Anion Gap

Understanding the Anion Gap

The anion gap represents the difference between positively charged ions (cations) and negatively charged ions (anions) in the blood. It cannot be directly measured, but is calculated from electrolyte panel blood test results.

What is the Anion Gap?

The anion gap is calculated using the concentrations of:

Major cations: sodium (and optionally potassium)
Major anions: chloride and bicarbonate

While potassium can be included in the calculation, it's commonly omitted since its concentration is much lower compared to the other ions and has minimal effect on the result.

In a healthy body, the total positive charge from cations equals the total negative charge from anions to maintain neutrality. However, blood tests don't measure all ion types. The anion gap helps identify unmeasured anions and cations, with normally more unmeasured anions leading to a small gap.

Clinical Uses

The anion gap is primarily used to:

Evaluate acid-base disorders
Help diagnose metabolic acidosis
Assess electrolyte imbalances
Monitor kidney function

Normal Range Values

Normal ranges can vary between laboratories due to differences in equipment and techniques. Values are reported in milliequivalents per liter (mEq/L):

Modern tests: 3-11 mEq/L
Older test methods: 8-16 mEq/L

High Anion Gap (>11 mEq/L)

Common causes include:

Medical Conditions:

Lactic acidosis
Diabetic ketoacidosis
Kidney failure
Uremia
Thiamine deficiency
Starvation

Toxins/Medications:

Carbon monoxide
Ethylene glycol (antifreeze)
Methanol
Aspirin overdose
Metformin
Ibuprofen

Low Anion Gap (<3 mEq/L)

Much less common. Causes include:

Laboratory error (most common)
Low albumin levels (hypoalbuminemia)
Multiple myeloma
High calcium or magnesium levels
Lithium toxicity
Bromide intoxication

Important Considerations

Results should always be interpreted by a healthcare professional
A single measurement may not reflect overall health status
Normal results don't rule out all medical conditions
Other factors like medications can affect results

When to Seek Medical Care

Contact your healthcare provider if:

Your anion gap is significantly outside normal range
You have unexplained symptoms
You're being monitored for acid-base or electrolyte disorders
You take medications that affect electrolyte levels

Anion Gap Calculator