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Serum Anion Gap Calculation (with bicarbonate and chloride measurement)

Published: August 9, 2026Updated: August 10, 2026

This test measures the anion gap in your blood, which helps assess how well your kidneys are filtering waste products and how your body is balancing acids and bases.

Quick summary

["The anion gap helps identify metabolic acidosis and other imbalances in your blood chemistry."]

Medically reviewed by WellAlly Editorial TeamLast reviewed August 10, 2026Editorial policy

Key facts at a glance

LOINC
94011-4
Short code
AG3V2
Synonyms
Anion Gap3 2h p Vaso SerPl-sCnc
Category
chemistry
Sample
serum
Container
Serum separator tube (SST, tiger top) or plain red top tube
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 (Na+ + K+ - Cl- - HCO3-) via ion-selective electrode (ISE) potentiometry
CPT
80051 (Electrolyte panel); 83970 (Parathormone/vasopressin related assays may apply separately)
SNOMED
1211000122103 | 271057005
Specimen volume
1-2 mL serum (minimum 0.5 mL)
Stability
Serum stable at room temperature for 4 hours, refrigerated (2-8°C) for 24 hours, frozen at -20°C for up to 2 weeks. Avoid hemolysis as it falsely elevates potassium and alters anion gap calculation.
Hemolysis max
50
Lipemia max
100
Icterus max
25
TAT
1–4 hours
Order
routine

What Is the Anion Gap?

The anion gap is a calculated value derived from your blood test results. It represents the difference between positively charged ions (like sodium) and negatively charged ions (like chloride and bicarbonate) in your serum. While not a standalone test, it’s often calculated alongside chloride and bicarbonate levels to diagnose conditions like diabetic ketoacidosis or kidney failure.

How Is It Measured?

This test involves measuring your serum chloride and bicarbonate levels, then calculating the gap between sodium and the sum of these two ions. Your healthcare provider will use these values to determine if your anion gap is within normal limits or elevated, which may signal an underlying health issue.

Why Might You Need This Test?

Doctors may order an anion gap calculation when they suspect metabolic acidosis (a buildup of acid in your blood) or other conditions affecting electrolyte balance, such as kidney disease, severe dehydration, or diabetic emergencies.

What Does an Elevated Anion Gap Mean?

An elevated anion gap often indicates metabolic acidosis caused by conditions like diabetic ketoacidosis, lactic acidosis, or poisoning from certain toxins. A normal anion gap suggests other causes of acidosis (like kidney problems retaining bicarbonate) may be at play.

Reference range

Anion gap: 8-16 mEq/L (calculated). 2 hours post-vasopressin challenge, normal response is minimal change (<2 mEq/L increase) from baseline, suggesting intact urinary concentrating ability.N/A

Specimen: serum

No fasting required. Patient should maintain adequate hydration prior to testing. Vasopressin (ADH) analog administered as prescribed by physician; blood draw must be timed precisely at 2 hours post-dose. Patient should remain seated or recumbent for at least 15 minutes prior to collection.

Clinical interpretation

How abnormal results are interpreted and what conditions they suggest.

◆Associated findings1

Short name

Anion Gap Test

Frequently asked questions

What causes an elevated anion gap?
Elevated anion gaps are commonly linked to conditions that increase unmeasured anions (e.g., ketones, lactate, or toxins) or decrease measured anions (e.g., hypochloremia). Common causes include diabetic ketoacidosis, methanol or ethylene glycol poisoning, or severe infections leading to lactic acidosis.
How soon should I expect results?
Results for this test are typically available within 24 to 48 hours, depending on your lab’s processing time. Your healthcare provider will discuss the results with you during your follow-up visit.
Can I prepare for this test?
No special preparation is needed for this test. You may have a simple blood draw, often done at your doctor’s office or a lab. If you’re on certain medications or have a history of bleeding disorders, let your provider know before the test.
  1. 1
    MedlinePlus — Laboratory TestsSource
  2. 2
    Lab Tests OnlineSource
  3. 3
    Mayo Clinic — Tests & ProceduresSource

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.

Medical Disclaimer

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 10, 2026. · Read our editorial policy