Anion Gap Calculator
Calculate serum anion gap, albumin-corrected anion gap, and delta-delta ratio to classify metabolic acidosis and identify mixed acid-base disorders.
Calculate Anion Gap
Enter serum electrolytes to calculate the anion gap, albumin-corrected AG, and delta-delta ratio. Albumin is optional but recommended — the corrected AG accounts for hypoalbuminaemia, which is common in critically ill patients and can mask a true high-AG acidosis.
The anion gap is an estimation tool and does not replace a comprehensive metabolic evaluation. Reference ranges vary by laboratory — some institutions use a normal AG of 8–12 mEq/L, while others use 10–14 mEq/L depending on whether potassium is included in the calculation. Verify with your own laboratory’s reference range.
Understanding the Anion Gap
The anion gap represents the difference between routinely measured cations and anions in serum. In plasma, the total number of positive charges always equals the total negative charges (the principle of electroneutrality). However, standard chemistry panels only measure a subset of these ions — leaving a “gap” that is primarily accounted for by unmeasured anions such as albumin, phosphate, sulphate, and organic acids.
An elevated anion gap indicates the presence of excess unmeasured anions in the blood, most commonly from organic acid accumulation (lactate, ketoacids, uraemic toxins, or ingested toxins). The concept was first formalised by Emmett and Narins in 1977 and remains one of the most widely used tools in acid-base assessment.
Anion Gap Formula
AG = Na⁺ − (Cl⁻ + HCO₃⁻)
Normal range: 8–12 mEq/L (without K⁺). Some laboratories include potassium: AG = (Na⁺ + K⁺) − (Cl⁻ + HCO₃⁻), with a normal range of 10–14 mEq/L.
Albumin-Corrected AG
Corrected AG = AG + 2.5 × (4.0 − Albumin)
Each 1 g/dL drop in albumin below 4.0 reduces the measured AG by approximately 2.5 mEq/L. The correction is essential in hypoalbuminaemic patients (ICU, cirrhosis, nephrotic syndrome) to unmask a hidden HAGMA.
Delta-Delta Ratio (Δ-Δ)
Δ-Δ = (AG − 12) / (24 − HCO₃⁻)
Compares the rise in AG above normal (12) to the fall in HCO₃⁻ below normal (24). Helps identify concurrent non-anion-gap metabolic acidosis or metabolic alkalosis superimposed on a HAGMA.
Worked Example
Na⁺ = 140, Cl⁻ = 100, HCO₃⁻ = 10, Albumin = 2.5 g/dL
AG = 140 − (100 + 10) = 30 mEq/L
Corrected AG = 30 + 2.5 × (4.0 − 2.5) = 33.75 mEq/L
Δ-Δ = (30 − 12) / (24 − 10) = 18 / 14 = 1.29 → Pure HAGMA
Key distinction: The anion gap differentiates between high-AG metabolic acidosis (HAGMA) — caused by accumulation of unmeasured acids — and normal-AG metabolic acidosis (NAGMA) — caused by bicarbonate loss or impaired renal acid excretion, with a compensatory rise in chloride.
Interpretation & Categories
The anion gap should be interpreted in the context of the full clinical picture — including arterial blood gas, serum lactate, ketones, renal function, and toxicology screening where appropriate. The table below summarises the key categories.
| Anion Gap | Category | Clinical Significance |
|---|---|---|
| < 3 mEq/L | Low AG | Hypoalbuminaemia, bromide intoxication, lithium toxicity, myeloma (cationic paraproteins) |
| 3–12 mEq/L | Normal AG | If metabolic acidosis present → NAGMA (hyperchloraemic acidosis) |
| 13–20 mEq/L | Mildly elevated AG | Early HAGMA, mild lactic acidosis, or baseline variation — correlate with clinical context |
| > 20 mEq/L | Significantly elevated AG | Strong HAGMA — consider ketoacidosis, lactic acidosis, renal failure, or toxic ingestion |
Delta-Delta Ratio Interpretation
| Delta-Delta Ratio | Interpretation | What This Suggests |
|---|---|---|
| < 1.0 | Combined HAGMA + NAGMA | A concurrent non-AG metabolic acidosis is present (e.g., diarrhoea, RTA superimposed on DKA) |
| 1.0–2.0 | Pure HAGMA | The bicarbonate drop is proportional to the AG rise — consistent with a single high-AG process |
| > 2.0 | HAGMA + Metabolic alkalosis | A concurrent metabolic alkalosis is raising the bicarbonate (e.g., vomiting, diuretic use) |
The delta-delta ratio is most useful when the AG is clearly elevated (typically > 20 mEq/L). When the AG is only mildly elevated (13–20 mEq/L), the delta-delta ratio can be unreliable because small changes in the numerator or denominator produce large swings in the ratio. Always correlate with the clinical picture.
Causes of Metabolic Acidosis
Once you have identified a metabolic acidosis using the pH, pCO₂, and HCO₃⁻, the anion gap divides it into two major categories. The GOLDMARK mnemonic is the preferred modern framework for high-AG causes, replacing the older MUDPILES mnemonic.
