Anion Gap Calculator
Calculate serum anion gap and albumin-corrected anion gap. Differentiate metabolic acidosis causes with clinical interpretation.
Acid-Base Disorder Calculator
Interpret ABG results with step-by-step acid-base analysis. Calculates anion gap, delta-delta ratio, Winter's formula, and identifies primary and mixed disorders.
⚠️ Medical Disclaimer: ABG interpretation must always be done in full clinical context. This tool is for educational purposes only.
mmHg
mEq/L
mEq/L
mEq/L
g/dL
Metabolic Acidosis
Anion Gap: 20 (corrected: 20) — ELEVATED
Primary Disorder⧉
Metabolic Acidosis
Based on pH, pCO₂, and HCO₃⁻ analysis
Anion Gap⧉
20
Na⁺ − Cl⁻ − HCO₃⁻ = 140 − 105 − 15
Corrected AG⧉
20
Adjusted for albumin: AG + 2.5 × (4 − albumin)
AG Elevated?⧉
Yes (>12)
Consider MUDPILES: Methanol, Uremia, DKA, Propylene glycol, INH/Iron, Lactic acidosis, Ethylene glycol, Salicylates
Delta-Delta Ratio⧉
0.89
Concurrent non-AG met acidosis
pH Status⧉
Acidemic
pH 7.30 (normal: 7.35-7.45)
Step-by-Step Interpretation
- Low pH + Low HCO₃⁻ → Metabolic Acidosis
- Anion Gap = 20 (corrected for albumin: 20)
- ⚠ Elevated anion gap → consider: DKA, lactic acidosis, uremia, toxins (MUDPILES)
- Delta-delta ratio 0.89 < 1 → concurrent non-AG metabolic acidosis
- Winter's formula: expected pCO₂ = 30.5 ± 2
- pCO₂ within expected range → appropriate compensation
Primary Acid-Base Disorders
| Disorder | pH | pCO₂ | HCO₃⁻ | Common Causes |
|---|---|---|---|---|
| Metabolic Acidosis | ↓ | ↓ (comp) | ↓ | DKA, lactic acidosis, renal failure, toxins |
| Metabolic Alkalosis | ↑ | ↑ (comp) | ↑ | Vomiting, diuretics, antacid overuse |
| Respiratory Acidosis | ↓ | ↑ | ↑ (comp) | COPD, hypoventilation, sedation, airway obstruction |
| Respiratory Alkalosis | ↑ | ↓ | ↓ (comp) | Hyperventilation, anxiety, PE, pregnancy, sepsis |
Compensation Rules
| Disorder | Expected Compensation |
|---|---|
| Met Acidosis (acute) | Expected pCO₂ = 1.5 × HCO₃⁻ + 8 (±2) (Winter's) |
| Met Alkalosis | Expected pCO₂ = 0.7 × HCO₃⁻ + 21 (±2) |
| Resp Acidosis (acute) | Expected HCO₃⁻ = 24 + (pCO₂ − 40)/10 |
| Resp Acidosis (chronic) | Expected HCO₃⁻ = 24 + 3.5 × (pCO₂ − 40)/10 |
| Resp Alkalosis (acute) | Expected HCO₃⁻ = 24 − 2 × (40 − pCO₂)/10 |
| Resp Alkalosis (chronic) | Expected HCO₃⁻ = 24 − 5 × (40 − pCO₂)/10 |
Planning notes, formulas, and examples
About the Acid-Base Disorder Calculator
The Acid-Base Disorder Calculator provides systematic, step-by-step interpretation of arterial blood gas (ABG) results. It identifies the primary acid-base disorder (metabolic or respiratory acidosis/alkalosis), calculates the anion gap with albumin correction, applies Winter's formula for compensation assessment, and computes the delta-delta ratio to detect mixed disorders.
Acid-base interpretation is one of the most challenging skills in clinical medicine. The systematic approach requires evaluating pH to determine acidemia or alkalemia, identifying the primary disturbance from pCO₂ and HCO₃⁻, checking compensation adequacy, and calculating the anion gap for metabolic acidosis workup. This calculator automates all steps and presents findings in clinical decision-making order.
The tool supports complex mixed disorders by combining primary disorder identification with compensation analysis and the delta-delta ratio. It includes clinical presets for common presentations (DKA, COPD exacerbation, vomiting, hyperventilation) and reference tables for primary disorders and compensation formulas. Every finding is explained step-by-step, making it valuable for both clinical practice and medical education.
When This Page Helps
ABG interpretation involves several sequential checks that are easy to miss under time pressure. This calculator keeps the steps in order, shows the compensation and anion gap calculations together, and makes mixed disorders easier to review in ICU, emergency, and teaching settings.
How to Use the Inputs
- Enter pH, pCO₂, and HCO₃⁻ from the arterial blood gas result.
- Enter Na⁺ and Cl⁻ from the basic metabolic panel for anion gap calculation.
