About the ABG Interpreter
This arterial blood gas (ABG) interpreter works through the standard stepwise approach used in emergency medicine, critical care and nursing education. It decides whether the pH shows acidemia or alkalemia, identifies the primary disorder (respiratory or metabolic), calculates the expected compensation with established bedside formulas, and flags a second process when the measured value falls outside the expected range.
Add sodium and chloride (and albumin, if known) to calculate the anion gap, the albumin-corrected gap and the delta ratio, which separate high anion gap acidosis from normal-gap (hyperchloraemic) acidosis and reveal hidden mixed disorders. The calculator also checks the internal consistency of the gas with the Henderson–Hasselbalch equation, which helps catch transcription or sampling errors.
Reference values used: pH 7.35–7.45, PaCO₂ 35–45 mmHg and HCO₃⁻ 22–26 mEq/L. The rules are approximations derived from adult data, so interpret the result alongside the history, examination and your own laboratory’s ranges. It is an educational and decision-support tool, not a substitute for clinical judgement.
How to use the abg interpreter
- 1Enter pH, PaCO₂ (mmHg) and bicarbonate from the blood gas.
- 2For a respiratory disorder, choose whether it is acute or chronic.
- 3Add sodium, chloride and albumin to calculate the anion gap and delta ratio.
- 4Read the primary disorder, then check whether compensation is appropriate.
- 5Use the step table and consistency check to review the full interpretation.
Formula and method
Step 1 classifies the pH. Step 2 finds the primary process: in acidemia a high PaCO₂ means respiratory acidosis and a low HCO₃⁻ metabolic acidosis; in alkalemia a low PaCO₂ means respiratory alkalosis and a high HCO₃⁻ metabolic alkalosis. If both point the same way the disorder is mixed. Step 3 predicts the compensating value — Winters’ formula for metabolic acidosis, a rise of 0.7 mmHg PaCO₂ per mEq/L of HCO₃⁻ in metabolic alkalosis, and the acute and chronic renal responses to changes in PaCO₂. A measured value outside the predicted range signals a second primary disorder.
The anion gap is corrected upwards by 2.5 mEq/L for each 1 g/dL that albumin is below 4.0. When the corrected gap exceeds 12 and HCO₃⁻ is below 24, the delta ratio (ΔAG ÷ ΔHCO₃⁻) is calculated: below 0.4 suggests a normal-gap acidosis, 0.4–0.8 a combined high-gap and normal-gap acidosis, 0.8–2 an uncomplicated high-gap acidosis, and above 2 a coexisting metabolic alkalosis (or pre-existing high bicarbonate). Finally the Henderson–Hasselbalch equation, pH = 6.1 + log₁₀(HCO₃⁻ ÷ (0.03 × PaCO₂)), checks that the three values are compatible.
- PaCO₂
- Arterial carbon dioxide tension (mmHg)
- HCO₃⁻
- Bicarbonate (mEq/L)
- AG
- Anion gap, corrected for albumin (mEq/L)
- Δ ratio
- (AG − 12) ÷ (24 − HCO₃⁻)
Worked examples
Diabetic ketoacidosis pattern
pH 7.26 is acidemic and HCO₃⁻ 12 is low, so the primary process is metabolic acidosis. Winters’ formula predicts PaCO₂ 1.5 × 12 + 8 = 26 ± 2; the measured 28 is within range. The anion gap is 140 − 102 − 12 = 26, and the delta ratio (26 − 12) ÷ (24 − 12) = 1.17 indicates a pure high-gap acidosis.
Acute respiratory acidosis (opioid overdose)
Acidemia with a PaCO₂ of 60 is respiratory acidosis. Acutely HCO₃⁻ rises about 1 mEq/L per 10 mmHg, so 24 + 2 = 26 ± 2 is expected; the measured 26 fits an acute, uncomplicated respiratory acidosis.
Chronic COPD retainer
Chronic renal compensation raises HCO₃⁻ about 3.5 mEq/L per 10 mmHg: 24 + 0.35 × 20 = 31 ± 2. The measured 31 matches, so this is a compensated chronic respiratory acidosis.
Vomiting with metabolic alkalosis
Alkalemia with HCO₃⁻ 34 is a metabolic alkalosis. Expected PaCO₂ = 40 + 0.7 × 10 = 47 ± 5, and the measured 46 shows appropriate hypoventilation.
Cardiac arrest: mixed acidosis
Both a high PaCO₂ (50) and a low HCO₃⁻ (15) push the pH down, so this is a combined respiratory and metabolic acidosis with no compensation.
Frequently asked questions
What are the steps of ABG interpretation?+
Check the pH for acidemia or alkalemia, identify whether PaCO₂ or HCO₃⁻ explains it (primary disorder), calculate the expected compensation, compare with the measured value to detect a second disorder, and in metabolic acidosis calculate the anion gap and delta ratio.
What is Winters’ formula?+
Winters’ formula predicts the PaCO₂ expected in metabolic acidosis: PaCO₂ = 1.5 × HCO₃⁻ + 8 ± 2. A higher measured PaCO₂ suggests an additional respiratory acidosis; a lower one suggests an additional respiratory alkalosis.
What are normal ABG values?+
Typical adult arterial values are pH 7.35–7.45, PaCO₂ 35–45 mmHg (4.7–6.0 kPa), HCO₃⁻ 22–26 mEq/L and PaO₂ 75–100 mmHg on room air. Laboratories may use slightly different ranges.
How do I tell acute from chronic respiratory acidosis?+
In acute respiratory acidosis HCO₃⁻ rises only about 1 mEq/L per 10 mmHg PaCO₂ rise; after 3–5 days the kidneys raise it about 3.5–4 mEq/L per 10 mmHg. Comparing the measured HCO₃⁻ with both predictions helps date the process.
Can a normal pH hide an acid-base disorder?+
Yes. A normal pH with abnormal PaCO₂ and HCO₃⁻, or a raised anion gap with a normal bicarbonate, often means two opposing disorders are present, such as metabolic acidosis with a concurrent metabolic alkalosis.
Why check the Henderson–Hasselbalch pH?+
Blood gas analysers measure pH and PaCO₂ and calculate bicarbonate, but values copied from different samples or mistyped will not fit the equation. A difference of more than about 0.05 pH units suggests an error.
For healthcare professionals and education only. This calculator implements a published formula or score but is not a diagnosis and does not replace clinical judgment. Verify inputs and results against the original source before using them in patient care.