Anion Gap Blood Test

An anion gap blood test checks the acid-base balance of your blood and if the electrolytes in your blood are properly balanced. Healthcare providers most commonly use anion gap to identify cases of metabolic acidosis — when you have higher-than-normal amounts of acid in your blood.


A needle puncturing a vein in the inner side of a person's elbow. The needle is connected to a syringe that's filled with blood.
In blood test results, anion gap is a measurement to check for the acid-base balance of your blood and/or an electrolyte imbalance in your blood.

What does anion gap mean in a blood test?

In blood test results, anion gap (also known as serum anion gap) is a measurement to check for the acid-base balance of your blood and/or an electrolyte imbalance in your blood. The anion gap is a calculation of the difference between the amounts of some negatively charged electrolytes (such as chloride and bicarbonate) and the amount of positively charged electrolytes (such as sodium) in your blood.

The anion gap measurement is based on the results of individual electrolyte blood tests, which are commonly included in the following routine bloodwork panels:


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What are electrolytes?

Electrolytes are minerals that carry an electric charge when they are dissolved in a liquid. The routinely measured electrolytes in your blood include:

  • Chloride.
  • Sodium.
  • Potassium.
  • Bicarbonate (also known as CO2).

Together, these electrolytes help control nerve and muscle function and maintain the acid-base balance (pH balance) of your blood and your water balance.

Some of the electrolytes in your blood have a positive electric charge, and others have a negative electric charge. The anion gap is a measurement of the difference — or gap — between the negatively charged and positively charged electrolytes. Each of the above electrolytes is individually measured in a comprehensive metabolic blood test panel and electrolyte blood test panel. Laboratory scientists then calculate the anion gap based on those measurements.

What is the normal pH level of blood?

In chemistry, pH is a scale that describes how acidic or basic a water solution is. A pH level of 7.0 is neutral and represents the middle of the scale. Acidic solutions have a pH value lower than 7, and basic or alkaline solutions have a pH level higher than 7.

Blood, which is over 90% water, is normally slightly basic, with a normal pH range of about 7.35 to 7.45. Usually, your body maintains the pH of your blood close to 7.4. Your blood's pH balance is critical to your health, and there are several ways in which your body resists change to your blood’s pH. When the balance is off, even by a small amount, it can make you sick.


What is an anion gap blood test used for?

The anion gap reveals whether your blood has an imbalance of electrolytes or if your blood is too acidic or too basic. If you have too much acid in your blood, it’s called acidosis. If your blood is too basic, you may have a condition called alkalosis.

One common application of the anion gap is classifying cases of metabolic acidosis (having a lower-than-normal blood pH), which is a potentially life-threatening condition that requires medical treatment.

When would I need an anion gap blood test?

Your healthcare provider may order a blood panel that includes an anion gap test if you have signs and symptoms of an imbalance in your blood acid levels — either acidosis (too acidic) or alkalosis (too basic).

Signs and symptoms of acidosis include:

Signs and symptoms of alkalosis include:

  • Feeling lightheaded and easily confused.
  • Having a hand tremor.
  • Experiencing muscle twitching.
  • Experiencing nausea and vomiting.
  • Experiencing numbness or tingling in your face, hands or feet.
  • Experiencing prolonged muscle spasms (tetany).

The anion gap is also commonly included in routine bloodwork and screening, including:

  • Basic metabolic panel (BMP).
  • Comprehensive metabolic panel (CMP).
  • Electrolyte panel.
  • Renal function panel (RFP).

Who performs an anion gap blood test?

A healthcare provider called a phlebotomist usually performs blood draws, including those for an anion gap blood test, but any healthcare provider who is trained in drawing blood can perform this task. A provider then sends the sample to a lab where a medical laboratory scientist prepares the sample and performs the test on machines known as analyzers.

Test Details

How do I prepare for an anion gap blood test?

If you’re experiencing symptoms of having a blood pH imbalance, such as shortness of breath or vomiting, your provider will likely have you undergo blood tests as soon as possible. There’s nothing you need to do to prepare in this case.

An anion gap can also be measured from blood test results from a routine comprehensive metabolic panel (CMP). If you’re undergoing a blood draw for a CMP, you may need to not eat or drink (fast) for several hours before your blood draw. In any case, your healthcare provider will let you know if you need to follow any special instructions.

What should I expect during my blood test?

Laboratory scientists calculate the anion gap from the results of an electrolyte panel or a comprehensive metabolic panel (CMP), which are blood tests.

