Anion Gap Calculator
Calculate the anion gap to assess acid-base disorders and metabolic acidosis.
Include potassium if your laboratory method requires it. Many modern methods exclude K⁺.
Most UK laboratories report anion gap without potassium. Confirm your lab’s reference range.
Reference ranges vary by laboratory — always verify with your institution’s reference intervals.
Calculation Breakdown
Differential Diagnosis — High Anion Gap Metabolic Acidosis
Albumin-Corrected Anion Gap (Optional)
Low albumin can mask an elevated anion gap because hypoalbuminaemia lowers the measured AG. Each 1 g/dL decrease in albumin lowers AG by approximately 2.5 mmol/L. This is an optional clinical adjustment for critically ill patients.
Anion Gap Calculator — Clinical Tool for Healthcare Professionals
Our Anion Gap Calculator helps clinicians quickly calculate the anion gap from serum electrolytes to assess acid-base disorders. Input sodium, chloride, bicarbonate, and optionally potassium to determine whether a patient has a normal, high, or low anion gap — important when assessing metabolic acid-base disorders.
What Is the Anion Gap?
The anion gap represents the difference between measured cations (positively charged ions) and measured anions (negatively charged ions) in serum. It reflects the concentration of unmeasured anions such as proteins, phosphate, sulphate, and organic acids. An elevated anion gap indicates the presence of additional unmeasured anions, often suggesting metabolic acidosis.
How to Use This Anion Gap Calculator
- Enter serum electrolyte values: Sodium (Na⁺), Chloride (Cl⁻), and Bicarbonate (HCO₃⁻) in mmol/L.
- Optionally add Potassium (K⁺): Select the formula method based on your laboratory’s reporting convention.
- Click Calculate: The calculator displays the anion gap, reference range, and clinical interpretation.
- Optional albumin correction: Enter albumin if available to calculate an albumin-corrected anion gap.
- Review the differential: The calculator provides relevant differential diagnoses based on the result.
Anion Gap Formula
Two common formulas are used depending on whether potassium is included:
- Without Potassium (most common):
Anion Gap = Na⁺ – (Cl⁻ + HCO₃⁻) - With Potassium:
Anion Gap = (Na⁺ + K⁺) – (Cl⁻ + HCO₃⁻)
Most UK laboratories report anion gap without potassium, with a normal range of 8–12 mmol/L. When potassium is included, the normal range shifts to 12–16 mmol/L. Always confirm with your local laboratory reference intervals.
Interpreting Anion Gap Results
| Anion Gap Value | Interpretation | Clinical Implications |
|---|---|---|
| <8 mmol/L (no K⁺) | Low Anion Gap | Rare. May occur with hypoalbuminaemia, paraproteinaemia, lithium toxicity, or bromide intoxication. |
| 8–12 mmol/L (no K⁺) | Within Reference Range | Within the example reference range. A normal anion gap does not exclude acid-base disorders; interpret alongside pH, bicarbonate and clinical context. |
| 13–20 mmol/L | Elevated Anion Gap | High anion gap metabolic acidosis (HAGMA) may be present. Investigate for toxic alcohols, DKA, lactic acidosis, renal failure, or salicylates. |
| >20 mmol/L | Markedly Elevated | A substantially elevated anion gap warrants prompt clinical assessment and correlation with pH, bicarbonate, lactate, ketones and the clinical picture. |
Albumin-Corrected Anion Gap
Hypoalbuminaemia can mask an elevated anion gap because albumin is a major unmeasured anion. For every 1 g/dL (10 g/L) decrease in albumin below normal, the anion gap decreases by approximately 2.5 mmol/L. This means a patient with severe illness and low albumin may have a “normal” anion gap despite having significant unmeasured anions.
Corrected Anion Gap = Measured AG + 2.5 × (Normal Albumin − Patient Albumin) ÷ 10
Many critically ill patients have hypoalbuminaemia, making albumin correction particularly important in ICU and emergency settings. Note that this is an optional clinical adjustment — the corrected value is not universally superior and should be interpreted alongside the uncorrected AG.
