About

Arterial blood gas (ABG) interpretation errors occur in up to 30% of clinical assessments. Misclassifying a mixed acid-base disorder as a simple one can delay treatment of sepsis, diabetic ketoacidosis, or salicylate poisoning. This calculator applies the Henderson-Hasselbalch equation (pH = 6.1 + log₁₀(HCO₃⁻ ÷ (0.03 × PaCO₂))) to compute expected pH, then runs a six-step clinical algorithm: primary disorder classification, expected compensation via Boston rules, anion gap with albumin correction, and delta-delta ratio analysis. It approximates steady-state conditions and assumes sea-level barometric pressure. Results do not replace arterial puncture or venous blood gas correlation in critically ill patients.

Edge cases matter. A PaCO₂ below 10 mmHg suggests measurement artifact or extreme hyperventilation. An anion gap above 30 mEq/L nearly always indicates organic acidosis regardless of pH. This tool flags these scenarios. Pro tip: always check the internal consistency of reported ABG values. If the measured pH deviates from the Henderson-Hasselbalch calculated pH by more than 0.04, suspect a laboratory error.

Formulas

The core equation governing acid-base chemistry in blood is the Henderson-Hasselbalch equation:

pH = 6.1 + log₁₀ ( HCO₃⁻0.03 × PaCO₂ )

Where HCO₃⁻ is bicarbonate concentration in mEq/L, PaCO₂ is arterial carbon dioxide partial pressure in mmHg, and 0.03 is the solubility coefficient of CO₂ in plasma at 37°C.

Winter's Formula predicts expected respiratory compensation in metabolic acidosis:

Expected PaCO₂ = 1.5 × HCO₃⁻ + 8 ± 2

The Anion Gap quantifies unmeasured anions:

AG = Na⁺ − (Cl⁻ + HCO₃⁻)

Albumin-corrected anion gap accounts for hypoalbuminemia, which lowers the apparent AG:

AG_corrected = AG + 2.5 × (4.0 − Albumin)

The Delta-Delta Ratio detects mixed metabolic disorders when the corrected AG is elevated:

ΔΔ = AG_corrected − 1224 − HCO₃⁻

Where ΔΔ < 1 suggests concurrent non-anion-gap metabolic acidosis, 1 - 2 suggests pure anion gap metabolic acidosis, and ΔΔ > 2 suggests concurrent metabolic alkalosis.

Reference Data

Disorder	Primary Change	Expected Compensation	Typical pH Range	Common Causes
Metabolic Acidosis	HCO₃⁻ ↓	PaCO₂ = 1.5 × HCO₃⁻ + 8 ± 2 (Winter's)	< 7.35	DKA, lactic acidosis, renal failure, diarrhea
Metabolic Alkalosis	HCO₃⁻ ↑	PaCO₂ ↑ by 0.7 mmHg per 1 mEq/L HCO₃⁻ rise	> 7.45	Vomiting, diuretics, hyperaldosteronism
Acute Respiratory Acidosis	PaCO₂ ↑	HCO₃⁻ ↑ by 1 per 10 mmHg PaCO₂ rise	< 7.35	COPD exacerbation, opioid overdose, pneumothorax
Chronic Respiratory Acidosis	PaCO₂ ↑	HCO₃⁻ ↑ by 3.5 per 10 mmHg PaCO₂ rise	7.32-7.38	COPD (stable), obesity hypoventilation, kyphoscoliosis
Acute Respiratory Alkalosis	PaCO₂ ↓	HCO₃⁻ ↓ by 2 per 10 mmHg PaCO₂ fall	> 7.45	Anxiety, pain, PE, early sepsis, high altitude
Chronic Respiratory Alkalosis	PaCO₂ ↓	HCO₃⁻ ↓ by 5 per 10 mmHg PaCO₂ fall	7.42-7.48	Chronic liver disease, pregnancy, CNS lesions
Normal ABG	None	None	7.35-7.45	Healthy individual
High Anion Gap Acidosis	AG > 12	Check delta-delta	Variable	MUD PILES: Methanol, Uremia, DKA, Propylene glycol, INH/Iron, Lactic acid, Ethylene glycol, Salicylates
Normal Anion Gap Acidosis	AG 8-12, Cl⁻ ↑	Check urine AG	< 7.35	Diarrhea, RTA types I/II/IV, saline infusion
Delta-Delta > 2	Concurrent metabolic alkalosis	HCO₃⁻ higher than expected	Variable	Vomiting + DKA, diuretics + lactic acidosis
Delta-Delta < 1	Concurrent non-AG acidosis	HCO₃⁻ lower than expected	Variable	DKA + diarrhea, RTA + organic acidosis
Normal Albumin	4.0 g/dL	AG correction: +2.5 per 1 g/dL below 4	-	Hypoalbuminemia masks elevated AG
Critical pH < 6.80	Severe acidemia	Life-threatening	< 6.80	Cardiac arrest, massive lactic acidosis
Critical pH > 7.70	Severe alkalemia	Life-threatening	> 7.70	Severe emesis, iatrogenic bicarbonate
PaCO₂ Normal	35-45 mmHg	-	-	Reference range
HCO₃⁻ Normal	22-26 mEq/L	-	-	Reference range
Na⁺ Normal	136-145 mEq/L	-	-	Reference range
Cl⁻ Normal	98-106 mEq/L	-	-	Reference range

