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.
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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Electrolytes are minerals that carry an electric charge when they are dissolved in a liquid. The routinely measured electrolytes in your blood include:
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.
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.
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.
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:
The anion gap is also commonly included in routine bloodwork and screening, including:
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.
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.
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:
The entire procedure usually takes less than five minutes.
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.
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.
Blood test reports, including anion gap blood test reports, usually provide the following information:
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.
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:
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.
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.
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.
Last reviewed by a Cleveland Clinic medical professional on 11/09/2021.
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Cleveland Clinic is a non-profit academic medical center. Advertising on our site helps support our mission. We do not endorse non-Cleveland Clinic products or services. Policy