hydroxybutyrate determination ^[13] ; however, in studies using state-of-the art equipment, this effect seems to be too small to be clinically significant. ^[9]

Related tests are as follows:

• Serum ketones

• Serum acetoacetate

• Plasma glucose

• ABG

• Urine ketones

β-hydroxybutyrate is one of the ketone bodies. See the image below.

The term ketone body describes any of 3 molecules: acetoacetate, β-hydroxybutyrate, or acetone. ^[2] Acetoacetate is produced by acetyl-CoA metabolism, β-hydroxybutyrate is the result of acetoacetate reduction, and acetone is produced by the spontaneous decarboxylation of acetoacetate. ^[2] Ketone bodies are fundamental for metabolic homeostasis during periods of prolonged starvation. ^[2] The brain cannot use fatty acids for energy production and usually depends on glucose to meet its metabolic needs. In cases of fasting or starvation, ketone bodies become a major fuel for brain cells, sparing amino acids from being catabolized to gluconeogenesis precursors to be used to supply the brain with energy. ^{[2, 14, 15]} After prolonged  starvation, ketone bodies can provide as much as two thirds of the brain's energy needs. ^[2]

Ketone bodies are strong organic acids that fully dissociate in blood. When ketone body production becomes uncontrollable, the buffering systems are saturated, and blood pH drops; this is a condition known as ketoacidosis. ^{[2, 16]} The two common clinical scenarios for ketoacidosis are diabetic ketoacidosis and alcoholic ketoacidosis.

Diabetic ketoacidosis

The most clinically relevant application of β-hydroxybutyrate determination involves the diagnosis, management, and monitoring of diabetic ketoacidosis. During states of insulin deficiency, lipolysis at the adipose tissue (stimulated by insulin deficiency) provides a huge fatty acid load to the liver. ^[2] Fatty acids are initially metabolized to acetyl-coenzyme A that cannot enter the citric acid cycle in the mitochondria due to oxaloacetate deficiency. Thus, acetyl-coenzyme A is diverted to ketone body production through the activity of several enzymes, producing acetoacetate. Acetoacetate is then reduced to 3-β-hydroxybutyrate by 3-β-hydroxybutyrate dehydrogenase.

The ratio of acetoacetate to 3-β-hydroxybutyrate depends on the redox status in the liver mitochondria (ie, the NAD+/NADH ratio). ^[2] Under normal circumstances, the β-hydroxybutyrate to acetoacetate ratio is around 1; however, in diabetic ketoacidosis, this may increase to 7-10. ^[2] Acetone is produced by the spontaneous decarboxylation of acetoacetate. ^[2]

Traditionally, the diagnosis of diabetic ketoacidosis was based on the detection of ketones in urine using the Legal reaction, during which acetoacetate reacts in the presence of alkali with nitroprusside to produce a purple-colored complex on a test strip. ^[2] However, this method has significant drawbacks. It is semiquantitative and not equally sensitive for urine and blood. ^[2]

Moreover, not all the patients with diabetic ketoacidosis are able to provide a urine sample upon presentation, and ketones in urine are not a precise estimation of blood ketones. ^[2] Most importantly, the most abundant ketone body during diabetic ketoacidosis is β-hydroxybutyrate, with a concentration 3-10 times higher of that of acetoacetate. As diabetic ketoacidosis is treated, serum β-hydroxybutyrate is transformed to acetoacetate due to the correction of the mitochondrial redox status, elevating urine acetoacetate levels and giving the false impression that the patient has not responded to treatment. ^[2]

Lastly, urine ketone strips can give false positive results in patients receiving drugs with sulfhydryl groups and false-negative results when they have been exposed to air for a long period of time or when the urine is acidic. ^[2] These disadvantages necessitate the evolution of a more reliable method for the diagnosis and management of diabetic ketoacidosis.

A significant number of studies have evaluated the ability of point-of-care β-hydroxybutyrate to detect patients with diabetic ketoacidosis. In various studies, the cut-off value for diabetic ketoacidosis diagnosis ranges from 1.5-3.5 mmol/L, and the blood volume necessary for β-hydroxybutyrate measurement is 5-10 μL. ^{[17, 18, 19]}

A β-hydroxybutyrate level of more than 1.5 mmol/L had sensitivity ranging from 98-100% and specificity ranging from 78.6-93.3% for the diagnosis of diabetic ketoacidosis in diabetic patients presenting to the ED with blood glucose levels of more than 250mg/dL. ^{[17, 18, 19]} In 2 other large studies, a cut-off value of 3 mmol/L in patients presenting to the ED with hyperglycemia had sensitivity of almost 100% and specificity of 92.89-94% for the diagnosis of diabetic ketoacidosis. ^{[10, 20]} Although most manufacturers propose a threshold of 1.5 mmol/L, authors have proposed an increase of the cut-off value to 2 mmol/L or even 3.5 mmol/L to maximize the diagnostic yield of the test. ^{[18, 19]}

Numerous studies have demonstrated the superiority of blood β-hydroxybutyrate versus urine ketones in the patient with diabetes and hyperglycemia with possible diabetic ketoacidosis. ^{[2, 10, 17, 18, 19, 20]} The point-of-care β-hydroxybutyrate test is much faster than the classic urine ketone test, can be easily obtained on arrival, and does not depend on the patient producing urine. Whereas the sensitivity of urine ketones is similar to that of β-hydroxybutyrate, the latter has been persistently shown to be more specific for both the 1.5 mmol/L and 3 mmol/L endpoints and also has a greater positive predictive value. ^{[10, 19, 20]}

As far as treatment monitoring is concerned, β-hydroxybutyrate concentrations correlate more strongly compared with acetoacetate with anion gap, pCO2, and anion gap in patients with diabetic ketoacidosis. ^[21] In a pediatric study, the use of a β-hydroxybutyrate end-point for diabetic ketoacidosis resolution (< 1 mmol/L) has led to earlier ICU discharge (17 h vs 28 h) compared with standard management using urine samples. ^[11] In a similar, medico-economic investigation, the use of a β-hydroxybutyrate end-point in children with diabetic ketoacidosis resulted in a 6.5-hour reduction in ICU stay duration, 375 less laboratory investigations per patient, and an accumulative decrease of 2,950 Euros per patient without compromising patient safety. ^[22]

In a pediatric study, routine implementation of blood β-hydroxybutyrate monitoring for the management of sick days and impedance of ketoacidosis decreased the need for hospitalization compared with the usual practice of urine ketone measurement. ^[23]

Alcoholic ketoacidosis

This is the second most common cause of ketoacidosis, although significantly less common than diabetic ketoacidosis. In most cases, patients report significant alcohol consumption accompanied by fasting. ^[2] From a biochemical point of view, ethanol is metabolized to acetoacetate and then acetate, producing significant amounts of NADH. In order to regenerate NAD+, pyruvate is metabolized to lactate and oxaloacetate is consumed to produce malate, depleting gluconeogenesis precursors. ^[2] During starvation, insulin levels are extremely low and facilitate acyl-CoA entry into mitochondria, producing significant amounts of acetyl-CoA that cannot be metabolized in the Krebs cycle and is diverted towards ketone body synthesis. ^[2]

In alcoholic ketoacidosis, the β-hydroxybutyrate to acetoacetate ratio is extremely high and β-hydroxybutyrate levels may be useful in the diagnosis and management of alcoholic ketoacidosis. ^[2] However, no studies have been performed to actually compare β-hydroxybutyrate (serum or blood) with the traditional diagnostic parameters for alcoholic ketoacidosis and thus no recommendations can be made in favor or against its use in this setting.