Does low bicarbonate (HCO3) level always indicate an acidotic state?

From the Guidelines

No, a low bicarbonate (HCO3) level does not always indicate an acidotic state. While low bicarbonate is commonly associated with metabolic acidosis, as seen in conditions like diabetic ketoacidosis where bicarbonate levels can be significantly reduced 1, it can also occur in other contexts. For instance, in respiratory alkalosis, the primary issue is low carbon dioxide (CO2) levels due to hyperventilation, and the kidneys respond by excreting bicarbonate to maintain acid-base balance, potentially resulting in low bicarbonate levels despite an alkalotic state.

Key Considerations

• Metabolic acidosis, characterized by low bicarbonate levels, can result from various causes including failure to excrete acid, increased acid production, or direct loss of bicarbonate, as discussed in the context of chronic kidney disease where bicarbonate supplementation is recommended for patients with serum bicarbonate concentrations <22 mmol/l 1.
• Respiratory acidosis, on the other hand, involves an increase in CO2 levels, which can lead to a compensatory increase in bicarbonate to buffer the acidity, as outlined in the BTS guideline for oxygen use in adults 1.
• The interpretation of bicarbonate levels must consider the clinical context, including pH, pCO2, and the presence of compensatory mechanisms, to accurately assess acid-base status.
• Laboratory errors, certain medications, or dilutional effects from excessive IV fluid administration can also cause falsely low bicarbonate readings without true acidosis.

Assessment and Management

To properly assess acid-base status, a comprehensive approach including arterial blood gas analysis is crucial, as it measures pH directly and allows for the determination of whether the primary disorder is metabolic or respiratory in nature, and whether compensatory mechanisms are at work. This approach is supported by guidelines and studies that emphasize the importance of understanding the underlying cause of acid-base disturbances to guide appropriate management 1. Therefore, bicarbonate levels should always be interpreted alongside pH, pCO2, and clinical context to avoid misdiagnosis and ensure appropriate treatment.

From the FDA Drug Label

In cardiac arrest, a rapid intravenous dose of one to two 50 mL vials (44.6 to 100 mEq) may be given initially and continued at a rate of 50 mL (44. 6 to 50 mEq) every 5 to 10 minutes if necessary (as indicated by arterial pH and blood gas monitoring) to reverse the acidosis. Sodium Bicarbonate Injection, USP is indicated in the treatment of metabolic acidosis which may occur in severe renal disease, uncontrolled diabetes, circulatory insufficiency due to shock or severe dehydration, extracorporeal circulation of blood, cardiac arrest and severe primary lactic acidosis

Low bicarbonate (HCO3) level does not always indicate an acidotic state, but it can be a sign of metabolic acidosis. The FDA drug label for sodium bicarbonate (IV) indicates that it is used to treat metabolic acidosis, which can occur in various conditions, including severe renal disease, uncontrolled diabetes, and circulatory insufficiency due to shock or severe dehydration 2. However, the label also notes that bicarbonate therapy should be planned in a stepwise fashion, as the degree of response from a given dose is not precisely predictable 2.

• Key points:
  • Low bicarbonate level can be a sign of metabolic acidosis
  • Sodium bicarbonate (IV) is used to treat metabolic acidosis
  • Bicarbonate therapy should be planned in a stepwise fashion
  • The degree of response from a given dose is not precisely predictable 2, 2

From the Research

Acidotic State and Bicarbonate Levels

• A low bicarbonate (HCO3) level can indicate an acidotic state, but it is not the only factor to consider 3, 4, 5.
• Metabolic acidosis can result from an increase of acid in the body or by excess loss of bicarbonate, and it is the most common disorder encountered in clinical practice 5.
• The diagnosis of metabolic acidosis is established clinically by the detection of a metabolic acidosis of the "unmeasured anion gap" type in the absence of significant renal failure, poison intake or a strongly positive clinical test for ketones 3.

Relationship Between Bicarbonate and Acidosis

• The blood concentrations of bicarbonate and organic acid anions are related, and a decrease in bicarbonate excretion can play an important role in the pathogenesis of metabolic alkalosis 3.
• Uncorrected severe metabolic acidosis, defined by serum bicarbonate concentrations less than 20 mmol/L, is associated with a high overall mortality in patients on renal replacement therapy 6.
• The treatment of acute metabolic acidosis should be directed at improving both extracellular and intracellular pH, and administration of base, primarily in the form of sodium bicarbonate, may be necessary in severe cases 4, 5.

Diagnosis and Treatment

• The diagnosis of metabolic acidosis includes assessment of acid-base parameters, including pH, partial pressure of CO(2) and HCO(3)(-) concentration in arterial blood, and calculation of the serum anion gap 4, 5.
• Treatment of metabolic acidosis must be aimed at diagnosis and correction of the underlying disease process, and specific treatment may be required when changes in pH are severe 5.
• Selective sodium-hydrogen exchanger 1 (NHE1) inhibitors have been shown to improve haemodynamics and reduce mortality in animal studies of acute lactic acidosis, and their value as a form of treatment is worth further investigation 4.