What Is the Anion Gap?

The anion gap (AG) is a calculated value. It estimates unmeasured acids in the blood.

Your blood contains:

Positively charged ions (cations):

• Sodium (Na⁺)
• Potassium (K⁺)

Negatively charged ions (anions):

• Chloride (Cl⁻)
• Bicarbonate (HCO₃⁻)

Not all ions are measured directly. The anion gap helps estimate the difference between measured cations and measured anions.

When the gap is high, it usually means extra acids are present.

Standard Anion Gap Formula

The most common formula is:

Anion Gap = Sodium − (Chloride + Bicarbonate)

In symbols:

AG = Na⁺ − (Cl⁻ + HCO₃⁻)

Example:

• Sodium = 140 mEq/L
• Chloride = 102 mEq/L
• Bicarbonate = 24 mEq/L

AG = 140 − (102 + 24)
AG = 140 − 126
AG = 14 mEq/L

Normal Anion Gap Range

The normal anion gap range depends on the lab and reference system.

Common ranges:

• Standard range: 8–16 mEq/L
• Extended range: 6–20 mEq/L
• Pediatric range: 10–18 mEq/L

Your calculator allows selecting different reference ranges, including custom values.

What Does a High Anion Gap Mean?

A high anion gap usually suggests metabolic acidosis caused by extra acids in the body.

Common causes include:

• Diabetic ketoacidosis (DKA)
• Lactic acidosis
• Renal failure
• Toxic ingestion (methanol, ethylene glycol)
• Salicylate overdose
• Sepsis

Why It Happens

When acids build up, bicarbonate drops. The difference between sodium and the measured anions widens. That increase shows up as a high anion gap.

Severe elevations (above 20 mEq/L) often indicate serious metabolic acidosis and may require urgent care.

What Does a Low Anion Gap Mean?

A low anion gap is less common.

Possible causes:

• Hypoalbuminemia (low albumin)
• Lab error
• Multiple myeloma
• Bromide or iodide excess

Most low values should be rechecked before assuming disease.

Advanced Anion Gap Calculations

Your calculator includes five calculation modes. Each serves a specific purpose.

1. Standard Anion Gap

This is the basic calculation:

AG = Na − (Cl + HCO₃)

Use this for routine electrolyte interpretation.

Best for:

• General metabolic assessment
• Initial acid-base evaluation

2. Corrected Anion Gap

Albumin affects the anion gap. Since albumin is negatively charged, low albumin can falsely lower the gap.

Albumin Correction Formula

Corrected AG = Standard AG + 2.5 × (4 − Albumin)

Where:

• Albumin is measured in g/dL
• 4 g/dL is considered normal

Example:

• Standard AG = 12
• Albumin = 2 g/dL

Correction = 2.5 × (4 − 2) = 5
Corrected AG = 12 + 5 = 17

This reveals hidden metabolic acidosis.

Use corrected AG in:

• Liver disease
• Malnutrition
• Critical illness
• ICU patients

3. Albumin-Adjusted Anion Gap

This is similar to the corrected method. It specifically adjusts the gap for low albumin to prevent misdiagnosis.

It confirms whether a high anion gap is real or falsely elevated due to protein changes.

4. Delta Anion Gap

The delta anion gap compares current and previous values.

Formula

Delta AG = Current AG − Previous AG

If the change is greater than 2 mEq/L, it is usually clinically significant.

Use delta AG to:

• Monitor DKA treatment
• Track lactic acidosis
• Assess response to therapy
• Evaluate worsening acidosis

A rising delta suggests deterioration. A falling delta suggests improvement.

5. Comprehensive Anion Gap Analysis

This mode includes:

• Sodium
• Chloride
• Bicarbonate
• Potassium
• Magnesium
• Phosphate
• Calcium
• Albumin

It also evaluates:

• Electrolyte balance
• Hypokalemia
• Hypophosphatemia
• Clinical recommendations

This mode is helpful for:

• Critical care patients
• ICU monitoring
• Severe metabolic disorders
• Complex electrolyte imbalances

Clinical Context Selection

The calculator allows selecting clinical context:

• General assessment
• Diabetic ketoacidosis
• Renal failure
• Lactic acidosis
• Toxic ingestion
• Sepsis

This helps tailor interpretation and recommendations.

For example:

In DKA, the anion gap trend is more important than a single value.
In renal failure, elevated AG reflects reduced acid clearance.
In toxic ingestion, a high gap may indicate methanol or ethylene glycol poisoning.

Patient Category Selection

Different patient groups have different normal ranges:

• Adult
• Pediatric
• Geriatric
• Critical ill

Children often have slightly different reference ranges. Critical patients require closer monitoring.

Units: mEq/L vs mmol/L

For sodium, chloride, and bicarbonate:

1 mEq/L = 1 mmol/L

So no conversion is required for these ions.

Why the Anion Gap Matters

The anion gap helps:

• Detect hidden metabolic acidosis
• Identify life-threatening conditions early
• Guide emergency treatment
• Monitor ICU patients
• Track disease progression

It is simple to calculate but powerful in clinical decision-making.

Step-by-Step: How to Use the Anion Gap Calculator

1. Choose calculation mode
2. Enter electrolyte values
3. Select clinical context
4. Choose patient category
5. Select reference range
6. Click "Calculate"
7. Review interpretation and recommendations

The calculator provides:

• Anion gap result
• Interpretation
• Electrolyte balance
• Differential diagnosis
• Clinical significance
• Recommendations

Limitations of Anion Gap Calculation

An anion gap calculator is a support tool. It does not replace clinical judgment.

Limitations include:

• Lab measurement error
• Albumin variation
• Mixed acid-base disorders
• Delayed lab sampling

Always interpret results with full clinical context.

Key Takeaways

• The anion gap measures unmeasured acids in the blood.
• Normal range is usually 8–16 mEq/L.
• High AG suggests metabolic acidosis.
• Albumin correction is essential in low-protein states.
• Delta AG helps monitor disease progression.
• Comprehensive analysis improves accuracy in critical care.