Anion Gap Blood Test
Reviewed
Also called: Serum Anion Gap, AG Blood Test
An anion gap blood test measures the acid-base balance (pH balance) in your blood by calculating the difference between negatively and positively charged electrolytes. It helps detect if your blood is too acidic (acidosis) or not acidic enough (alkalosis).
Specimen: Blood
Why this test may be ordered
- To check for an electrolyte imbalance
- If you have symptoms of acidosis such as shortness of breath, vomiting, confusion, fatigue, or fast heartbeat
- If you have symptoms of alkalosis such as arrhythmia, irritability, hand tremor, muscle twitching, nausea, vomiting, or numbness/tingling in the face, hands, or feet
Preparation and collection
No special preparation is necessary for this test. If your provider has ordered more tests on your blood sample, you may need to fast for several hours before the test. Your provider will let you know if there are any special instructions.
A health care professional will take a blood sample from a vein in your arm using a small needle. After the needle is inserted, a small amount of blood is collected into a test tube. The procedure usually takes less than five minutes.
There is very little risk to having this test. You may feel slight pain or bruising at the spot where the needle was put in, but most symptoms go away quickly.
Understanding results
Your anion gap results are calculated from an electrolyte panel, basic metabolic panel (BMP), comprehensive metabolic panel (CMP), or renal function panel (RFP). Normal results may vary by lab. Your provider will use these results along with your medical history and other tests to make a diagnosis.
- High anion gap: Your blood may be more acidic than normal (acidosis). This could be due to dehydration, diarrhea, too much exercise, kidney disease, diabetes or diabetic ketoacidosis, salicylate poisoning (e.g., from too much aspirin), or antifreeze poisoning. Limitation: A high anion gap result is not a diagnosis by itself; it indicates a possible acid-base imbalance that requires further evaluation. Next step: Your provider will use this result along with your medical history and other tests to determine the underlying cause.
- Low anion gap: Your blood may not be acidic enough (alkalosis). However, this result is rare, and the most common cause is a low level of albumin (a protein in the blood), which may be a sign of not eating enough protein, kidney problems, liver disease, heart disease, or some types of cancer. Limitation: A low anion gap result is uncommon, and your provider will usually have you tested again to make sure the results are accurate before drawing conclusions. Next step: Your provider may repeat the test and use this result along with other tests to investigate possible causes.
- There is a wide range of normal results and many possible causes of abnormal results.
- The anion gap test is a calculation based on other electrolyte tests, so its accuracy depends on those measurements.
- A low anion gap result is rare and often requires repeat testing to confirm.
What to do next
Talk with your provider about what your test results mean and whether you need additional testing to make a diagnosis.
- What do my anion gap results indicate about my acid-base balance?
- Do I need any follow-up tests based on these results?
- What symptoms should I watch for that might relate to acidosis or alkalosis?
- How do my other health conditions or medications affect these results?
- Should I make any changes to my diet or lifestyle based on this test?