High Anion Gap Metabolic Acidosis (HAGMA) — GOLDMARK
Normal Anion Gap Metabolic Acidosis (NAGMA)
NAGMA occurs when bicarbonate is lost or acid excretion is impaired, with chloride rising to maintain electroneutrality (hence “hyperchloraemic acidosis”). The urine anion gap (UAG) helps distinguish renal from extrarenal causes.
Diarrhoea is the most common cause of NAGMA worldwide. The small bowel and colon secrete bicarbonate-rich fluid, and high-volume losses lead to a hyperchloraemic acidosis. Other GI causes include pancreatic fistulae, biliary drains, ileostomy output, and ureterosigmoidostomy.
The urine anion gap (UAG = Na⁺ + K⁺ − Cl⁻) is typically negative in GI losses, reflecting appropriate renal ammonium (NH₄⁺) excretion — the kidneys are working correctly to compensate for the bicarbonate loss.
Type 1 (distal RTA): Inability to secrete H⁺ in the distal tubule. Urine pH remains > 5.5 despite systemic acidosis. Associated with nephrocalcinosis, hypokalaemia, and conditions such as Sjögren syndrome, SLE, and amphotericin B use.
Type 2 (proximal RTA): Defective proximal bicarbonate reabsorption. Urine pH may be < 5.5 once the serum HCO₃⁻ falls below the lowered reabsorptive threshold. Often part of Fanconi syndrome (glycosuria, aminoaciduria, phosphaturia).
Type 4 (hypoaldosteronism): The most common RTA in adults. Caused by aldosterone deficiency or resistance, leading to hyperkalaemia and impaired NH₄⁺ excretion. Seen in diabetic nephropathy, ACE inhibitors, spironolactone, and trimethoprim use.
The UAG in RTA is typically positive, indicating impaired renal ammonium excretion.
Large-volume normal saline (0.9% NaCl) infusion is a well-recognised cause of iatrogenic NAGMA. The high chloride content (154 mEq/L, above plasma) causes a dilutional and hyperchloraemic acidosis. This is sometimes called “dilutional acidosis” but is mechanistically a chloride-driven phenomenon.
Balanced crystalloids (e.g., Ringer’s lactate, Plasma-Lyte) have a lower chloride load and are less likely to cause this effect. Carbonic anhydrase inhibitors (acetazolamide) also cause NAGMA by impairing proximal bicarbonate reabsorption.
When you find a NAGMA, calculate the urine anion gap (UAG = urine Na⁺ + K⁺ − Cl⁻). A negative UAG (typically −20 to −50) suggests appropriate renal NH₄⁺ excretion → GI loss is the likely cause. A positive UAG (> 0) suggests impaired renal acid excretion → think RTA.
Special Populations & Considerations
Key clinical point: In any critically ill or hospitalised patient, always correct the anion gap for albumin before concluding it is normal. A “normal” measured AG with an albumin of 2.0 g/dL may actually represent a corrected AG of 17 mEq/L — a clinically significant HAGMA that would otherwise be missed.
Stepwise Acid-Base Approach
A systematic approach to acid-base interpretation ensures that mixed disorders are not missed. Follow these steps whenever you encounter an abnormal arterial blood gas or chemistry panel.
Look at the pH first: acidaemia (pH < 7.35) or alkalaemia (pH > 7.45)? Then determine whether the primary process is metabolic (HCO₃⁻ is the primary change) or respiratory (pCO₂ is the primary change). The pH always points toward the primary disorder — if pH is low, the primary process is either metabolic acidosis or respiratory acidosis.
Use Winter’s formula for metabolic acidosis: Expected pCO₂ = 1.5 × HCO₃⁻ + 8 (± 2). If the measured pCO₂ is higher than expected, a concurrent respiratory acidosis is present. If lower than expected, there is a superimposed respiratory alkalosis. For metabolic alkalosis: Expected pCO₂ = 0.7 × HCO₃⁻ + 21 (± 2).
Compensation is never complete for metabolic disorders (pH does not fully normalise). If the pH is perfectly normal with abnormal HCO₃⁻ and pCO₂, suspect a mixed disorder.
If a metabolic acidosis is present, calculate the AG. Always correct for albumin: Corrected AG = AG + 2.5 × (4.0 − Albumin). An elevated corrected AG indicates HAGMA. A normal corrected AG with metabolic acidosis indicates NAGMA.
Even if the primary disorder is respiratory, calculate the AG — it may reveal a concurrent metabolic acidosis.
If the AG is elevated, determine whether the bicarbonate drop is proportional: Δ-Δ = (AG − 12) / (24 − HCO₃⁻). A ratio < 1 suggests a concurrent NAGMA. A ratio > 2 suggests a concurrent metabolic alkalosis. A ratio of 1–2 is consistent with a pure HAGMA.
