Evaluation of Acid-Base Disorders

Last updated: April 8, 2026, 12:35 PM Pacific Time (PDT)

Interactive bedside framework for metabolic and respiratory acid-base disorders. Start with pH, then identify the primary process, check whether compensation fits, and look for mixed disorders before anchoring.

Interactive diagnostic assistant

Activate the assistant to estimate the primary disorder, expected compensation, anion gap pattern, and mixed-disorder clues from partial ABG/VBG and chemistry data.

Blood gas

pH
pCO2 (mmHg)
HCO3 from blood gas (mEq/L)
PaO2 or O2 clue (optional)

ABG mode uses blood gas HCO3 for compensation checks.

Chemistry Na (mEq/L)
Cl (mEq/L)
Chemistry CO2 / serum bicarbonate (mEq/L)
Albumin (g/dL)
Lactate (optional)
Glucose / ketone clue (optional)

History and bedside clues Diarrhea / GI bicarbonate loss Vomiting / NG suction CKD / uremia COPD / chronic hypercarbia Sepsis / shock Toxin / salicylate / toxic alcohol concern Diuretics / contraction alkalosis

Primary process

Mixed-disorder clues

Calculated anchors

Anion gap formula: AG = Na - Cl - HCO3. Albumin-corrected anion gap: corrected AG = AG + 2.5 x (4 - albumin) when albumin is below 4 g/dL.

Teaching pearl: acid-base interpretation is strongest when you combine the pH direction, the dominant pCO2/HCO3 abnormality, the expected compensation, and the bedside story.

Flowchart

Step 1

Start with pH and name the dominant direction

Decide whether the overall picture is acidemia, alkalemia, or near-normal pH with a possible mixed disorder before interpreting any single number in isolation.

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Step 2

Check whether compensation fits

If the measured pCO2 or HCO3 does not match the expected compensatory response, assume there is a second process until proven otherwise.

Acidosis branch

Metabolic

Metabolic acidosis

Low HCO3 drives the process. Separate high anion gap from non-gap causes, then check compensation.

Respiratory

Respiratory acidosis

High pCO2 drives the process. Decide whether this looks acute, chronic, or acute-on-chronic.

Alkalosis branch

Metabolic

Metabolic alkalosis

High HCO3 drives the process. Think chloride depletion, diuretics, mineralocorticoid states, or alkali load.

Respiratory

Respiratory alkalosis

Low pCO2 drives the process. Sort hyperventilation from true hypoxemic/PE/sepsis physiology.

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Step 3

Look for mixed disorders

Normal-ish pH, out-of-range compensation, or discordant gap/delta-gap patterns often mean two or more acid-base processes are present at once.

Teaching pearl: a near-normal pH does not rule out major acid-base pathology. It often means the patient has more than one process.