Differential Diagnosis — High Anion Gap Metabolic Acidosis (HAGMA)
Use the mnemonic MUDPILES to remember causes of elevated anion gap:
- Methanol poisoning
- Uraemia (chronic kidney disease/acute kidney injury)
- Diabetic ketoacidosis (Type 1 and Type 2 diabetes)
- Paraldehyde (historical, rarely encountered)
- Iron tablets or Isoniazid overdose
- Lactic acidosis (tissue hypoxia, sepsis, metformin, liver failure)
- Ethylene glycol (antifreeze) poisoning
- Salicylates (aspirin overdose)
A more modern mnemonic is MASTERS:
- Methanol
- Alcoholic ketoacidosis
- Salicylates
- Toluene (glue sniffing)
- Ethylene glycol
- Renal failure
- Starvation or diabetic ketoacidosis
Non-Anion Gap Metabolic Acidosis (NAGMA)
If the anion gap is within reference range but acidosis is suspected clinically, consider causes of hyperchloraemic metabolic acidosis:
- Gastrointestinal bicarbonate loss (severe diarrhoea, ileostomy)
- Renal tubular acidosis (types 1, 2, and 4)
- Acetazolamide therapy
- Ammonium chloride ingestion
- Total parenteral nutrition (TPN)
- Early renal failure
Common Clinical Scenarios
Diabetic Ketoacidosis (DKA)
Patients typically present with an elevated anion gap (often >20 mmol/L), low bicarbonate (<15 mmol/L), and ketones positive in urine or blood. Euglycaemic DKA can occur, particularly with SGLT2 inhibitor use, so do not rely solely on blood glucose. Treatment involves insulin infusion, fluid resuscitation, and electrolyte monitoring.
Lactic Acidosis
May be type A (due to tissue hypoperfusion/sepsis/shock) or type B (due to drugs, malignancy, or mitochondrial dysfunction). Lactate levels will be elevated. Address underlying cause promptly.
Toxic Alcohol Poisoning
Methanol and ethylene glycol cause severe high anion gap metabolic acidosis. An osmolal gap may be elevated, particularly early after ingestion. Suspect in altered mental status with unexplained acidosis. Antidotes include fomepizole or ethanol. Haemodialysis may be required.
Important Limitations
- Always correlate with clinical picture: Anion gap is one part of acid-base assessment. Review arterial blood gases, lactate, ketones, renal function, and patient history.
- Verify laboratory reference ranges: Different laboratories may use different methods and report slightly different reference intervals.
- Hypoalbuminaemia masks elevated anion gap: Consider albumin correction in critically ill patients.
- Delta-delta analysis: In complex cases, calculate the delta gap to identify mixed acid-base disorders.
Frequently Asked Questions
What is a normal anion gap value?
The normal anion gap is typically 8–12 mmol/L when calculated without potassium, or 12–16 mmol/L when potassium is included. However, reference ranges vary by laboratory and measurement method. Always use your local laboratory’s reference intervals for clinical interpretation.
Should I include potassium in the anion gap calculation?
Most modern UK laboratories report anion gap without potassium because it contributes minimally to the overall calculation and its concentration varies widely. However, some institutions still use the formula with potassium. Follow your laboratory’s convention and use their corresponding reference range. The choice does not affect clinical decision-making as long as you apply the correct reference range.
Why correct for albumin?
Albumin is a major unmeasured anion. When albumin levels are low (hypoalbuminaemia), the anion gap decreases artificially, potentially masking a true elevated anion gap metabolic acidosis. Each 1 g/dL (10 g/L) drop in albumin reduces the anion gap by approximately 2.5 mmol/L. In critically ill patients with hypoalbuminaemia, correcting for albumin can reveal hidden pathology. However, the corrected value is an optional clinical adjustment and should be interpreted alongside the uncorrected AG.
What does a low anion gap mean?
A low anion gap (<8 mmol/L) is uncommon. Possible causes include severe hypoalbuminaemia, paraproteinaemia (multiple myeloma), lithium toxicity, bromide or iodide intoxication, and hyperviscosity syndromes. A negative anion gap strongly suggests laboratory error or unusual interfering substances.
What is the delta gap or delta-delta?
The delta gap helps identify mixed acid-base disorders in high anion gap metabolic acidosis. It compares the change in anion gap to the change in bicarbonate. If the delta ratio (ΔAG/ΔHCO₃) is approximately 1, there is pure HAGMA. A ratio <1 suggests concurrent NAGMA, while a ratio >2 suggests concurrent metabolic alkalosis. This advanced calculation is beyond this basic calculator.
Can medications affect the anion gap?
Yes. Some medications can increase the anion gap, including salicylates, metformin (in lactic acidosis), acetazolamide, topiramate, propofol infusion syndrome, and toxic alcohols (methanol, ethylene glycol). Others like lithium can decrease it. Always review medication history when interpreting abnormal results.
Is the anion gap useful in respiratory acidosis/alkalosis?
The anion gap primarily evaluates metabolic acid-base disorders. In pure respiratory acidosis or alkalosis, the anion gap should be within reference range unless there is a concurrent metabolic disturbance. Always review the full arterial blood gas (ABG) and clinical context.