This step is critical for identifying mixed metabolic disorders — for example, a patient with DKA (HAGMA) who also has diarrhoea (NAGMA), or a patient with lactic acidosis (HAGMA) who is also vomiting (metabolic alkalosis).
For HAGMA, use the GOLDMARK mnemonic and check: lactate, ketones (serum BHB), renal function, osmolar gap (for toxic alcohols), salicylate level, and paracetamol level. For NAGMA, calculate the urine anion gap to differentiate GI losses from renal tubular acidosis.
Remember that multiple processes can coexist — a septic ICU patient may have lactic acidosis (HAGMA) from shock, a dilutional acidosis (NAGMA) from large-volume saline resuscitation, and a metabolic alkalosis from nasogastric suction, all at the same time.
Common Pitfalls & Limitations
This is the single most common error in AG interpretation. Albumin is the largest contributor to the normal anion gap (~75% of unmeasured anions). In hypoalbuminaemic patients — including most ICU admissions, patients with cirrhosis, nephrotic syndrome, or malnutrition — the measured AG may appear normal even in the presence of significant organic acid accumulation. Each 1 g/dL drop in albumin reduces the AG by approximately 2.5 mEq/L. Failure to correct leads to missed diagnoses and delayed treatment.
A normal anion gap does not exclude metabolic acidosis — it simply narrows the differential to NAGMA causes (diarrhoea, RTA, saline infusion). Additionally, mixed acid-base disorders can normalise the AG. A patient with a HAGMA and concurrent metabolic alkalosis may have a near-normal HCO₃⁻ and a misleadingly moderate AG. The delta-delta ratio helps unmask this scenario.
Methanol and ethylene glycol poisoning cause a HAGMA, but the AG may be normal early in the presentation (before the parent alcohol is metabolised). In the early phase, the osmolar gap is elevated. As the parent compound is metabolised to organic acids, the osmolar gap falls and the AG rises. Checking only the AG and missing the osmolar gap window can delay life-saving treatment (fomepizole, dialysis). Always check the osmolar gap when toxic alcohol ingestion is suspected.
The normal AG reference range varies by institution and analyser. Ion-selective electrode (ISE) analysers — now standard — tend to measure chloride slightly higher than older flame photometry methods, resulting in a lower “normal” AG (approximately 8–12 mEq/L versus the older 12 ± 4). Using an outdated reference range can lead to missed elevations. Always verify your laboratory’s specific reference range, and be cautious when comparing AG values across different hospital systems.
The delta-delta ratio assumes a normal baseline AG of 12 and a normal HCO₃⁻ of 24. In patients with chronic conditions (e.g., CKD with baseline HCO₃⁻ of 18, or chronic liver disease with baseline AG of 6), these assumptions fail. Furthermore, the ratio is unreliable when the AG elevation is mild (13–18 mEq/L) because small measurement errors produce large ratio swings. Use the delta-delta as a screening tool, not a definitive diagnosis.
Quick Reference Summary
Without K⁺
albumin drop
Pure HAGMA
HAGMA (mEq/L)
| Finding | Think… | Next Step |
|---|---|---|
| AG > 20, Lactate elevated | Lactic acidosis (Type A or B) | Identify and treat cause of hypoperfusion or metabolic dysfunction |
| AG > 20, Ketones positive | DKA, AKA, or starvation ketosis | Check glucose, serum BHB, consider insulin or dextrose |
| AG > 20, Osmolar gap elevated | Toxic alcohol (methanol, ethylene glycol) | Fomepizole, toxicology consult, consider haemodialysis |
| AG > 20, Elevated creatinine | Uraemic acidosis | Assess need for renal replacement therapy |
| Normal AG + Acidosis | NAGMA (GI loss vs. RTA) | Calculate urine anion gap to differentiate |
| Delta-Delta < 1.0 | Mixed HAGMA + NAGMA | Search for two concurrent processes |
| Delta-Delta > 2.0 | HAGMA + Metabolic alkalosis | Look for vomiting, NG suction, diuretics, or alkali administration |
The Golden Rule: Always correct the anion gap for albumin. A “normal” AG in a hypoalbuminaemic patient may be hiding a clinically significant high-AG metabolic acidosis. When in doubt, correct it out.
Disclaimer & References
For Educational Purposes Only. This calculator and the accompanying clinical information are intended as educational tools for healthcare professionals. They do not replace clinical judgement. Results should be interpreted in the full clinical context. Lab reference ranges vary by institution — verify with your own laboratory. Drug dosages should be confirmed against current prescribing information.
References
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- Rodríguez Soriano J. Renal tubular acidosis: the clinical entity. Journal of the American Society of Nephrology. 2002;13(8):2160-2170. DOI: 10.1097/01.ASN.0000023430.92674.E5
- Adeva-Andany M, López-Ojén M, Funcasta-Calderón R, et al. Comprehensive review on lactate metabolism in human health. Mitochondrion. 2014;17:76-100. DOI: 10.1016/j.mito.2014.05.007