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Kidneys and electrolytes · Blood
CO2 (bicarbonate)
The total amount of carbon dioxide in the blood, mainly bicarbonate; a measure of the acid-base balance.
At a glance
- Measured in blood
- In 1 test panel
- No special preparation
Explanation
CO2 (bicarbonate): what is measured?
This test, also called total CO2 or alkali reserve, measures all forms of carbon dioxide in the blood. Most of it is bicarbonate (HCO3-), a base that neutralizes acids in the blood. The lungs breathe out carbon dioxide and the kidneys retain or excrete bicarbonate; this keeps the acidity of the blood constant. An abnormal value shows that the body has difficulty maintaining that balance. The measurement from a vein is different from a blood gas test from an artery, which is done in hospital.
What can it detect?
- Metabolic acidosis (the blood is too acidic), for example with severely reduced kidney function
- Metabolic alkalosis (the blood is too alkaline), for example with prolonged vomiting or water pills
- Sign of a breathing problem that changes the acidity
Result
What does the result mean?
CO2 (bicarbonate) too high
A raised CO2 means that there is relatively much bicarbonate in the blood. This mainly occurs with loss of stomach acid due to prolonged vomiting, with water pills and with a potassium deficiency. In chronic lung diseases in which someone breathes out carbon dioxide poorly, the kidneys also retain extra bicarbonate. Rarely, adrenal hormones play a role. A slightly raised value in someone without symptoms usually has no meaning.
CO2 (bicarbonate) too low
A low CO2 means that there is little bicarbonate, usually because the body uses it to neutralize acids. Causes are prolonged diarrhea, severely reduced kidney function, poorly controlled diabetes and rapid breathing (hyperventilation). According to the Dutch medical specialists' guideline (FMS), metabolic acidosis due to kidney damage usually only develops with an eGFR below 30. Heavy exercise just before the blood draw can also temporarily lower the value. A slightly low value in someone without symptoms is often a result of the sample collection or the transport of the sample.
After an abnormal result
With a slightly abnormal value without symptoms, the GP usually repeats the measurement, together with the other salts and kidney function. With a clearly abnormal value, the cause is looked for, for example in medicines, diarrhea, vomiting, diabetes or the kidneys. With chronic kidney damage and a low bicarbonate, the GP consults the internist-nephrologist; with symptoms or a strongly abnormal value, referral to hospital for a blood gas test follows.
Reliability
How reliable is the test?
According to the Dutch Society for Clinical Chemistry (NVKC), the result depends on the measurement method and the reference values differ between laboratories. The value is sensitive to how the sample is handled: if the tube is in transit for a long time or is not properly filled, carbon dioxide escapes and the result is falsely low. An abnormal CO2 does not itself show what the cause is; for that, the other salts, kidney function and sometimes a blood gas test are needed. In chronic kidney damage, the Dutch medical specialists' guideline (FMS) uses bicarbonate to decide whether supplementation is needed; in people without symptoms, a value on its own says little.
What can affect the result?
- Long transport, a tube that is not properly filled or a finger prick can give a falsely low value because carbon dioxide escapes.
- Heavy exercise, rapid breathing, vomiting and diarrhea temporarily shift the value.
- Water pills, laxatives and sodium bicarbonate (for example as an antacid) affect the result.
- Reference values differ between laboratories and measurement methods.
Who it is for
Who is the test useful for?
Useful for
Useful with severely reduced kidney function, prolonged vomiting or diarrhea, poorly controlled diabetes, shortness of breath or use of medicines that affect the acid-base balance, always together with sodium, potassium and chloride. In healthy adults without symptoms, a slightly abnormal result in broad panels is often a result of the sample collection or transport.
How common is it?
There are no Dutch figures on how often an abnormal CO2 or bicarbonate occurs in people without symptoms. Metabolic acidosis due to kidney damage mainly occurs with severely reduced kidney function, which is the case in a small proportion of people with chronic kidney damage. According to the Dutch GP guideline (NHG), chronic kidney damage itself occurs in about 1 in 10 adults.
FMS guideline on chronic kidney damage; NHG guideline on chronic kidney damage; no Dutch registration figure for the test on its own
Drawbacks and risks
- A falsely low value due to the transport of the sample does occur and can cause unnecessary worry or lead to unnecessary follow-up tests.
Get tested
CO2 (bicarbonate): test panels
CO2 (bicarbonate) is part of these test panels. You order online and have your blood drawn at a collection point or collect the sample at home.
Evidence
Sources
This explanation is based on Dutch guidelines and independent information. It is general information, not a diagnosis. Discuss an abnormal result with your GP.
More about risks and resultsMore explanations
Other tests: Kidneys and electrolytes
- BUN/creatinine ratio
- Chloride
- Creatinine
- Creatinine (urine)
- Cystatin C
- eGFR
- Phosphate (inorganic)
- Potassium
- Sodium
- Urea
- Urea nitrogen (BUN)