How to Use This Calculator
- 1
Enter Sodium (Na⁺) (mEq/L)
Input the serum sodium concentration. Reference range is typically 135–145 mEq/L.
- 2
Enter Chloride (Cl⁻) (mEq/L)
Input the serum chloride concentration. Reference range is typically 96–106 mEq/L.
- 3
Enter Bicarbonate (HCO₃⁻) (mEq/L)
Input the serum bicarbonate concentration. Reference range is typically 22–26 mEq/L.
- 4
Enter Serum Albumin (g/dL)
Input the serum albumin level. Standard baseline is 4.0 g/dL. Low albumin depresses uncorrected anion gap values.
- 5
Review Your Results
View your Anion Gap, Albumin-Corrected AG, Delta Ratio, Delta Gap, Strong Ion Difference, and HCO₃ Deficit alongside actionable clinical insights.
Example Calculation
A patient presents with lab values of Sodium 140 mEq/L, Chloride 102 mEq/L, Bicarbonate 24 mEq/L, and Serum Albumin 4.0 g/dL.
Sodium (Na⁺) (mEq/L)
140
Chloride (Cl⁻) (mEq/L)
102
Bicarbonate (HCO₃⁻) (mEq/L)
24
Serum Albumin (g/dL)
4.0
Results
Anion Gap
14 mEq/L
Albumin-Corrected AG
14 mEq/L
Delta Gap
2 mEq/L
Delta Ratio
N/A
Tips
Always Correct for Albumin
Albumin carries a net negative charge and constitutes a significant portion of unmeasured anions. Hypoalbuminemia lowers the measured anion gap by approximately 2.5 mEq/L for every 1.0 g/dL drop below 4.0 g/dL.
Use Delta Ratio for Mixed Disorders
The Delta Ratio (ΔAG / ΔHCO₃⁻) helps identify concurrent acid-base disturbances. A ratio between 1.0 and 2.0 indicates a pure high-AG metabolic acidosis, while <0.4 indicates hyperchloremic acidosis.
Check Lab Error Potential
An unusually low or negative anion gap (<8 mEq/L) often points to hypoalbuminemia, hypercalcemia, severe hyperkalemia, lithium toxicity, or bromide intoxication interfering with chloride assays.
Understanding the Anion Gap & Acid-Base Physiology
The Anion Gap Calculator evaluates clinical electrolyte panels to identify and differentiate metabolic acid-base disorders in human and veterinary medicine.
By analyzing serum sodium, chloride, bicarbonate, and albumin, it delivers essential diagnostic metrics including the standard anion gap, albumin-corrected anion gap (Figge formula), delta gap, delta ratio, and strong ion difference.
For a patient with serum sodium 140 mEq/L, chloride 102 mEq/L, bicarbonate 24 mEq/L, and albumin 4.0 g/dL, the uncorrected anion gap is 14.0 mEq/L, representing a mild elevation above the typical baseline of 12 mEq/L in 2026 practice.
Core Formulas & Calculations
1. Standard Anion Gap
$$\text{Anion Gap} = \text{Na}^+ - (\text{Cl}^- + \text{HCO}_3^-)$$
Normal reference range:8 to 12 mEq/L (varies slightly depending on lab analyzers).
2. Albumin-Corrected Anion Gap (Figge Formula)
$$\text{Corrected AG} = \text{AG} + 2.5 \times (4.0 - \text{Serum Albumin})$$
Each 1.0 g/dL drop in serum albumin below 4.0 g/dL lowers the measured anion gap by 2.5 mEq/L.