You can expect to experience the following during a blood test, or blood draw:

  • You’ll sit in a chair, and a healthcare provider will check your arms for an easily accessible vein. This is usually in the inner part of your arm on the other side of your elbow.
  • Once they’ve located a vein, they’ll clean and disinfect the area.
  • They’ll then insert a small needle into your vein to take a blood sample. This may feel like a small pinch.
  • After they insert the needle, a small amount of blood will collect in a test tube.
  • Once they have enough blood to test, they’ll remove the needle and hold a cotton ball or gauze on the site to stop the bleeding.
  • They’ll place a bandage over the site, and you’ll be finished.

The entire procedure usually takes less than five minutes.

What should I expect after my anion gap blood test?

After a healthcare provider has collected a blood sample from you, they’ll send your blood to a laboratory for testing. Once the test results are back, your healthcare provider will share the results with you.

What are the risks of an anion gap blood test?

Blood tests are a very common and essential part of medical testing. There’s very little risk to having blood tests. You may have slight tenderness or a bruise at the site of the blood draw, but this will resolve quickly.

Results and Follow-Up

What do the results of an anion gap blood test mean?

Blood test reports, including anion gap blood test reports, usually provide the following information:

  • The name of the blood test or what was measured in your blood.
  • The number or measurement of your blood test result.
  • The normal measurement range for that test.
  • Information that indicates if your result is normal or abnormal or high or low.

What is a normal anion gap blood test result?

There’s no universal “normal” anion gap, partly because laboratories and healthcare providers can measure and compare different electrolytes in your blood. Laboratories may have different reference ranges for a normal anion gap. When you get your blood test results back, there will be information that indicates what that lab’s normal anion gap range is.

There’s also a wide range of “normal” anion gap results depending on different factors. For example, people who take lithium carbonate medication for bipolar disorder may have a reduced anion gap due to the medication.

What does a high anion gap blood test result mean?

If your blood test results show a high anion gap, you may have acidosis. This means you may have a higher-than-normal level of acid in your blood (a lower-than-normal blood pH).

Acidosis may be a sign of the following:

Having acidosis may also indicate more serious conditions, including:

  • Kidney disease: Healthy kidneys remove acid from your body through your pee, and they keep the right level of bicarbonate (an electrolyte) in your blood. But if you have kidney disease, your kidneys can't remove enough acid, which can lead to metabolic acidosis.
  • Diabetes: Acidosis could indicate that you have undiagnosed diabetes (more commonly Type 1 diabetes). When your pancreas no longer makes enough insulin (a hormone) to allow blood sugar into your cells for use as energy, it can make your blood acidic. This is known as diabetic ketoacidosis.
  • Diabetic ketoacidosis (DKA): If you have diabetes and your body is not getting enough synthetic insulin, it could lead to diabetic ketoacidosis, which is a life-threatening medical condition.
  • Salicylate poisoning (salicylate toxicity): Salicylates are a type of drug found in many over-the-counter and prescription medications. Aspirin is the most common type of salicylate. Intentional ingestion or accidental overdose of salicylates can cause salicylate poisoning, which is life-threatening.

What does a low anion gap blood test result mean?

Having a low anion gap blood test result is rare. Because of this, your provider will likely redo the test to make sure the results are accurate or they may do further testing.

If your blood results reveal a low anion gap, it may mean you have a lower-than-normal level of albumin (hypoalbuminemia). Albumin is an essential protein in your blood. Low albumin may indicate the following conditions:

Again, low anion gap results are uncommon. Be sure to talk to your provider if want more information on your test results.

Should I be concerned if I have a low or high anion gap blood test result?

If your blood test results reveal that you have a high or low anion gap, it doesn’t necessarily mean you have a medical condition. Many factors can affect your electrolyte levels, which affect your anion gap measurement. There could’ve also been an error in the collection, transport or processing of the blood tests.

When your healthcare provider interprets your blood test results, they take into consideration many factors, such as your medical history and medications. Your provider may recommend additional testing if your anion gap results are abnormal to determine a diagnosis.

If you have any questions about your results, be sure to ask your provider.

When should I know the results of an anion gap blood test?

If you have acute signs and symptoms of acidosis, such as shortness of breath and vomiting, your provider will draw your blood for blood tests as soon as possible. Your provider will likely have the results within hours.

If your anion gap test is part of a routine bloodwork panel, such as a basic metabolic panel, it could take one to two business days before you get the results.

A note from Cleveland Clinic

Seeing an abnormal test result can be stressful. Know that having a high or low anion gap doesn’t necessarily mean you have a medical condition and need treatment. Approximately one in 20 healthy individuals will have an abnormal test result. Many other factors can affect your electrolyte levels, which affect your anion gap measurement. Your healthcare provider will let you know if you need to undergo further tests to determine the cause of the abnormal level. Don’t be afraid to ask your provider questions. They’re there to help you.

Medically Reviewed

Last reviewed by a Cleveland Clinic medical professional on 11/09/2021.

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