Anion Gap: Why It’s Tested and What It Detects

What Is Anion Gap?

The anion gap is a number that shows whether your body has the right balance of acids and bases. It helps doctors check if something is wrong with how your body controls its pH level, which is how acidic or basic your blood is.

Your body needs the right mix of electrolytes, which are minerals like sodium, chloride, and bicarbonate. These minerals carry tiny electrical charges and help your body stay hydrated, control muscles and nerves, and keep your blood from getting too acidic or too basic.

The anion gap is not something that shows up directly in your blood. Instead, it’s calculated from other blood test results. The most common formula is:

Anion Gap = Sodium - (Chloride + Bicarbonate)

• Sodium helps control water levels in the body.
• Chloride helps maintain proper blood pressure and pH.
• Bicarbonate keeps the acid levels in your blood balanced.

Doctors use the anion gap to find out if your body is having problems removing or handling acid. It can help diagnose issues like diabetes problems, kidney disease, or poisoning.

When Is Anion Gap Testing Recommended?

Your doctor might ask for an anion gap test if they think there’s an acid or base problem in your body. It usually comes as part of a blood chemistry test, like metabolic panels.

You may need this test if you:

• Have trouble breathing or are breathing too fast
• Feel confused, weak, or dizzy for no clear reason
• Have uncontrolled diabetes
• Have kidney problems
• Are suspected of swallowing something toxic (like antifreeze or strong cleaners)
• Need monitoring for certain ongoing health problems

How Is Anion Gap Testing Conducted?

Most of the time, anion gap is checked through a blood test. A nurse or technician will draw a small sample of blood from your arm. 

This sample is sent to a lab to measure sodium, chloride and bicarbonate (also called CO₂ on some test reports). These numbers are then used to calculate your anion gap.

In some cases, doctors also use a urine test to calculate the anion gap. This is mostly done when your blood test shows a normal anion gap, but you still seem to have an acid problem. The urine anion gap tells doctors how well your kidneys are removing acids from your body.

Test Types That Include Anion Gap

The anion gap is part of these common blood test panels:

• Comprehensive Metabolic Panel (CMP)
• Basic Metabolic Panel (BMP)
• Electrolyte Panel

These panels measure the electrolytes needed to calculate the anion gap.

How to Prepare

Usually, you don’t need to do much to prepare for this test. But to help get accurate results:

• Stay hydrated by drinking water the day before
• Avoid alcohol for 24 hours before the test
• Tell your doctor about any medicines, vitamins, or supplements you're taking

What Are the Normal Anion Gap Levels?

• Blood Anion Gap: 4 to 12 mEq/L
• Urine Anion Gap: 0 to slightly positive (less than 10 mEq/L)

Note: The units of measurement and the normal ranges can differ depending on the laboratory and the specific methods used.

What Do Abnormal Levels Mean?

High Anion Gap

When your anion gap is too high, it usually means there is too much acid in your blood. This condition is called metabolic acidosis.

Causes of high anion gap include:

• Diabetic ketoacidosis - when people with diabetes burn fat for energy and produce harmful acids called ketones
• Lactic acidosis - when lactic acid builds up in your body, usually due to infection, lack of oxygen, or intense exercise
• Kidney failure - when the kidneys stop removing acid from the body properly
• Toxin ingestion - when harmful substances like antifreeze or certain alcohols are swallowed
• Severe dehydration - when your body loses too much water, affecting how your blood works

If your anion gap is high:

• Seek medical care quickly, especially if you feel very sick
• Follow your doctor’s plan for managing diabetes or kidney disease
• Drink enough water (unless advised otherwise)
• Avoid toxins like alcohol or unknown chemicals
• Review your diet and medication plan with your doctor

Low Anion Gap

A low anion gap is less common. It may mean there is too little albumin, which is a protein in your blood, or too much of another charged substance.

Causes of low anion gap include:

• Low albumin levels - this may happen with liver disease, inflammation, or malnutrition
• Multiple myeloma - a blood disease where abnormal proteins build up
• High calcium levels - can affect the anion gap calculation
• Lab error - sometimes the low result is caused by a mistake during testing

If your anion gap is low:

• Ask for a repeat test to double-check the result
• Check your albumin level
• Talk to your doctor about possible liver or bone health issues
• Eat a well-balanced diet with enough protein
• Follow up if you have any known health conditions

Which Specialist Should You Consult?

Depending on what is causing the abnormal levels, you may need to see:

• A nephrologist if your kidneys aren’t working well
• An endocrinologist for diabetes or other hormonal problems
• A toxicologist or emergency doctor if poisoning is suspected
• A general doctor for basic tests and treatment
• A gastroenterologist if liver issues are involved