ABG Calculator – Arterial Blood Gas Interpretation
Enter pH, PaCO₂ and HCO₃⁻ from an arterial blood gas sample to classify the acid–base disturbance: respiratory or metabolic acidosis or alkalosis, and whether it is acute, partly compensated or fully compensated.
ABG Analysis and Acid-Base Imbalance Checker
Arterial Blood Gas Interpretation Formula
Where:
- \( \mathrm{pH} \) — acidity of arterial blood; normal range 7.35–7.45
- \( \mathrm{PaCO_2} \) — partial pressure of arterial carbon dioxide, the respiratory component; normal range 35–45 mmHg
- \( \mathrm{HCO_3^-} \) — bicarbonate concentration, the metabolic component; normal range 22–26 mEq/L
The three values are not independent: they are linked by the Henderson–Hasselbalch equation \( \mathrm{pH} = 6.1 + \log_{10}\left( \frac{[\mathrm{HCO_3^-}]}{0.03 \times \mathrm{PaCO_2}} \right) \). The calculator compares each measured value with its normal range and then checks whether the opposing system is compensating for the primary disturbance.
Arterial Blood Gas – Calculation Example
Example: an arterial sample from a healthy adult at rest returns the calculator's default values: pH 7.40, PaCO₂ 40 mmHg and HCO₃⁻ 24 mEq/L.
\[ \mathrm{pH} = 7.40 \in [7.35,\ 7.45] \\ \mathrm{PaCO_2} = 40\ \mathrm{mmHg} \in [35,\ 45] \\ \mathrm{HCO_3^-} = 24\ \mathrm{mEq/L} \in [22,\ 26] \]
Result: Normal Arterial Blood Gas — all three values lie inside their reference ranges, so acid–base balance is intact.
For contrast, entering pH 7.30, PaCO₂ 40 mmHg and HCO₃⁻ 15 mEq/L gives Acute Metabolic Acidosis: the pH is low, the PaCO₂ is still normal (no respiratory compensation has started yet) and the bicarbonate is far below range.
How to Use the ABG Calculator
- Enter the measured pH in the pH field (accepted values 0–14, e.g. 7.40).
- Enter the carbon dioxide value in the paCO₂ field, in mmHg.
- Enter the bicarbonate value in the HCO₃⁻ field, in mEq/L (many labs report mmol/L, which is numerically identical).
- Read the classification in the ABG Analysis result — for example "Acute Respiratory Acidosis", "Compensated Metabolic Acidosis", "Mixed Disorders" or "Normal Arterial Blood Gas".
What Is Arterial Blood Gas Interpretation?
An arterial blood gas (ABG) is one of the most informative single tests in acute medicine. It shows how well the lungs oxygenate the blood and how well the lungs and kidneys together maintain acid–base balance. Three of the reported values define the acid–base status: pH (the overall acidity), PaCO₂ (the respiratory component, controlled by ventilation) and HCO₃⁻ (the metabolic component, controlled by the kidneys).
Every acid–base disorder is either respiratory or metabolic, and either acidosis or alkalosis — four primary patterns in total. The quickest bedside rule: if the pH moves in the opposite direction to PaCO₂ (low pH with high PaCO₂, or high pH with low PaCO₂), the primary problem is respiratory; if the pH moves together with HCO₃⁻ (both low or both high), the primary problem is metabolic.
Because two systems control one variable, the healthy system always tries to correct the failing one. Lungs respond within minutes — hyperventilating to blow off CO₂ in metabolic acidosis — while kidneys need hours to days, for example retaining bicarbonate in chronic respiratory acidosis. The calculator's wording reflects this timeline: Acute means no compensation has begun, Partly Compensated means the second system is moving but the pH is still outside 7.35–7.45, and Compensated means the pH has been pulled back into the normal range. Physiological compensation never overcorrects, so a pH on the "wrong" side of 7.40, or a second value moving in an unexpected direction, should raise suspicion of a mixed disorder — which the calculator reports as Mixed Disorders, meaning both components are abnormal in a way a single primary process with compensation cannot explain.
Because the kidneys maintain bicarbonate, metabolic acidosis is a hallmark of advanced renal failure. If the ABG shows a metabolic acidosis, it is worth estimating kidney function — for example with the Cockcroft-Gault creatinine calculator or the BUN to creatinine ratio calculator. In diabetic ketoacidosis, the acidosis is compounded by hyperglycemia-driven sodium shifts, which you can assess with the corrected sodium calculator.
ABG Pattern Reference Table
| Primary disorder | pH | PaCO₂ | HCO₃⁻ |
|---|---|---|---|
| Respiratory acidosis — acute | ↓ | ↑ | Normal |
| Respiratory acidosis — compensated | Normal | ↑ | ↑ |
| Respiratory alkalosis — acute | ↑ | ↓ | Normal |
| Respiratory alkalosis — compensated | Normal | ↓ | ↓ |
| Metabolic acidosis — acute | ↓ | Normal | ↓ |
| Metabolic acidosis — compensated | Normal | ↓ | ↓ |
| Metabolic alkalosis — acute | ↑ | Normal | ↑ |
| Metabolic alkalosis — compensated | Normal | ↑ | ↑ |
Note that a normal pH with a low PaCO₂ and a low HCO₃⁻ fits both compensated metabolic acidosis and compensated respiratory alkalosis; the calculator reports both possibilities, and the patient's history decides which is the true primary disorder.
