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ABG Interpreter

Systematic arterial blood gas interpretation: identify the primary acid-base disorder, calculate expected compensation, and detect mixed disturbances.

Inputs

pH Normal: 7.35–7.45

pCO₂ (mmHg) Normal: 35–45 mmHg

HCO₃⁻ (mEq/L) Normal: 22–26 mEq/L

Na⁺ (mEq/L) — optional For anion gap calculation

Cl⁻ (mEq/L) — optional For anion gap calculation

Albumin (g/dL) — optional For corrected anion gap (default 4.0 if blank)

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Enter pH, pCO₂, and HCO₃⁻ above.

Primary disorder—

Expected compensation—

Compensation status—

Anion gap—

Disclaimer: For educational purposes only. ABG interpretation requires clinical context. Always correlate with patient presentation, history, and other labs.

Systematic approach

Step 1: Assess pH → acidemia (< 7.35) or alkalemia (> 7.45)
Step 2: Identify primary disorder (respiratory vs metabolic)
Step 3: Calculate expected compensation
Step 4: If metabolic acidosis → calculate anion gap
Step 5: If elevated AG → delta-delta ratio

Compensation formulas

Metabolic acidosis: Expected pCO₂ = 1.5 × HCO₃ + 8 (±2) (Winter's)
Metabolic alkalosis: Expected pCO₂ = 0.7 × HCO₃ + 21 (±2)
Acute respiratory acidosis: Expected HCO₃ = 24 + (pCO₂ − 40) × 0.1
Chronic respiratory acidosis: Expected HCO₃ = 24 + (pCO₂ − 40) × 0.35
Acute respiratory alkalosis: Expected HCO₃ = 24 − (40 − pCO₂) × 0.2
Chronic respiratory alkalosis: Expected HCO₃ = 24 − (40 − pCO₂) × 0.5

Delta-delta interpretation

< 1: Combined AG + non-AG metabolic acidosis
1–2: Pure AG metabolic acidosis
> 2: AG metabolic acidosis + metabolic alkalosis

Clinical pearls

AG = Na − (Cl + HCO₃); normal ~12 ± 4. Corrected for albumin: add 2.5 × (4.0 − albumin).
Common AG acidosis causes: MUDPILES (Methanol, Uremia, DKA, Propylene glycol, INH/Iron, Lactic acidosis, Ethylene glycol, Salicylates).
Non-AG (hyperchloremic) acidosis: RTA, diarrhea, normal saline excess.
Respiratory compensation occurs within minutes (acute) to days (chronic).

References

Emmett M, Narins RG. Clinical use of the anion gap. Medicine. 1977;56(1):38–54.
Berend K, et al. Physiological approach to assessment of acid-base disturbances. N Engl J Med. 2014;371(15):1434–1445.