Anion Gap Calculator
Clinical acid-base diagnostic system calculating standard serum anion gap, albumin-corrected anion gap, delta gap, and delta-delta ratio with complete electrolyte electroneutrality analysis.
Patient Serum Electrolytes
Include Potassium (K+) in Calculation
Modern ISE practice omits K+; older reference uses [Na+ + K+] − [Cl− + HCO3−]
Albumin-Corrected AG
12.0 mEq/L
Δ from measured: 0.0 mEq/L
Anion Gap with K+
16.0 mEq/L
Normal: 12 – 16 mEq/L
Delta Gap (ΔAG)
0.0 mEq/L
AG − 12.0 normal reference
Delta Ratio (ΔΔ)
1.00
Equilibrium (0.0 / 0.0)
Electrolyte & Gap Breakdown
| Ion / Parameter | Measured | Reference Range | Status |
|---|---|---|---|
| Sodium (Na+) | 140.0 mEq/L | 135 – 145 mEq/L | Normal |
| Chloride (Cl−) | 104.0 mEq/L | 96 – 106 mEq/L | Normal |
| Bicarbonate (HCO3−) | 24.0 mEq/L | 22 – 28 mEq/L | Normal |
| Serum Albumin | 4.0 g/dL | 3.5 – 5.0 g/dL | Normal |
| Measured Cations | 140.0 mEq/L | Na+ (or Na+ + K+) | Base |
| Measured Anions | 128.0 mEq/L | Cl− + HCO3− | Base |
| Standard Anion Gap | 12.0 mEq/L | 8.0 – 12.0 mEq/L (ISE) | Normal |
| Albumin-Corrected AG | 12.0 mEq/L | 8.0 – 12.0 mEq/L | Normal |
2D Electroneutrality Cation-Anion Balance & Risk Dial
Direct comparison of measured serum charges and unmeasured anion gap
Sodium (Na⁺)
Chloride (Cl⁻)
Bicarbonate (HCO₃⁻)
Anion Gap
Live Mathematical Formula Substitution:
1. AG = [Na⁺] − ([Cl⁻] + [HCO₃⁻]) = 140 − (104 + 24) = 140 − 128 = 12.0 mEq/L
2. Albumin-Corrected AG = 12.0 + 2.5 × (4.0 − 4.0) = 12.0 + 0.0 = 12.0 mEq/L
3. Delta Gap (ΔAG) = 12.0 − 12.0 = 0.0 mEq/L | ΔHCO₃⁻ = 24.0 − 24.0 = 0.0 mEq/L
Delta Gap (ΔAG) & Delta-Delta Ratio (ΔΔ) Analysis
The Delta-Delta ratio evaluates mixed acid-base disorders by comparing the excess anion gap (ΔAG) to the deficit in serum bicarbonate (ΔHCO3−). In a pure High Anion Gap Metabolic Acidosis (HAGMA), each 1 mEq/L increase in unmeasured anions titrates approximately 1 mEq/L of bicarbonate.
Excess Anion Gap (ΔAG)
0.0 mEq/L
Corrected AG − 12.0
Bicarbonate Deficit (ΔHCO3−)
0.0 mEq/L
24.0 − Measured HCO3−
Delta Ratio (ΔΔ)
1.00
Equilibrium (0.0 / 0.0)
Ratio < 0.4
Pure NAGMA
Hyperchloremic acidosis. Bicarbonate falls without an equivalent rise in unmeasured organic anions. Causes: severe diarrhea, RTA, high-volume 0.9% saline.
Ratio 0.4 – 0.8
Mixed HAGMA + NAGMA
Both high anion gap and normal anion gap processes coexist. Classic example: DKA with heavy vomiting and saline resuscitation, or DKA + diarrhea.
Ratio 0.8 – 2.0
Pure HAGMA / Normal
Uncomplicated high anion gap acidosis (or normal equilibrium). 1:1 buffer consumption. Seen in uncomplicated DKA, lactic acidosis, uremia.
Ratio > 2.0
Mixed HAGMA + Metabolic Alkalosis
Bicarbonate is unexpectedly elevated relative to ΔAG. Seen in DKA or lactic acidosis with severe vomiting, NG tube suction, diuretic therapy, or chronic COPD compensation.
