Arterial blood gas interpreter

What is this for?

Acid–base interpretation follows a fixed three-step sequence: establish the direction of the pH, classify the respiratory and metabolic values independently, then identify which of the two matches the pH. Whichever matches is the primary disorder; the other, if it has moved in the opposite direction, is compensating.

How to use it

1. Enter the pH, pCO₂, and bicarbonate from the arterial blood gas report.
2. Optionally add oxygen saturation, and sodium and chloride if you want the anion gap calculated at the same time.
3. Work through the three steps in the worked solution — they mirror the sequence you would use manually.

Worked example

A patient with a COPD exacerbation has pH 7.28, pCO₂ 55 mmHg, HCO₃ 25 mEq/L. Interpret the blood gas.

Answer: pH 7.28 is acidosis. pCO₂ 55 is above 45, so respiratory acidosis. HCO₃ 25 is normal. The respiratory value matches the pH direction, so this is an uncompensated respiratory acidosis.

Clinical pearls & pitfalls

• The pCO₂ relationship is inverted relative to intuition: a high pCO₂ produces acidosis, because carbon dioxide forms carbonic acid. Bicarbonate behaves in the expected direction.
• Compensation always moves the pH back toward normal, never past it. If the pH has overshot, there is a second primary disorder rather than compensation.
• Compensation is never complete. A pH that has returned fully to 7.40 in the presence of markedly abnormal pCO₂ and HCO₃ indicates a mixed disorder.
• Winter's formula tells you whether respiratory compensation for a metabolic acidosis is adequate. A pCO₂ higher than predicted means the patient is also failing to ventilate — often a sign of impending respiratory failure.
• Always calculate the anion gap in a metabolic acidosis. Gap and non-gap acidoses have entirely different differential diagnoses.

Assumptions & limitations

• Uses the simple three-step directional method. It does not compute the full set of expected-compensation formulae for every disorder, nor the delta-delta ratio for identifying a second metabolic process alongside a gap acidosis.
• Reference ranges vary by laboratory and by altitude. Those used here are the conventional sea-level values.
• Assumes an arterial sample. Venous gases have a systematically higher pCO₂ and lower pH, and cannot be interpreted against these ranges.
• Interpretation of the numbers is not a diagnosis. The clinical context determines the cause and the management.

References

• Berend K, de Vries AP, Gans RO. Physiological approach to assessment of acid–base disturbances. N Engl J Med. 2014;371(15):1434-1445.
• Albert MS, Dell RB, Winters RW. Quantitative displacement of acid–base equilibrium in metabolic acidosis. Ann Intern Med. 1967;66(2):312-322. (Winter's formula.)
• Seifter JL. Integration of acid–base and electrolyte disorders. N Engl J Med. 2014;371(19):1821-1831.

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