Anion Gap (Blood)
Does this test have other names?
Serum anion gap
What is this test?
This test looks at electrically charged particles in your blood. This helps your health
care provider diagnose acid-base problems. The test results are done from the results
of an electrolyte panel, another blood test.
Your blood contains sodium, chloride, and bicarbonate. All of these are charged particles.
The value for the anion gap tells your health care provider something about which
other charged particles must be in your blood to make it neutral.
This test gives clues about different types of acidosis, when your blood is too acidic.
It also tells your provider about alkalosis, when your blood is not acidic enough.
Acidosis in particular can be life-threatening, so it's important to find the cause
and treat it as soon as possible.
Metabolic acidosis may be caused when:
Your body makes too much acid. For example, lactic acidosis may happen if you exercise
too much. People with diabetes may get ketoacidosis. This means their body makes acids
called ketones. And acidosis can also be caused by certain poisons, such as methanol
(wood alcohol), too much aspirin, or antifreeze.
You lose too much bicarbonate. This can happen if you have diarrhea. It can also happen
with a condition called proximal renal tubular acidosis. In this condition the kidneys
do not take in enough bicarbonate and it is lost in urine.
Your body doesn't get rid of enough acid in your urine because of problems with your
Why do I need this test?
You might need this test if you have symptoms of metabolic acidosis. Signs and symptoms
Shortness of breath
Nausea and vomiting
Rapid heart rate
Low blood pressure
What other tests might I have along with this test?
Your health care provider may also order other blood tests because many conditions
can affect the acid-base chemistry of your blood. Possible tests include:
If you may have swallowed a poison, such as wood alcohol, salicylate (in aspirin),
and ethylene glycol (in antifreeze), your provider may test your blood for it.
If your provider thinks you have ketoacidosis, you might need a urine dipstick test
for ketone compounds. Ketoacidosis is a health emergency.
What do my test results mean?
Many things may affect your lab test results. These include the method each lab uses
to do the test. Even if your test results are different from the normal value, you
may not have a problem. To learn what the results mean for you, talk with your health
Results are given in milliequivalents per liter (mEq/L). Normal results are 3 to 10 mEq/L,
although the normal level may vary from lab to lab.
If your results are higher, it may mean that you have metabolic acidosis.
Hypoalbuminemia means you have less albumin protein than normal. If you have this
condition, your expected normal result must be lower.
How is this test done?
The test requires a blood sample, which is drawn through a needle from a vein in your
Does this test pose any risks?
Taking a blood sample with a needle carries risks that include bleeding, infection,
bruising, or feeling dizzy. When the needle pricks your arm, you may feel a slight
stinging sensation or pain. Afterward, the site may be slightly sore.
What might affect my test results?
Being dehydrated or retaining water in your body can affect your results. Antibiotics
such as penicillin can also affect your results.
How do I get ready for this test?
You don't need to prepare for this test. But be sure your health care provider knows
about all medicines, herbs, vitamins, and supplements you are taking. This includes
medicines that don't need a prescription and any illicit drugs you may use.