Winters' Formula: Expected Respiratory Compensation
Evaluates whether the respiratory center is mounting appropriate hyperventilation compensation for metabolic acidosis: Expected PaCO2 = 1.5 × [HCO3−] + 8 ± 2 mmHg.
Expected PaCO2 Range
42 – 46 mmHg
Mean: 44.0 mmHg (based on HCO3− = 24 mEq/L)
If measured PaCO2 > expected: Concomitant Respiratory Acidosis (e.g. respiratory fatigue, COPD).
If measured PaCO2 < expected: Concomitant Respiratory Alkalosis (e.g. early sepsis, pain, aspirin toxicity).
If measured PaCO2 < expected: Concomitant Respiratory Alkalosis (e.g. early sepsis, pain, aspirin toxicity).
Comprehensive Etiologies & Modern Diagnostic Mnemonics
| Category | Mnemonic & Cause | Primary Unmeasured Anion / Ion | Key Confirmatory Diagnostic Tests |
|---|---|---|---|
| HAGMA | G – Glycols (Ethylene Glycol, Propylene Glycol) | Glycolate, Oxalate, L-lactate | Osmolal gap > 10, toxic alcohol panel, calcium oxalate crystals in urine |
| HAGMA | O – Oxoproline (5-Oxoproline / Pyroglutamic Acid) | 5-Oxoproline | Urine organic acid analysis; history of chronic paracetamol/acetaminophen in malnourished/septic females |
| HAGMA | L – L-Lactate (Tissue Hypoperfusion, Sepsis, Shock) | L-Lactate | Serum lactate level (> 2.0 mmol/L indicates cellular hypoperfusion/shock) |
| HAGMA | D – D-Lactate (Short Bowel Syndrome, Bacterial Overgrowth) | D-Lactate | Specific D-lactate stereoisomer assay (standard clinical lactate assay only detects L-lactate) |
| HAGMA | M – Methanol (Windshield Washer Fluid, Moonshine) | Formate (Formic Acid) | Osmolal gap > 10, serum methanol chromatography, visual impairment / optic papillitis |
| HAGMA | A – Aspirin / Salicylates (Salicylate Intoxication) | Salicylate, Lactate, Ketones | Serum salicylate level; mixed respiratory alkalosis + high anion gap metabolic acidosis |
| HAGMA | R – Renal Failure (Acute or End-Stage Uremia) | Sulfates, Phosphates, Urate, Hippurate | BUN, Serum Creatinine, eGFR < 15 mL/min, oliguria |
| HAGMA | K – Ketoacidosis (DKA, Alcoholic AKA, Starvation) | β-Hydroxybutyrate, Acetoacetate | Serum quantitative β-hydroxybutyrate, blood glucose, urine acetoacetate test |
| NAGMA | U – Ureteroenterostomy / Ileal Conduit | Hyperchloremia (Cl− absorbed for HCO3−) | Surgical history of urinary diversion; elevated serum chloride |
| NAGMA | S – Saline Resuscitation (0.9% Normal Saline Infusion) | Hyperchloremic Acidosis | Infusion history of normal saline (Cl− concentration is 154 mEq/L vs plasma 104) |
| NAGMA | E – Endocrine (Hypoaldosteronism, Addison's) | Type 4 RTA (Hyperkalemic Acidosis) | Elevated serum K+, low serum aldosterone, low morning cortisol |
| NAGMA | D – Diarrhea (GI Bicarbonate Loss) | Direct enteric loss of HCO3− | Stool history; Negative Urinary Anion Gap (UAG = Na+ + K+ − Cl− < 0) |
| NAGMA | C – Carbonic Anhydrase Inhibitors (Acetazolamide) | Renal HCO3− wasting | Medication list (glaucoma or altitude prophylaxis); alkaline urine pH > 7.0 |
| NAGMA | A – Ammonium Chloride / Parenteral Nutrition | Acid loading with chloride | Hyperalimentation review, total parenteral nutrition (TPN) chloride load |
| NAGMA | R – Renal Tubular Acidosis (Types 1, 2, 4) | Defective H+ excretion or HCO3− reabsorption | Positive Urinary Anion Gap (UAG > 0); urine pH > 5.5 despite systemic acidemia |
| NAGMA | P – Pancreatic / Biliary Fistula | Direct loss of bicarbonate-rich digestive fluid | Surgical drain output analysis, fluid electrolyte panel |
| LOW GAP | Hypoalbuminemia (Sepsis, Cirrhosis, Nephrotic) | Deficit of normal unmeasured negative charges | Serum Albumin < 3.5 g/dL; AG drops by 2.5 mEq/L per 1.0 g/dL drop in albumin |
