Anion Gap (Serum/Plasma, Calculated)
This test measures the difference between measured cations (like sodium) and unmeasured anions (like chloride, bicarbonate) in your blood. It helps assess metabolic imbalances, such as acid-base disorders or kidney function.
["The anion gap helps identify potential metabolic issues by comparing key blood chemistry components."]
Key facts at a glance
- LOINC
- 10466-1
- Short code
- AGAP
- Synonyms
- Anion Gap SerPl Calculated.3Ions-sCnc
- Category
- chemistry
- Sample
- serum
- Container
- Serum separator tube (SST) or red-top tube; plasma from lithium-heparin or green-top tube also acceptable
- Units
- N/A
- Ref low
- 8
- Ref high
- 16
- Critical range
- 6 N/A – 20 N/A
- Fasting
- false
- Population
- adult
- Sex
- all
- Age range
- ≥ 0 years
- Method
- calculated-ions
- CPT
- 80051
- SNOMED
- 121006004
- Specimen volume
- 1-2 mL of serum or plasma
- Stability
- Room temperature: 8 hours; Refrigerated (2-8°C): 7 days; Frozen (-20°C): 30 days. Separated serum/plasma preferred for extended storage.
- Hemolysis max
- 50
- Lipemia max
- 100
- Icterus max
- 50
- TAT
- 1–4 hours
- Order
- routine
What Is the Anion Gap Used For?
The anion gap is primarily used to diagnose conditions affecting acid-base balance, such as diabetic ketoacidosis, kidney failure, or poisoning from certain toxins. It can also help determine whether a patient’s acidity is due to a metabolic or respiratory cause.
How Is the Anion Gap Calculated?
Your blood sample is analyzed to measure sodium (Na⁺) and chloride (Cl⁻) levels, along with bicarbonate (HCO₃⁻). The anion gap is calculated by subtracting the sum of chloride and bicarbonate from sodium. This value reflects the presence of unmeasured anions in your blood.
Common Causes of Abnormal Anion Gaps
An elevated anion gap often suggests conditions like diabetic ketoacidosis, lactic acidosis, or kidney problems that fail to clear certain acids. A normal gap may indicate metabolic or respiratory causes like hyperchloremic acidosis.
What Does a High or Low Anion Gap Mean?
A high anion gap may signal dangerous metabolic imbalances, while a low gap is less common but can occur with certain medications or fluid shifts. Your doctor will interpret results in the context of your symptoms and other test results.
Reference range
8-16 mmol/L (with potassium); 8-12 mmol/L (without potassium, calculated from Na+ - (Cl- + HCO3-))N/A
No special preparation required. Fasting is not necessary. Patient should be in a steady state without recent intravenous fluid administration affecting electrolyte concentrations.
Clinical interpretation
How abnormal results are interpreted and what conditions they suggest.
Clinical Significance: The Anion Gap Is a Key Diagnostic Tool
Short name
Anion Gap
Frequently asked questions
Why might my doctor order an anion gap test?
What should I expect during or after the test?
Can I prevent issues related to an abnormal anion gap?
Sources
- 1
- 2Lab Tests OnlineSource
- 3
Click any link to access the full source. PubMed Central (PMC) links provide free full-text access.
This information is for educational purposes only and is not a substitute for professional medical advice, diagnosis, or treatment.
This content is for general informational and educational purposes only and is not a substitute for professional medical advice, diagnosis, or treatment. Always seek the guidance of a qualified healthcare provider with any questions you have about a medical condition, and never disregard professional medical advice or delay seeking it because of something you read here.
If you think you may have a medical emergency, call your local emergency number or go to the nearest emergency department immediately.
Medically reviewed by WellAlly Editorial Team Last reviewed August 15, 2026. · Read our editorial policy