Free tool
ABG Interpreter.
Most ABG tools just label the gas. The part new grads miss is what comes next: is the compensation appropriate, or is a second disorder hiding underneath it? Enter the pH, PaCO₂, and HCO₃ and this works the full sequence — primary disorder, expected compensation (Winter's formula and the respiratory rules), mixed-disorder detection, and — if you add the electrolytes — the anion gap and delta-delta. Add a PaO₂ and FiO₂ for the A–a gradient and oxygenation read.
Arterial blood gas
The three required values come straight off the gas. Everything below the line is optional and unlocks extra analysis.
Optional — anion gap & delta-delta
Add sodium and chloride (off the BMP) to fold the anion gap into the read and screen for a second metabolic disorder.
Optional — oxygenation & A–a gradient
Add the PaO₂ and the FiO₂ the patient was on to get the A–a gradient, P/F ratio, and an oxygenation read.
Enter the pH, PaCO₂, and HCO₃ to begin.
The 5-step read (ROME / tic-tac-toe) [1]
| 1. pH | Acidemia < 7.35, alkalemia > 7.45. This is the patient's net state. |
| 2. Primary | Which value moved with the pH? Respiratory Opposite (pH & PaCO₂ move opposite ways), Metabolic Equal (pH & HCO₃ move the same way). |
| 3. Compensation | Is the other system shifting to pull the pH back? Use Winter's / the respiratory rules to ask whether the shift is the right size. |
| 4. Mixed? | If compensation is too much or too little, a second disorder is present. Anion gap + delta-delta catch metabolic ones. |
| 5. Oxygenation | Separate axis. PaO₂, P/F ratio, and the A–a gradient tell you about gas exchange, not acid-base. |
Expected-compensation formulas [1][2]
| Primary disorder | Expected compensation |
|---|---|
| Metabolic acidosis | Winter's: expected PaCO₂ = (1.5 × HCO₃) + 8 ± 2 |
| Metabolic alkalosis | Expected PaCO₂ ≈ (0.7 × HCO₃) + 21 (rises ~0.7 mmHg per 1 mEq/L HCO₃) |
| Resp. acidosis — acute | HCO₃ rises ~1 per 10 mmHg PaCO₂ above 40 |
| Resp. acidosis — chronic | HCO₃ rises ~3.5–4 per 10 mmHg PaCO₂ above 40 |
| Resp. alkalosis — acute | HCO₃ falls ~2 per 10 mmHg PaCO₂ below 40 |
| Resp. alkalosis — chronic | HCO₃ falls ~4–5 per 10 mmHg PaCO₂ below 40 |
The body never over-compensates back to a normal pH — if the pH is fully corrected, suspect a mixed picture. Delta-delta (Δgap/Δbicarb) screens a high-gap acidosis for a coexisting normal-gap acidosis or metabolic alkalosis.
References — the evidence behind every read
This tool only ever cites the vetted sources below. Each interpretation in your result carries a numbered marker that links straight to the matching reference here, so you can see exactly what every line is grounded in.
- Berend K, de Vries APJ, Gans ROB. Physiological approach to assessment of acid–base disturbances. N Engl J Med. 2014;371(15):1434–1445. nejm.org — stepwise approach; identifying the primary disorder; compensation rules.
- Albert MS, Dell RB, Winters RW. Quantitative displacement of acid–base equilibrium in metabolic acidosis. Ann Intern Med. 1967;66(2):312–322. acpjournals.org — Winter's formula (expected PaCO₂ = 1.5 × HCO₃ + 8 ± 2).
- Seifter JL. Integration of acid–base and electrolyte disorders. N Engl J Med. 2014;371(19):1821–1831. nejm.org — mixed disorders; the delta-delta (Δ/Δ) ratio.
- Kraut JA, Madias NE. Serum anion gap: its uses and limitations in clinical medicine. Clin J Am Soc Nephrol. 2007;2(1):162–174. doi.org — anion gap; albumin correction ≈ 2.5 mEq/L per 1 g/dL below 4.0.
- O'Driscoll BR, Howard LS, Earis J, Mak V; British Thoracic Society. BTS guideline for oxygen use in adults in healthcare and emergency settings. Thorax. 2017;72(Suppl 1):ii1–ii90. brit-thoracic.org.uk — target SpO₂ 94–98% (most patients) and 88–92% (hypercapnia risk).
- Matthay MA, Arabi Y, Arroliga AC, et al. A new global definition of acute respiratory distress syndrome. Am J Respir Crit Care Med. 2024;209(1):37–47. atsjournals.org — PaO₂/FiO₂ ≤ 300 mmHg oxygenation thresholds.
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