Base Excess - Interpretation

Interpretation

Base excess beyond the reference range indicates

• metabolic alkalosis if too high (more than +2 mEq/L)
• metabolic acidosis if too low (less than −2 mEq/L)

Blood pH is determined by both a metabolic component, measured by base excess, and a respiratory component, measured by pCO₂ (partial pressure of carbon dioxide). Often a disturbance in one triggers a partial compensation in the other. A secondary (compensatory) process can be readily identified because it opposes the observed deviation in blood pH.

For example, inadequate ventilation, a respiratory problem, causes a buildup of CO₂, hence respiratory acidosis; the kidneys then attempt to compensate for the low pH by raising blood bicarbonate. The kidneys only partially compensate, so the patient may still have a low blood pH, i.e. acidosis. In summary, the kidneys partially compensate for respiratory acidosis by raising blood bicarbonate.

A high base excess, thus metabolic alkalosis, usually involves an excess of bicarbonate. It can be caused by

• Compensation for primary respiratory acidosis
• Excessive loss of HCl in gastric juice by vomiting
• Renal overproduction of bicarbonate, in either contraction alkalosis or Cushing's disease

A base deficit (a below-normal base excess), thus metabolic acidosis, usually involves either excretion of bicarbonate or neutralization of bicarbonate by excess organic acids. Common causes include

• Compensation for primary respiratory alkalosis
• Diabetic ketoacidosis, in which high levels of acidic ketone bodies are produced
• Lactic acidosis, due to anaerobic metabolism during heavy exercise or hypoxia
• Chronic renal failure, preventing excretion of acid and resorption and production of bicarbonate
• Diarrhea, in which large amounts of bicarbonate are excreted
• Ingestion of poisons such as methanol, ethylene glycol, or excessive aspirin

The serum anion gap is useful for determining whether a base deficit is caused by addition of acid or loss of bicarbonate.

• Base deficit with elevated anion gap indicates addition of acid (e.g., ketoacidosis).
• Base deficit with normal anion gap indicates loss of bicarbonate (e.g., diarrhea). The anion gap is maintained because bicarbonate is exchanged for chloride during excretion.