- Enter the pH, pCO₂ (mmHg), and HCO₃⁻ (mEq/L) from the ABG report.
- For respiratory disorders, select Acute or Chronic to apply the correct compensation formula.
- Optionally enter Na⁺ and Cl⁻ (mEq/L) to calculate the anion gap. Add albumin (g/dL) for albumin-corrected AG.
- Results appear instantly: primary disorder, compensation adequacy, mixed-disorder flag, and — if Na/Cl entered — the AG and delta ratio.
- Use the output to guide clinical decision-making in the context of the full history and examination.
All computation runs in your browser; no values are stored or transmitted.
When to Use
Use this tool to systematically interpret arterial (or venous, with caution) blood gas results in any patient with a suspected acid-base disturbance — including acute kidney injury, chronic kidney disease, sepsis, pulmonary disorders, diabetic ketoacidosis, or medication toxicity. The stepwise output (primary disorder → compensation → mixed disorder → AG/delta ratio) mirrors the physiological approach described by Berend et al. (NEJM 2014).
Appropriate use
Adults with an arterial blood gas drawn in the appropriate clinical context (respiratory failure, metabolic crisis, AKI, CKD with acidosis, DKA, toxicology). Particularly useful for detecting mixed disorders — e.g., a patient with COPD developing an intercurrent metabolic acidosis — where a single primary label would be misleading.
Limitations
HCO₃⁻ on an ABG is calculated (not measured); use the measured HCO₃⁻ from a concurrent electrolyte panel when possible. The tool assumes the ABG reflects a true arterial sample. Compensation formulas assume a single primary disorder — in severe or rapidly evolving mixed disorders, multiple iterations may be needed. Always interpret results in clinical context.
Pearls & Pitfalls
Use Winter's formula for metabolic acidosis
In metabolic acidosis, the expected pCO₂ = 1.5 × HCO₃⁻ + 8 (±2). If the actual pCO₂ is higher than expected, an additional respiratory acidosis is present; if lower, an additional respiratory alkalosis is superimposed. This is one of the most clinically powerful mixed-disorder detectors in medicine.
Correct the anion gap for albumin
The normal AG is defined at albumin = 4.0 g/dL. For each 1 g/dL drop in albumin, the AG falls by ~2.5 mEq/L. In critically ill or malnourished patients (low albumin), a seemingly "normal" AG may actually conceal a high-AG acidosis — always use the albumin-corrected AG when albumin is available.
Pitfalls
(1) Do not rely on calculated HCO₃⁻ alone — measure it from electrolytes. (2) The delta ratio assumes a single high-AG metabolic acidosis; triple disorders require clinical synthesis beyond any formula. (3) "Normal" pH does not mean no disorder — a mixed acidosis and alkalosis can yield a pH in the normal range. (4) Respiratory compensation is time-dependent: use Acute vs Chronic appropriately (acute = hours; chronic = days).
Why Use It
A systematic ABG interpretation reduces errors from pattern-matching to a single label. Detecting a mixed disorder changes management: a patient with COPD and DKA needs both respiratory support and volume resuscitation; a patient with CKD acidosis superimposed on a respiratory alkalosis from sepsis needs a different workup than pure metabolic acidosis. The anion gap and delta ratio add further resolution, directing the differential toward MUDPILES causes (elevated AG) vs hyperchloremic/normal-AG causes — critical in CKD where both patterns are common.
ABG Acid-Base Analyzer
Enter pH, pCO₂, and HCO₃⁻ for the primary interpretation and compensation check. Add Na⁺, Cl⁻, and albumin for the anion gap and delta ratio.
⚕ ABG interpretation uses Winter's formula (metabolic acidosis), standard respiratory compensation rules, and albumin-corrected anion gap. This is a decision-support tool for licensed clinicians — always interpret in full clinical context. References: Winters RW (1960); Berend K et al., NEJM 2014.
Next Steps
Use the result to support — not replace — clinical judgment.
- For metabolic acidosis: determine whether AG is elevated — if so, apply the delta ratio and work through the MUDPILES differential (Methanol, Uremia, DKA, Propylene glycol, Isoniazid, Lactic acidosis, Ethylene glycol, Salicylates). If normal AG, consider RTA, diarrhea, saline resuscitation.
- For metabolic alkalosis: assess volume status, chloride responsiveness (urine Cl⁻), and medications (diuretics, steroids, antacids).
- For respiratory acidosis: differentiate obstructive (COPD, asthma) from central/neuromuscular causes; assess for hypoxemia.
- For respiratory alkalosis: consider anxiety, pain, sepsis, liver failure, early salicylate toxicity, or iatrogenic over-ventilation.
- Repeat the ABG after intervention (bronchodilators, NaHCO₃, dialysis) to reassess compensation and trajectory.
- Escalate to nephrology or intensivists when mixed disorders, severe pH derangements (<7.10 or >7.60), or clinical deterioration are present.
Evidence & References
Formulas & Equations
| Disorder | Expected Compensation |
|---|---|
| Metabolic acidosis (Winter's) | Expected pCO₂ = 1.5 × HCO₃⁻ + 8 (±2) Actual pCO₂ > expected → added respiratory acidosis; < expected → added respiratory alkalosis |
| Metabolic alkalosis | Expected pCO₂ ≈ 0.7 × HCO₃⁻ + 21 (±2) |
| Respiratory acidosis — acute | Expected HCO₃⁻ = 24 + 1 × ((pCO₂ − 40) / 10) |
| Respiratory acidosis — chronic | Expected HCO₃⁻ = 24 + 3.5 × ((pCO₂ − 40) / 10) |
| Respiratory alkalosis — acute | Expected HCO₃⁻ = 24 − 2 × ((40 − pCO₂) / 10) |
| Respiratory alkalosis — chronic | Expected HCO₃⁻ = 24 − 4 × ((40 − pCO₂) / 10) |
| Anion gap | AG = Na⁺ − (Cl⁻ + HCO₃⁻); normal 8–12 mEq/L |
| Albumin-corrected AG | Corrected AG = AG + 2.5 × (4.0 − albumin g/dL) |
| Delta ratio (high-AG acidosis) | (AG − 12) / (24 − HCO₃⁻); <0.4 concurrent normal-AG acidosis; 1–2 pure HAGMA; >2 concurrent metabolic alkalosis or chronic respiratory acidosis |
References
- Winters RW. Terminology of acid-base disorders. Ann Intern Med. 1965;63(5):873–884. [Winter's formula source]
- Berend K, de Vries APJ, Gans ROB. Physiological approach to assessment of acid-base disturbances. N Engl J Med. 2014;371(15):1434–1445.
- Narins RG, Emmett M. Simple and mixed acid-base disorders: a practical approach. Medicine (Baltimore). 1980;59(3):161–187.