3. Delta Gap & Delta Ratio
$$\text{Delta Gap (\Delta AG)} = \text{AG} - 12$$ $$\text{Delta Bicarbonate (\Delta HCO}_3^-) = 24 - \text{HCO}_3^-$$ $$\text{Delta Ratio} = \frac{\Delta AG}{\Delta HCO_3^-} = \frac{\text{AG} - 12}{24 - \text{HCO}_3^-}$$
Step-by-Step Worked Example
Consider a patient presenting with potential metabolic acidosis and the following lab values:
- Sodium (Na⁺):
140 mEq/L - Chloride (Cl⁻):
102 mEq/L - Bicarbonate (HCO₃⁻):
24 mEq/L - Serum Albumin:
4.0 g/dL
Calculation Steps:
- Standard Anion Gap:
$$\text{AG} = 140 - (102 + 24) = 140 - 126 = 14.0\text{ mEq/L}$$ - Albumin Correction:
$$\text{Corrected AG} = 14.0 + 2.5 \times (4.0 - 4.0) = 14.0\text{ mEq/L}$$ - Delta Gap:
$$\Delta \text{AG} = 14.0 - 12 = 2.0\text{ mEq/L}$$ - HCO₃ Deficit:
$$\Delta \text{HCO}_3^- = 24 - 24 = 0.0\text{ mEq/L}$$ - Delta Ratio:
$$\Delta \text{Ratio} = \frac{2.0}{0.0} \rightarrow \text{Undefined / N/A (bicarbonate delta is zero)}$$ - Apparent Strong Ion Difference (SID):
$$\text{Na}^+ - \text{Cl}^- = 140 - 102 = 38.0\text{ mEq/L}$$
Clinical Interpretation of High Anion Gap Acidosis (MUDPILES / KUSSMAUL)
When an elevated anion gap (>12 mEq/L) is detected, clinicians evaluate common etiologies of high-AG metabolic acidosis:
- K — Ketoacidosis (Diabetic, Alcoholic, Starvation)
- U — Uremia (Renal Failure)
- S — Salicylates (Aspirin Overdose)
- S — Sodium Penicillin / Paracetamol (Acetaminophen toxicity)
- M — Methanol ingestion
- A — Alcohol ketoacidosis / Aldehydes
- U — Unmeasured organic acids (e.g., D-lactic acidosis)
- L — Lactic Acidosis (Type A tissue hypoxia, Type B metabolic)
Delta Ratio Clinical Reference Table
| Delta Ratio | Interpretation | Diagnostic Suspicions |
|---|---|---|
| < 0.4 | Hyperchloremic Normal-AG Acidosis | Diarrhea, Renal Tubular Acidosis (RTA), Saline infusion |
| 0.4 – 1.0 | Mixed High-AG and Normal-AG Acidosis | RTA + Lactic acidosis, Renal failure + Diarrhea |
| 1.0 – 2.0 | Pure High-AG Metabolic Acidosis | DKA, Lactic acidosis, Uremia, Toxic ingestions |
| > 2.0 | High-AG Acidosis + Concurrent Metabolic Alkalosis | Vomiting, Nasogastric suction, Diuretic therapy |
Frequently Asked Questions
What is the anion gap and why is it measured?
The anion gap (AG) represents the difference between measured serum cations (sodium) and measured serum anions (chloride and bicarbonate). It is a diagnostic tool used to identify and categorize metabolic acidosis by detecting the presence of unmeasured organic and inorganic anions.
How does hypoalbuminemia affect the anion gap?
Serum albumin is the primary unmeasured anion in human blood. When albumin levels fall, the measured anion gap drops artificially. Using the Figge formula [Corrected AG = Measured AG + 2.5 × (4.0 - Albumin)], clinicians adjust for low albumin to avoid missing high-anion gap metabolic acidosis.
What does an elevated anion gap mean?
An elevated anion gap (>12 mEq/L) indicates an excess of unmeasured anions in the serum, commonly caused by conditions summarized by mnemonics like MUDPILES or KUSSMAUL (e.g., ketoacidosis, lactic acidosis, renal failure, or toxic ingestions such as ethylene glycol or methanol).
What is the Delta Ratio and how is it interpreted?
The Delta Ratio compares the change in anion gap from normal baseline (AG - 12) to the change in bicarbonate from normal baseline (24 - HCO₃⁻). A ratio of 1.0–2.0 indicates pure high-AG metabolic acidosis, <0.4 indicates hyperchloremic (normal-AG) acidosis, 0.4–1.0 indicates a mixed high-AG and normal-AG acidosis, and >2.0 suggests concurrent metabolic alkalosis or pre-existing high bicarbonate.