What is the role of arterial blood gas analysis in the diagnosis of metabolic acidosis?

Updated: Oct 05, 2018
  • Author: Antonia Quinn, DO; Chief Editor: Romesh Khardori, MD, PhD, FACP  more...
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A low HCO3 level found on an automated sequential multiple analyzer (SMA) (eg, serum chemistries) is often the first clue to the presence of a metabolic acidosis; however, it cannot be the only consideration in the diagnosis of metabolic acidosis. A low HCO3 level can be caused by metabolic acidosis, a metabolic compensation of a respiratory alkalosis, or a laboratory error.

The HCO3 level that is calculated by the arterial blood gas (ABG) machine, which uses the Henderson-Hasselbalch equation, represents a more accurate measure of the plasma HCO3 level than the SMA measurement. It is suggested that the HCO3 level that is determined from the ABG be used in the anion gap calculation instead of the HCO3 level found using the SMA.

Measurement of pH and PCO2 by ABG in a patient with a low HCO3 level makes it possible to differentiate a metabolic compensation of a respiratory alkalosis from a primary metabolic acidosis. Measurement of PCO2 also makes it possible to judge the appropriateness of respiratory compensation of a metabolic acidosis, and to detect respiratory acidosis, which is signified by an elevated PCO2 level.

Oxygenation does not affect the acid-base status of a patient and generally should not be part of the discussion unless severe hypoxia is leading to ischemia. In that case, measurement of PO2 can identify severe hypoxia as a precipitant of lactic acidosis.

ABGs also measure base excess/base deficit (BE/BD), which is the best indicator of the degree of acidosis/alkalosis. BE/BD is measured by gauging the amount of acid or base that is required to titrate the patient's blood sample to a pH of 7.40, given a PCO2 level of 40 mm Hg at 37 degrees Celsius. BE/BD is a more accurate reflection of the body's state, and it is recommended over calculations using the HCO3 level.

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