ABG Interpreter Calculator
Enter pH, PaCO2 and HCO3 to interpret the arterial blood gas — the disorder and its compensation.
Educational reference — not medical advice. This tool is a simplified educational ABG interpreter for nursing and student practice. It is not medical advice or a diagnosis, and it does not replace clinical assessment of the patient. Always interpret blood gases in full clinical context with a qualified clinician.
An arterial blood gas is read in three steps: is the pH acidotic (below 7.35) or alkalotic (above 7.45); is PaCO2 (normal 35–45) driving it, indicating a respiratory cause; and is HCO3 (normal 22–26) driving it, indicating a metabolic cause. Compensation is present when the other value shifts to push pH back toward normal.
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How to Use This Calculator
Enter the three core values from the blood gas: the arterial pH, the PaCO2 in millimetres of mercury, and the bicarbonate (HCO3) in milliequivalents per litre. The tool reports whether the blood is acidotic or alkalotic, whether the cause is respiratory or metabolic, and whether the body is compensating.
It follows the standard student method of reading a gas, and shows its reasoning so you can learn the pattern rather than just read an answer.
How the Calculation Works
First the pH sets the direction: below 7.35 is acidosis, above 7.45 is alkalosis. Then the tool checks which value explains it. A high PaCO2 with acidosis, or a low PaCO2 with alkalosis, points to a respiratory cause, because carbon dioxide forms acid. A low HCO3 with acidosis, or a high HCO3 with alkalosis, points to a metabolic cause.
Compensation is judged by whether the other system has shifted in the direction that pushes pH back toward normal — full, partial, or none.
acidosis pH<7.35, alkalosis pH>7.45; CO2 = respiratory, HCO3 = metabolic
Worked Example
Given pH 7.30, PaCO2 60, HCO3 26: the pH is below 7.35, so it is an acidosis. The PaCO2 is high at 60 (normal 35–45), which causes acidosis, so the primary problem is respiratory acidosis.
The HCO3 is normal at 26, so the kidneys have not yet compensated — this is an uncompensated respiratory acidosis, the classic pattern of acute hypoventilation.
Assumptions and Limitations
- Uses the standard three-value method (pH, PaCO2, HCO3)
- Does not assess oxygenation (PaO2), the anion gap, or mixed disorders in depth
- Interpretation must always be matched to the clinical picture
- For education and student practice — not a diagnosis
When to Use an ABG Calculator
Nursing and medical students use this to interpret an arterial blood gas — spotting acidosis or alkalosis and whether it is compensated. It is built for exam preparation and building interpretation skills. Use it to learn and check your reasoning, not to replace clinical interpretation by a qualified provider.
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Open calculator →Frequently Asked Questions
How do you interpret an ABG?
Check the pH for acidosis (<7.35) or alkalosis (>7.45), then see whether PaCO2 (respiratory) or HCO3 (metabolic) explains it, then judge whether the other value has shifted to compensate.
What are normal ABG values?
pH 7.35–7.45, PaCO2 35–45 mmHg, and HCO3 22–26 mEq/L. Values outside these ranges signal an acid-base disturbance whose cause the pattern reveals.
What is respiratory versus metabolic?
Respiratory disorders are driven by PaCO2 (carbon dioxide, an acid handled by the lungs); metabolic disorders are driven by HCO3 (bicarbonate, a base handled by the kidneys).
What is compensation?
It is the body's attempt to correct pH by shifting the other system — for example the kidneys raising HCO3 to offset a chronically high CO2. It can be none, partial, or full.
Can this diagnose a patient?
No. It is an educational interpreter for practising the method. Real blood gases must be read in full clinical context by a qualified clinician.
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