Frequently Asked Questions

The calculator derives pH from the Henderson-Hasselbalch equation using the PaCO₂ and HCO₃⁻ you enter. If your lab-measured pH differs by more than 0.04 from the calculated value, the ABG values may be internally inconsistent - possibly due to a sample processing delay, air contamination in the syringe, or a transcription error. Always compare calculated pH against measured pH as a quality check.

Albumin is the dominant unmeasured anion in plasma. Each 1 g/dL decrease in albumin below the normal 4.0 g/dL reduces the apparent anion gap by approximately 2.5 mEq/L. A critically ill patient with albumin of 2.0 g/dL and a "normal" AG of 12 actually has a corrected AG of 17 - consistent with a hidden high-AG metabolic acidosis. This correction is clinically mandatory in ICU patients.

Winter's Formula estimates expected PaCO₂ compensation only in primary metabolic acidosis. It does not apply to metabolic alkalosis (which uses a different compensation ratio of ~0.7 mmHg PaCO₂ rise per 1 mEq/L HCO₃⁻ increase), nor to respiratory disorders. Additionally, it assumes steady-state - compensation takes 12-24 hours to fully develop, so in acute settings the predicted PaCO₂ may not yet be reached.

The delta-delta ratio compares the rise in the corrected anion gap (above normal of 12) to the fall in HCO₃⁻ (below normal of 24). A ratio between 1 and 2 suggests a pure high-AG metabolic acidosis - each mEq/L of unmeasured acid consumes one mEq/L of HCO₃⁻. A ratio below 1 means HCO₃⁻ has dropped more than expected, indicating a concurrent non-AG metabolic acidosis (e.g., diarrhea alongside DKA). A ratio above 2 means HCO₃⁻ is higher than expected, suggesting a concurrent metabolic alkalosis (e.g., vomiting alongside lactic acidosis).

Yes. The calculator applies different compensation rules for acute versus chronic respiratory conditions. In acute respiratory acidosis, HCO₃⁻ rises by 1 mEq/L per 10 mmHg PaCO₂ increase. In chronic respiratory acidosis (>3-5 days), renal compensation raises HCO₃⁻ by 3.5 mEq/L per 10 mmHg. You select the chronicity in the tool, which changes the expected compensation range and therefore the classification of whether compensation is adequate or a mixed disorder is present.

A pH below 7.10 is associated with impaired cardiac contractility, vasodilation, and catecholamine resistance. A pH below 6.80 is generally incompatible with life. On the alkalemic side, pH above 7.55 increases risk of arrhythmias and seizures, and pH above 7.70 carries very high mortality. This calculator flags values outside the 7.35-7.45 normal range and provides specific warnings at critical thresholds.