| LOW GAP | Multiple Myeloma (IgG Paraproteinemia) | Positively charged unmeasured monoclonal IgG proteins | Serum protein electrophoresis (SPEP), serum free light chains, bone marrow biopsy |
| LOW GAP | Lithium Toxicity (Cation Accumulation) | Unmeasured monovalent cation (Li+) | Serum lithium level (> 1.5 mEq/L indicates clinical toxicity) |
| LOW GAP | Bromide / Iodide Toxicity (Halide Interference) | False lab chloride elevation (ISE cross-reactivity) | Extremely low or negative AG (< 0 mEq/L); serum bromide levels |
| LOW GAP | Severe Hypercalcemia / Hypermagnesemia | Excess unmeasured divalent cations (Ca2+, Mg2+) | Ionized calcium, total serum calcium, serum magnesium level |
Principles of Serum Anion Gap & Electroneutrality in Clinical Practice
The principle of electroneutrality mandates that in any aqueous biological compartment, the total concentration of positive electrical charges (cations) must exactly equal the total concentration of negative electrical charges (anions). In human serum, the principal cations are Sodium (Na+), Potassium (K+), Calcium (Ca2+), and Magnesium (Mg2+). The principal anions are Chloride (Cl−), Bicarbonate (HCO3−), Albumin (carrying net negative charges at physiological pH 7.40), Phosphate (PO43−), Sulfate (SO42−), and organic acids.
Modern Ion-Selective Electrode (ISE) Reference Range Notice
Historical textbooks cited a normal anion gap range of 8 to 16 mEq/L, which was established using older colorimetric and flame photometry methodologies. Modern hospital clinical chemistry laboratories utilize Ion-Selective Electrode (ISE) analyzers that measure serum chloride 2 to 4 mEq/L higher than older techniques. Consequently, the modern consensus normal reference interval for serum anion gap without potassium is 8 to 12 mEq/L (or 3 to 11 mEq/L in some institutional analyzers). Any anion gap > 12 mEq/L warrants investigative clinical evaluation.
The Figge-Jabor-Kazda-Fencl Albumin Correction Equation
Serum albumin is the single largest contributor to the normal unmeasured anion pool. Under normal conditions (serum albumin 4.0 g/dL), albumin accounts for approximately 10 to 12 mEq/L of negative charge (roughly 2.5 to 2.8 mEq/L of negative charge per gram of albumin). When a patient develops hypoalbuminemia—a pervasive finding in intensive care units, sepsis, liver cirrhosis, starvation, and nephrotic syndrome—the baseline measured anion gap plummets.
For every 1.0 g/dL drop in serum albumin below 4.0 g/dL, the observed anion gap drops by approximately 2.5 mEq/L (or 0.25 mEq/L per 1 g/L drop below 40 g/L). If a clinician fails to correct for low albumin, an occult, life-threatening High Anion Gap Metabolic Acidosis (such as lactic acidosis or diabetic ketoacidosis) may appear deceptively “normal” on routine lab panels.
Clinical Case Walkthrough: Unmasking Occult Acidosis in Sepsis
Patient Presentation: A 64-year-old female in the intensive care unit with septic shock has the following laboratory profile:
- Sodium (Na+): 138 mEq/L
- Chloride (Cl−): 108 mEq/L
- Bicarbonate (HCO3−): 18 mEq/L
- Serum Albumin: 1.6 g/dL (severely reduced)
1. Standard Anion Gap Calculation:
AG = 138 − (108 + 18) = 138 − 126 = 12.0 mEq/L (Appears deceptively normal on uncorrected glance!)
2. Albumin-Corrected Anion Gap Calculation:
AGcorrected = 12.0 + 2.5 × (4.0 − 1.6) = 12.0 + 2.5 × 2.4 = 12.0 + 6.0 = 18.0 mEq/L
Interpretation: The patient actually has a marked High Anion Gap Metabolic Acidosis (unmasking occult severe hyperlactatemia of 6 mEq/L).