- Enter serum albumin for corrected anion gap (defaults to 4.0 if normal).
- Review the primary disorder identification and step-by-step findings.
- Check the anion gap and delta-delta ratio for mixed disorder detection.
- Use presets to study common clinical scenarios.
- Reference the compensation rules table to verify adequacy.
Formula used
Anion Gap = Na⁺ − Cl⁻ − HCO₃⁻
Corrected AG = AG + 2.5 × (4 − albumin)
Winter's Formula: Expected pCO₂ = 1.5 × HCO₃⁻ + 8 (±2)
Delta-Delta Ratio = (AG − 12) / (24 − HCO₃⁻)Example Calculation
Result: Metabolic Acidosis, AG = 30 (elevated), delta-delta 1.38 (pure AG acidosis)
Low pH (7.20) with low HCO₃⁻ (10) = metabolic acidosis. Elevated AG (30) points to organic acids (DKA, lactic acidosis). Winter's expected pCO₂ = 23 ±2; measured 25 = appropriate respiratory compensation.
Tips & Best Practices
- Always start with pH to determine acidemia vs. alkalemia.
- Check pCO₂ and HCO₃⁻ to identify the primary driver.
- Calculate the anion gap for ALL metabolic acidoses — it changes management.
- Always correct the AG for albumin in ICU and malnourished patients.
- If the AG is elevated, always calculate the delta-delta ratio.
- A normal pH does not mean normal — two opposing disorders can normalize pH.
Systematic Approach to ABG Interpretation
Step 1: Look at pH (acidemia <7.35, alkalemia >7.45). Step 2: Identify the primary process — does pCO₂ or HCO₃⁻ explain the pH? Step 3: Assess compensation using expected formulas. Step 4: Calculate anion gap if metabolic acidosis is present. Step 5: If AG is elevated, calculate delta-delta ratio for hidden second metabolic disorder.
MUDPILES: Differential for Elevated Anion Gap
The mnemonic MUDPILES helps recall causes of AG metabolic acidosis: Methanol, Uremia, Diabetic ketoacidosis, Propylene glycol, Isoniazid/Iron, Lactic acidosis, Ethylene glycol, Salicylates. Each has specific follow-up tests (osmolar gap for toxic alcohols, lactate, ketones, BUN/Cr).
Normal Anion Gap (Hyperchloremic) Acidosis
When HCO₃⁻ is low but the AG is normal, look for: diarrhea (GI bicarbonate loss), renal tubular acidosis, early renal failure, carbonic anhydrase inhibitors, and saline infusion dilutional acidosis. The urine anion gap (Na⁺ + K⁺ − Cl⁻) helps distinguish renal from GI causes.
Sources & Methodology
Last updated:
Methodology
This worksheet follows the usual stepwise acid-base review used at the bedside. It starts with pH to classify acidemia or alkalemia, then compares pCO2 and bicarbonate to identify the primary disturbance. For metabolic acidosis it calculates the anion gap, offers an albumin-corrected anion gap, applies Winter's formula to judge expected respiratory compensation, and uses the delta-delta ratio to look for an additional metabolic process.
The output is an interpretation aid, not a substitute for full clinical review. Arterial or venous sample quality, laboratory timing, mixed respiratory disorders, lactate, ketones, renal function, and toxicology data can all change the final interpretation.
Sources
- Physiology, Acid Base Balance (NCBI Bookshelf / StatPearls) — Public reference for primary disturbances, compensation, and mixed-disorder interpretation.
- Clinical Physiology of Acid-Base and Electrolyte Disorders (McGraw-Hill) — Classic reference text for systematic acid-base interpretation and compensation rules.
Frequently Asked Questions
The anion gap (Na⁺ − Cl⁻ − HCO₃⁻) estimates unmeasured anions. Normal is 8-12 mEq/L. Elevation indicates accumulation of unmeasured acids (lactate, ketoacids, toxic alcohols, uremia).
Albumin is the major unmeasured anion. Hypoalbuminemia (common in hospitalized patients) lowers the baseline AG, masking an elevated gap. Add 2.5 for each g/dL albumin below 4.
If there is an AG metabolic acidosis, the delta-delta ratio reveals if there is a concurrent non-AG metabolic acidosis (<1) or metabolic alkalosis (>2). A ratio of 1-2 indicates a pure AG acidosis.
Winter's formula predicts the expected pCO₂ in metabolic acidosis. If measured pCO₂ exceeds the expected range, a concurrent respiratory acidosis is present. If below, respiratory alkalosis coexists.
Yes, mixed disorders are common. A patient with DKA and pneumonia might have AG metabolic acidosis plus respiratory acidosis. Systematic analysis with compensation rules and delta-delta catches these.
When compensation is inadequate or excessive (per expected compensation formulas), when the delta-delta ratio falls outside 1-2, or when clinical context suggests multiple processes (e.g., sepsis + renal failure + vomiting).
More in this topic
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