🧪 Acid-Base Calculator

Arterial blood gas interpretation follows a strict order: check the pH, find which component matches it, then check whether compensation is appropriate. Skip a step and mixed disorders hide in plain sight. This calculator walks the standard sequence automatically.

Enter pH, PaCO2, bicarbonate, sodium, and chloride. You get the primary disorder, the expected compensation from Winter's formula and the respiratory rules, plus anion gap and delta gap to unmask a second metabolic disorder. It is built for students and clinicians reviewing the method, not for bedside decisions.

How to use this calculator

Enter the ABG values and the basic metabolic panel electrolytes. Venous gases can substitute roughly for pH and bicarbonate, but PaCO2 runs higher venously, so arterial values are preferred.

The compensation rules

Compensation never overcorrects: the pH always stays on the side of the primary disorder. Each disorder has an expected respiratory or metabolic response.

Winter's formula is the workhorse: in pure metabolic acidosis, the lungs blow off CO2 to a predictable level. If PaCO2 is higher than expected, there is a superimposed respiratory acidosis (think obtunded patient); if lower, a superimposed respiratory alkalosis (think sepsis or pain).

Worked example

pH 7.32, PaCO2 30, HCO3 15, Na 140, Cl 108. The pH is low (acidemia) and bicarbonate is low, so the primary disorder is metabolic acidosis. Winter's expected PaCO2 is 1.5 x 15 + 8 = 30.5, range 28.5 to 32.5. Actual PaCO2 is 30: compensation is appropriate, a simple disorder.

The anion gap is 140 - (108 + 15) = 17, which is high. The delta gap is (17 - 12) - (24 - 15) = 5 - 9 = -4, inside the -6 to +6 window, so there is no hidden second metabolic disorder. Final read: simple high-anion-gap metabolic acidosis with appropriate respiratory compensation. Classic DKA numbers.

Reading mixed disorders

Two red flags reveal mixed disorders. First, compensation that misses the expected range means a second respiratory disorder. Second, a delta gap outside -6 to +6 means a second metabolic disorder: below -6 is a combined high and normal anion gap acidosis; above +6 is high anion gap acidosis plus metabolic alkalosis (the vomiting DKA patient).

A normal pH never rules out serious pathology. Metabolic acidosis plus metabolic alkalosis can cancel out to pH 7.40 while the patient is in real trouble. For the oxygen side of the same ABG, see our A-a gradient calculator and arterial blood pH calculator.

Common exam traps

Direction errors are the classic mistake: in respiratory acidosis the CO2 is high and bicarbonate rises to compensate; students often flip which one is primary. Anchor on the pH first: whichever of PaCO2 or HCO3 moves in the direction that explains the pH is the primary disorder.

Another trap: chronic respiratory disorders need time. The "chronic" compensation numbers assume days of adaptation. A patient intubated yesterday with a PaCO2 of 70 is acute until proven otherwise, so use the acute expectations first.

A practice routine for ABG mastery

Reading about acid-base disorders builds familiarity; working cases builds skill. The fastest routine is ten minutes a day with real or realistic numbers. Cover the answer, run the three steps out loud (pH, matching abnormality, compensation), then check. After twenty cases the sequence becomes reflex.

Build your own cases by starting from the answer. Pick "metabolic alkalosis with appropriate compensation," choose HCO3 34, compute expected PaCO2 as 40 + 0.7 x 10 = 47, set PaCO2 46, then derive pH around 7.49. Reverse-engineering teaches the relationships faster than forward-solving alone.

Common beginner errors to watch: forgetting that compensation never normalizes pH fully, mixing up acute and chronic respiratory expectations, and skipping the delta gap when the anion gap is high. Every missed case is usually one of these three, so checking them explicitly catches most mistakes before they happen.

Metabolic acidosis: high vs normal anion gap

Not all metabolic acidoses are the same, and the anion gap sorts them. A high gap means unmeasured acids have arrived: lactate from shock, ketoacids from DKA, toxins like methanol. A normal gap means bicarbonate was lost and chloride took its place: diarrhea, renal tubular acidosis, or large volumes of normal saline.

The distinction drives treatment. High-gap acidosis demands finding and fixing the acid source: insulin for DKA, perfusion for lactate, dialysis for toxic alcohols. Normal-gap acidosis is usually fixed by stopping the loss and giving bicarbonate back, often as oral bicarbonate or just time. Giving bicarbonate to a high-gap acidosis without treating the cause is one of the classic board-exam traps.

The delta gap then checks for a second disorder hiding underneath, as the calculator shows. A vomiting DKA patient, for instance, carries high-gap acidosis plus metabolic alkalosis, and only the delta gap reveals the combination.

For informational purposes only, not medical advice. Consult a healthcare professional.