3. Delta Gap & Delta Ratio:
ΔAG = 18.0 − 12.0 = 6.0 mEq/L
ΔHCO3− = 24.0 − 18.0 = 6.0 mEq/L
Delta Ratio = 6.0 / 6.0 = 1.00 (Confirms a pure, uncomplicated High Anion Gap Metabolic Acidosis).
AG = 138 − (108 + 18) = 138 − 126 = 12.0 mEq/L (Appears deceptively normal on uncorrected glance!)
2. Albumin-Corrected Anion Gap Calculation:
AGcorrected = 12.0 + 2.5 × (4.0 − 1.6) = 12.0 + 2.5 × 2.4 = 12.0 + 6.0 = 18.0 mEq/L
Interpretation: The patient actually has a marked High Anion Gap Metabolic Acidosis (unmasking occult severe hyperlactatemia of 6 mEq/L).
3. Delta Gap & Delta Ratio:
ΔAG = 18.0 − 12.0 = 6.0 mEq/L
ΔHCO3− = 24.0 − 18.0 = 6.0 mEq/L
Delta Ratio = 6.0 / 6.0 = 1.00 (Confirms a pure, uncomplicated High Anion Gap Metabolic Acidosis).
Frequently Asked Questions (FAQ)
On modern clinical chemistry analyzers using Ion-Selective Electrodes (ISE), the standard reference interval for serum anion gap calculated without potassium is 8 to 12 mEq/L (or mmol/L). When potassium is included in the formula ([Na⁺ + K⁺] − [Cl⁻ + HCO₃⁻]), the reference interval shifts upward to 12 to 16 mEq/L.
Potassium is predominantly an intracellular cation (normal intracellular concentration ~140 mEq/L), with low and tightly regulated extracellular concentration (3.5 to 5.0 mEq/L). Fluctuations in extracellular potassium contribute little to the overall balance compared to sodium (140 mEq/L). Omitting potassium avoids minor artifacts from slight in vitro hemolysis during blood draw. Most clinical algorithms and medical reference benchmarks use the three-ion formula.
At normal arterial pH of 7.40, serum albumin molecules carry multiple negative charges on their amino acid residues, providing approximately 10 to 12 mEq/L of the normal unmeasured anion pool. When albumin drops, there are fewer unmeasured anions, causing the measured anions (Cl⁻ and HCO₃⁻) to rise to preserve electroneutrality, which mathematically shrinks the apparent anion gap. The correction formula adds 2.5 mEq/L to the anion gap for every 1.0 g/dL deficit in albumin below 4.0 g/dL.
An anion gap below 3 mEq/L or a negative value is almost never physiological. Primary clinical causes include: severe hypoalbuminemia; Multiple Myeloma (where excessive monoclonal IgG antibodies carry a net positive charge, adding unmeasured cations); Lithium intoxication (adding unmeasured monovalent Li⁺); severe hypercalcemia or hypermagnesemia; or laboratory analytical artifact such as Bromide ingestion, which interferes with chloride ISE sensors and produces false massive hyperchloremia.
The Delta Ratio (ΔAG / ΔHCO₃⁻) compares the accumulation of unmeasured organic acid anions to the depletion of serum bicarbonate buffer. A ratio between 0.8 and 2.0 indicates pure HAGMA (like classic diabetic ketoacidosis or lactic acidosis). A ratio below 0.4 indicates pure Normal Anion Gap Metabolic Acidosis (NAGMA), while 0.4 to 0.8 indicates a mixed HAGMA and NAGMA. A ratio above 2.0 signals a mixed disorder where an underlying metabolic alkalosis (e.g. from vomiting or diuretics) or pre-existing respiratory acidosis compensation has preserved bicarbonate levels despite severe high gap acid accumulation.
Medical Diagnostic Disclaimer: This clinical calculator is designed solely for educational, research, and informational support by qualified healthcare professionals and medical students. It does not replace comprehensive clinical judgment, arterial blood gas (ABG) analysis, physical examination, or laboratory consultation. Patient care decisions must incorporate the entirety of clinical findings, hemodynamics, toxicology testing, and organ function assessments.