Limitations and Practical Tips
- No oxygenation assessment. The calculator uses only pH, PaCO₂ and HCO₃⁻; it says nothing about PaO₂ or oxygen saturation, which are essential parts of a full ABG report.
- No expected-compensation formulas. For borderline or mixed results, calculate the expected compensation yourself — for metabolic acidosis, Winter's formula \( \mathrm{PaCO_2}^{\text{expected}} = 1.5 \times [\mathrm{HCO_3^-}] + 8 \pm 2 \) — and consider the anion gap, which this tool does not compute.
- "Mixed Disorders" is a broad category. It covers many combinations; common causes include sepsis, cardiac arrest, renal failure combined with respiratory failure, and vomiting in a patient on diuretics.
- Preanalytical errors matter. Air bubbles in the syringe, delayed analysis, excess heparin or an inadvertently venous sample can shift all three values and change the interpretation.
- Educational tool only. Always interpret an ABG together with the history, examination and other laboratory results; never base treatment decisions on the calculator output alone.
Frequently Asked Questions
How do I interpret an ABG step by step by hand?
First look at the pH: below 7.35 means acidosis, above 7.45 means alkalosis. Next identify the cause: if PaCO₂ moves opposite to the pH, the disorder is respiratory; if HCO₃⁻ moves in the same direction as the pH, it is metabolic. Then check whether the third value is normal (acute), abnormal with pH still outside range (partly compensated), or abnormal with pH back inside 7.35–7.45 (compensated). Finally, ask whether the compensation is physiologically possible — if not, suspect a mixed disorder.
My lab reports PaCO₂ in kPa — how do I convert it to mmHg?
Multiply the kPa value by 7.5 to get mmHg, or divide mmHg by 7.5 to get kPa. For example, a PaCO₂ of 5.3 kPa equals about 40 mmHg, the normal mid-range value. European and many other labs commonly use kilopascals, so always check the unit on the report before entering the number. Bicarbonate needs no conversion: mEq/L and mmol/L are numerically identical for HCO₃⁻.
Can I use this calculator with a venous blood gas (VBG)?
Not directly. Venous samples run about 0.03–0.05 lower in pH, 4–6 mmHg higher in PCO₂ and slightly higher in bicarbonate than arterial samples. For screening, pH and HCO₃⁻ from a VBG correlate reasonably well with arterial values, but PCO₂ agreement is weaker, so classification of respiratory disorders may be wrong. A definitive acid–base diagnosis requires an arterial sample.
What is the anion gap and why does it matter after classifying a metabolic acidosis?
The anion gap separates metabolic acidoses into two groups. Calculate it as Na⁺ − (Cl⁻ + HCO₃⁻), with a normal range of roughly 3–11 mEq/L (varies by lab). A high anion gap suggests added acids — diabetic ketoacidosis, lactic acidosis from sepsis or shock, renal failure, or toxic alcohols. A normal anion gap (hyperchloremic) acidosis points to bicarbonate loss, e.g. diarrhea or renal tubular acidosis. The treatment differs, so always take the next step after the ABG classification.
How much compensation should I expect — what are the formulas?
For metabolic acidosis use Winter's formula: expected PaCO₂ = 1.5 × HCO₃⁻ + 8 ± 2 mmHg. For metabolic alkalosis: expected PaCO₂ = 0.7 × (HCO₃⁻ − 24) + 40. For acute respiratory acidosis, HCO₃⁻ rises about 1 mEq/L per 10 mmHg PaCO₂ increase (chronic: 3–4 per 10). In acute respiratory alkalosis HCO₃⁻ falls about 2 per 10 mmHg drop (chronic: 4–5 per 10). If measured compensation deviates from the expected value, a mixed disorder is likely.
All three values are normal but the patient is clearly sick — what then?
A 'normal' acid–base result does not rule out serious illness. The calculator ignores oxygenation: a patient with severe hypoxemia or a high A–a gradient can have a perfectly normal pH, PaCO₂ and HCO₃⁻. It also misses pre-existing chronic disorders that have fully normalized, and subtle mixed disorders that cancel each other out. Always read the full ABG report, including PaO₂ and oxygen saturation, alongside the clinical picture.
Why can the same numbers mean two different diagnoses?
A normal pH with both PaCO₂ and HCO₃⁻ low can be either compensated metabolic acidosis or compensated respiratory alkalosis — the numbers alone cannot tell them apart. In the first case the kidney problem (or acid load) came first; in the second, hyperventilation came first. Only the history and setting distinguish them: a septic patient tachypneic for hours fits respiratory alkalosis, while a patient with diarrhea or renal failure fits metabolic acidosis.
I was not given the of bicarb.
I want to find out with these values:
PH--7.55
PaCo2--30
PaO2--65
What's this patient's condition?
good
WELL DONE
nice post