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Acid-Base Interpretation: A Practical Approach

This is a corrected version of the article that appeared in print.

Masahiro J. Morikawa, MD, MPH
Prakash R. Ganesh, MD, MPH

American Family Physician. 2025;111(2):148-155.

Author disclosure: No relevant financial relationships.

This clinical content conforms to AAFP criteria for CME.

Acid-base disorders are prevalent in critically ill patients, and a systematic approach is essential for evaluation. The first step is to determine the primary process based on a patient's pH, partial pressure of carbon dioxide, and bicarbonate measurements. After this is complete, the next step is to evaluate for respiratory or metabolic compensation. Deviations from expected compensation may indicate additional acid-base processes. For metabolic acidosis, anion gap calculation distinguishes between anion gap metabolic acidosis and non–anion gap metabolic acidosis. The evaluation for anion gap metabolic acidosis includes calculating the osmolal gap and conducting a gap-gap analysis. Evaluating non–anion gap metabolic acidosis involves urine anion gap calculation. These analyses identify potential etiologies and additional acid-base disturbances. Metabolic alkalosis assessment begins with measuring urine chloride levels to determine whether the process is a result of chloride depletion. Respiratory acidosis, caused by hypoventilation, often results from chronic lung disease or neuromuscular dysfunction. Respiratory alkalosis, due to hyperventilation, is common in sepsis, chronic liver disease, and acute pulmonary embolism.

Acid-base disorders are common in patients who are critically ill.1 A systematic approach to evaluating these disorders provides vital diagnostic information and guides timely treatment.2–5 Acidosis leads to acidemia (pH less than 7.38)6 and is typically due to one of three factors: (1) accumulation of acids (eg, salicylic acid, keto acids, lactic acid) or loss of alkali (as in diarrhea or proximal renal tubular acidosis); (2) decreased renal excretion of acid (as in kidney failure or distal renal tubular acidosis); or (3) accumulation of carbon dioxide in the body. Alkalosis is a process that leads to alkalemia (pH greater than 7.42) and is primarily caused by excessive carbon dioxide exhalation (respiratory alkalosis), the accumulation of alkali (sodium bicarbonate [HCO3] administration), or loss of acid (as with gastrointestinal loss from vomiting and diarrhea).7

SORT: KEY RECOMMENDATIONS FOR PRACTICE

Clinical recommendationEvidence ratingComments
Using a systematic approach to evaluating acid-base disorders provides vital diagnostic information and guides timely treatment in critically ill patients.2–5 CReviews based on usual practice, consensus
To determine the primary process of an acid-base disorder, the pH and Paco2 on blood gas should be evaluated, noting that abnormal values increasing or decreasing in the same direction indicate a metabolic process and those shifting in opposite directions indicate a respiratory process.3,5 CReview based on usual practice, consensus
When evaluating acid-base disorders, expected patterns of Paco2 compensation for metabolic disturbances and HCO3 compensation for respiratory disturbances should be reviewed to determine if an additional acid-base disorder is present.3,5 CUsual practice, consensus

HCO3 = bicarbonate.

A = consistent, good-quality patient-oriented evidence; B = inconsistent or limited-quality patient-oriented evidence; C = consensus, disease-oriented evidence, usual practice, expert opinion, or case series. For information about the SORT evidence rating system, go to https://www.aafp.org/afpsort.

APPROACH

The physiologic approach to acid-base analysis is based on the concept of the carbonic acid-HCO3 buffer system as a center of homeostatic control of blood pH.2–4,8,9 In addition to assessment of vital signs and fluid status, blood gas measurement and basic chemistry panel are required. Venous blood gas often is measured initially instead of arterial blood gas because it is easier to obtain and may reveal metabolic changes earlier in cases of circulatory insufficiency.10 An arterial blood gas measurement is preferred when assessing a patient's oxygenation status.

MASAHIRO J. MORIKAWA, MD, MPH, is the B. Lewis Barnett Jr Professor of Family Medicine at the University of Virginia, Charlottesville.

PRAKASH R. GANESH, MD, MPH, is an assistant professor in the Department of Family Medicine & Community Health at the University Hospitals Cleveland Medical Center, and at the Center for Community Health Integration, Case Western Reserve University School of Medicine, and Neighborhood Family Practice, Ohio

Address correspondence to Masahiro J. Morikawa, MD, MPH, at mm7jc@virginia.edu.

Author disclosure: No relevant financial relationships.

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