Anion Gap

The Anion Gap (AG) calculator is often used to determine if a patient's metabolic acidosis also has an elevated anion gap. Acidosis with an increased anion gap as a distinct differential diagnosis compared to non-gap acidosis.

What the Anion Gap Measures

The serum anion gap (AG) is a calculated value that estimates the difference between measured serum cations (sodium) and measured anions (chloride and bicarbonate):

AG = Na⁺ − (Cl⁻ + HCO₃⁻)

The normal range is typically 8–12 mEq/L, though this varies somewhat by lab and assay method (particularly with older vs. newer chloride electrodes).

The Science: Why This Works

The body maintains electroneutrality — total cations must equal total anions in the serum. But routine chemistry panels only measure Na⁺, Cl⁻, and HCO₃⁻; they don't measure the "unmeasured" ions like albumin, phosphate, sulfate, lactate, and other organic anions that also contribute to the anion side of the ledger.

The anion gap is essentially a proxy for these unmeasured anions. In health, this gap is small and stable because albumin (which is negatively charged) accounts for most of it. When a disease process generates excess organic or inorganic acids, those acid anions accumulate in the blood, widening the calculated gap — even though bicarbonate is what's actually being consumed to buffer the acid.

Why It's Clinically Important

1. Differentiating causes of metabolic acidosis — This is the calculator's primary use. A low bicarbonate alone doesn't tell you why it's low. Splitting metabolic acidosis into high-AG vs. normal-AG (hyperchloremic) categories dramatically narrows the differential:

   • High AG acidosis: think ketoacidosis (diabetic, alcoholic, starvation), lactic acidosis, renal failure, toxic ingestions (methanol, ethylene glycol, salicylates) — often remembered via mnemonics like MUDPILES or GOLD MARK.

   • Normal AG acidosis: think GI bicarbonate loss (diarrhea), renal tubular acidosis, early renal failure, or excess chloride administration.

2. Detecting mixed acid-base disorders — Calculating the "delta-delta" (change in AG relative to change in HCO₃⁻) can reveal a second, hidden acid-base process, such as a concurrent metabolic alkalosis masking the degree of acidosis.

3. Toxicology screening — An unexpectedly elevated AG in a patient without an obvious cause can be an early clue to toxic alcohol ingestion, prompting further workup (osmolar gap, specific levels) before it's confirmed.

4. Monitoring severity and trends — Serial AG measurements help track response to treatment (e.g., resolution of DKA) or worsening of an underlying process.

The Importance of Serum Albumin

The most clinically important caveat is that the AG is highly dependent on serum albumin, since albumin is the largest unmeasured anion. Hypoalbuminemia (common in critically ill, malnourished, or cirrhotic patients) lowers the "expected" baseline AG, which can mask a true high-AG acidosis if you don't correct for it.

The standard correction:
Corrected AG = Measured AG + 2.5 × (4.0 − patient's albumin in g/dL)

Formulas Included in This Tool

1. Anion Gap = Na - (Cl + HCO3-) or Anion gap, mEq/L = sodium, mEq/L - (chloride, mEq/L+ bicarbonate, mEq/L)

2. Delta Gap = the patient’s Anion Gap – the “normal” anion gap (considered to be 10 to 12)

3. Albumin corrected anion gap, mEq/L = anion gap + [ 2.5 × (4 - albumin, g/dL) ]

4. Albumin corrected delta gap, mEq/L = albumin corrected anion gap - “normal” anion gap (considered to be 10 to 12)

5. Delta ratio = delta anion gap / (24 - bicarbonate, mEq/L)

6. Albumin corrected delta ratio = albumin corrected delta gap / (24 - bicarbonate, mEq/L)

Interpretation

The delta ratio is the ratio of the amount of additional anion in a body to the amount of additional H+. The anion's volume of distribution and its excretion affect this ratio. Organic acids with a greater distribution may produce lower anion gaps compared to inorganic acids, which may be confined to the extracellular compartment.

Etiology Considerations

Management of an anion gap acidosis focuses on correcting the underlying cause and varies based on the specific etiology.

Anion Gap Metabolic Acidosis: MUDPILERS

• Methanol

• Uremia

• Diabetic ketoacidosis/alcoholic ketoacidosis

• Paraldehyde

• Isoniazid

• Lactic acidosis

• Ethanol/ethylene glycol

• Rhabdomyolysis/renal failure

• Salicylates

Non-Anion Gap Acidosis: HARDUPS

• Hyperalimentation

• Acetazolamide

• Renal tubular acidosis

• Diarrhea

• Ureteroenterostomy (or ureteroenteric fistula)

• Post-hypocapnia

• Spironolactone

Acute Respiratory Acidosis: 

• CNS depression (drugs/CVA)

• Airway obstruction

• Pneumonia

• Pulmonary edema

• Hemo/pneumothorax

• Myopathy

Chronic Respiratory Acidosis:

• COPD

• Restrictive lung disease

• Any hypoventilation state

Metabolic Alkalosis: CLEVER PD

• Contraction

• Licorice

• Endocrine causes (Conn's, Cushings, or Bartter's Syndrome)

• Vomiting

• Excess alkali

• Refeeding alkalosis

• Post-hypercapnia

• Diuretics

Respiratory Alkalosis: CHAMPS (think speed up breathing)

• Central Nervous System (CNS) disease

• Hypoxia

• Anxiety

• Mechanical ventilators

• Progesterone

• Salicylates/sepsis