Use the anion gap calculator to do a fast anion gap calculation from routine electrolytes. Enter Na⁺, Cl⁻, HCO₃⁻ (CO₂) to calculate anion gap and flag high-anion-gap acidosis; optionally include K⁺ and an albumin-corrected value. Normal AG (without K⁺) is typically ~8–12 mEq/L; with K⁺, ~12–16 mEq/L. Low albumin lowers the measured gap, so correct it when albumin is reduced.
What is the anion gap?
- The anion gap (AG) estimates “unmeasured” anions in blood:
AG (no K⁺) = Na⁺ − (Cl⁻ + HCO₃⁻)
AG (with K⁺) = (Na⁺ + K⁺) − (Cl⁻ + HCO₃⁻). - It helps distinguish between metabolic acidosis with high anion gap (e.g., lactate, ketones) and normal anion gap (hyperchloremic) causes.
Typical reference ranges
- Without K⁺: ~8–12 mEq/L (some modern analyzers report slightly lower ranges).
- With K⁺: ~12–16 mEq/L.
- If albumin is low, raise the AG by ~2.5 mEq/L for each 1 g/dL albumin below 4 (Figge correction). Hypoalbuminemia can mask a high AG.
How to use
- Do your anion gap calculation with the formula above
- If albumin is low, apply the albumin correction to avoid a false-normal result.
- If the anion gap is high, think “unmeasured acids” (lactate, ketones, toxins). If normal, think hyperchloremic causes (e.g., RTA, diarrhea).
Na⁺ 140, Cl⁻ 100, HCO₃⁻ 16 → AG = 140 − (100 + 16) = 24 mEq/L → high-anion-gap metabolic acidosis. If albumin were 2.0 g/dL, corrected AG ≈ 24 + 2.5 × (4 − 2) = 29 mEq/L.
Urine anion gap (UAG)
- UAG = (Urine Na⁺ + K⁺ − Cl⁻).
- Negative UAG ⇒ lots of urinary NH₄⁺ (balances Cl⁻): typical of GI bicarbonate loss (e.g., diarrhea).
- Positive/zero UAG with acidosis ⇒ low NH₄⁺ excretion, suggests renal tubular acidosis (RTA).
Anion Gap Calculation formula
There are two types of formulas we use:
- (Na) – (Cl + HCO3) or
- (Na + K) – (Cl + HCO3)
- Normal Anion Gap is between (8-12)mEq/L.
The Anion Gap saves time, and sometimes, it saves lives.
| Condition | Anion Gap Status | Common Causes |
|---|---|---|
| High | >12 mEq/L | Lactic acidosis, Diabetic ketoacidosis, Renal failure, Poisoning |
| Normal | 8-12 mEq/L | Healthy or simple acidosis without hidden acids |
| Low | <8 mEq/L | Hypoalbuminemia, Myeloma, Lab error |
Anion Gap in Pregnancy (First, Second, Third Trimester)
| Anion Gap Units | Nonpregnant Adult | First Trimester | Second Trimester | Third Trimester |
|---|---|---|---|---|
| Normal Range (mEq/L) | 8 – 12 mEq/L | 8 – 12 mEq/L | 7 – 11 mEq/L | 6 – 10 mEq/L |
Pro Tips for Mastering Anion Gap Interpretation
Memorize this:
- High Gap = Gain of Acid
- Low Gap = Loss of Bicarbonate
FAQ — For Anion Gap Calculator
How do you calculate an anion gap?
Use AG = Na⁺ − (Cl⁻ + HCO₃⁻) (most common). Some labs include K⁺: AG = (Na⁺ + K⁺) − (Cl⁻ + HCO₃⁻). Compare your result with your lab’s reference range.
How to calculate the anion gap (step-by-step)?
Take the sodium value, subtract the sum of chloride and bicarbonate (CO₂). If albumin is low, add ~2.5 mEq/L for each 1 g/dL below 4 g/dL to get a corrected AG.
How to calculate anion gap in DKA?
Use the same formula (AG = Na⁺ − (Cl⁻ + HCO₃⁻)). DKA typically shows a high anion gap because ketone anions accumulate. Some clinicians also track the delta ratio to see if a mixed disorder is present.
How to calculate anion gap for DKA (with albumin correction)?
First compute the measured AG, then correct for albumin if it’s low: AG_corr = AG + 2.5 × (4 − albumin[g/dL]). This prevents underestimating the gap in DKA patients with hypoalbuminemia.
How to calculate urine anion gap?
Use UAG = (Urine Na⁺ + K⁺ − Cl⁻). A negative UAG suggests high NH₄⁺ excretion (kidney responding properly; think diarrhea), while a positive/zero UAG suggests impaired NH₄⁺ excretion (e.g., RTA).
Notes & disclaimers
- Reference ranges vary by lab/method; always check your lab’s interval.
- This page is for education; it doesn’t replace clinical judgment.
Sources
- StatPearls — Anion Gap & Acid–Base: formula, typical ranges, causes of high AG.
- Medscape Calculator — Urine Anion Gap: formula and interpretation.
- LITFL: UAG overview; Delta ratio concept for mixed disorders.
- PMC study / Figge equation: albumin-corrected anion gap.
- Review on reference intervals: modern analyzers may report lower AG ranges.