Interpret an arterial blood gas: calculated pH, anion gap, Winter’s formula compensation check, and primary disorder.
Calculated pH
How to use this tool
These two values determine the calculated pH and the primary disorder.
They give the anion gap, which separates high-gap from normal-gap metabolic acidosis.
In metabolic acidosis, compare actual PaCO2 with the expected range to spot a second respiratory disorder.
A low albumin falsely lowers the anion gap; the corrected value is shown in Advanced options.
Formula & worked example
PaCO2 40 mmHg and HCO3 24 mEq/L give pH 7.40 by Henderson–Hasselbalch, with an anion gap of 16 (Na 140, Cl 100) — a normal gas.
Limitations — read this before you rely on it
Calculated pH assumes standard constants and a correctly measured gas; it does not replace a measured pH or clinical context (oxygenation, lactate, ketones, renal function, drugs). Compensation rules are approximations. This tool is not medical advice — ABG interpretation must be done by a clinician.
Frequently asked questions
In metabolic acidosis, the lungs should compensate by lowering PaCO2 to 1.5 × HCO3 + 8 (±2). If PaCO2 is higher, there is also respiratory acidosis; if lower, respiratory alkalosis.
Above about 12 mEq/L (lab-dependent). Common causes include lactic acidosis, ketoacidosis, renal failure and toxic alcohols.
Albumin is a major unmeasured anion, so low albumin lowers the gap and can hide a high-gap acidosis; the correction adds 2.5 per 1 g/dL below 4.
Yes — mixed disorders can cancel out, and compensated disorders bring pH near normal while HCO3 and PaCO2 stay abnormal.
No — it calculates pH from PaCO2 and HCO3. If your gas includes a measured pH, compare the two; a large mismatch suggests a measurement or entry error.