Bicarbonate (CO2) blood test (HCO₃⁻)
Given as total CO₂, most of which is bicarbonate
- Updated
- 2026-09-10
- Every number with its source
What bicarbonate is and why it is measured as CO₂
Bicarbonate is one of the body's most important buffer substances — it helps keep blood pH stable by neutralising the acids that form every day during metabolism. In the laboratory, the amount of bicarbonate is most often measured as total CO₂ as part of an electrolyte panel, because most of the carbon dioxide present in blood is in the form of bicarbonate ions (HCO₃⁻). The bicarbonate level is regulated by two body systems: the kidneys, which produce and reabsorb it, and the lungs, which remove carbon dioxide through breathing. Because of this, the bicarbonate test helps assess whether an acid-base imbalance has a respiratory or a metabolic cause. The test is usually done together with potassium, sodium and chloride, because together all of these electrolytes reflect the body's overall fluid and acid-base state.
Reference range
| Group | Range, mmol/l | Source |
|---|---|---|
| Adults Given as total CO₂, most of which is bicarbonate | 23–29 | MedlinePlus Reviewed 2025-05-19 |
MedlinePlus gives this range for adults. The exact interval used by your laboratory is printed on your own report, because it can differ slightly depending on the laboratory's method — see also how to read blood test results.
When bicarbonate is tested
- As part of a general electrolyte panel (potassium, sodium, chloride and bicarbonate) during a routine check-up
- When there is difficulty breathing, or frequent vomiting or diarrhoea over a longer period
- When a metabolic disorder is suspected, for example a complication of diabetes (ketoacidosis) or kidney disease
- To monitor an already known kidney, lung or liver disease, or to assess the effect of medicines that act on the electrolyte balance
- With weakness, confusion or unusual tiredness, when an electrolyte imbalance is suspected as the cause
How to prepare
- No special preparation is usually needed — tell your doctor about every prescription and over-the-counter medicine you take, because many of them can affect the result
- Do not stop any medicine before the test without your doctor's instruction
- If other tests are ordered at the same time, you may be asked not to eat before the blood draw — check with your own doctor
A raised result — above 29 mmol/l
According to MedlinePlus, the bicarbonate (CO₂) level in blood is considered raised when it exceeds 29 mmol/l. The most common cause is metabolic alkalosis, which can arise from prolonged vomiting, dehydration or certain hormonal disorders. The result is assessed together with the other electrolytes and the clinical picture. The causes listed for a raised result are:
- Metabolic alkalosis — from prolonged vomiting, dehydration or excessive sweating
- Cushing's syndrome and hyperaldosteronism (adrenal hormone disorders)
- Compensated respiratory acidosis — when the body has been trying for a long time to compensate for a chronic lung disease
- Heart failure and Bartter syndrome
- SIADH (syndrome of inappropriate antidiuretic hormone secretion) and burns
What is usually assessed alongside a raised result:
- The other electrolytes (potassium, sodium, chloride) — to make clear whether this is an isolated change or a wider imbalance
- Kidney and lung function markers, if chronic compensation is suspected
- Medicines being taken that could affect the acid-base balance — to discuss with a doctor
- When to repeat the test, so that the change over time can be seen
A low result — below 23 mmol/l
According to MedlinePlus, the bicarbonate (CO₂) level in blood is considered low when it is below 23 mmol/l. The most common cause is metabolic acidosis, which can arise from kidney disease, ketoacidosis or prolonged diarrhoea. It may show itself as difficulty breathing, weakness or confusion. The causes listed for a low result are:
- Metabolic acidosis — from kidney disease, ketoacidosis (for example in diabetes) or lactic acidosis
- Prolonged diarrhoea — bicarbonate is lost through the digestive tract
- Addison's disease (adrenal insufficiency) and renal tubular acidosis
- Compensated respiratory alkalosis, poisoning with ethylene glycol, methanol or salicylates
- Some medicines that reduce carbonic anhydrase activity
Symptoms that can occur:
- Difficulty breathing
- Confusion
- Weakness or tiredness
- Prolonged vomiting and/or diarrhoea
What is usually assessed alongside a low result:
- A blood gas test and the other electrolytes — to assess the acid-base balance in a wider context
- Kidney function markers (creatinine), if a kidney cause is suspected
- Glucose and ketone tests, if ketoacidosis is suspected
- When to repeat the test to assess the change
Related tests
Bicarbonate is most often tested together with potassium, sodium and chloride as a general electrolyte panel. If a specific cause is suspected, a doctor may additionally order creatinine or blood gas tests. Related pages: potassium, sodium, chloride, creatinine, calcium.
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Frequently asked questions
›What is the normal bicarbonate (CO2) level in blood?
According to MedlinePlus, the normal total CO₂ level in blood in adults — most of which is bicarbonate — is 23–29 mmol/l. You will find your laboratory's exact interval on your own report, because it can differ slightly depending on the laboratory's method.
›Are bicarbonate and CO2 in blood the same test?
In practice, yes — the laboratory “total CO2” test essentially measures the amount of bicarbonate (HCO₃⁻), because it makes up most of the carbon dioxide present in blood. That is why on a report this indicator is often called simply CO₂.
›Why does a doctor order a bicarbonate (electrolyte) test?
This test helps assess the acid-base balance and the electrolyte state, which are affected by kidney and lung function. It is often ordered during a routine check-up or when a kidney, lung or metabolic disorder is suspected.
›What symptoms can occur when bicarbonate is disturbed?
According to MedlinePlus, a disturbed electrolyte balance can cause difficulty breathing, confusion, weakness, tiredness, and also prolonged vomiting or diarrhoea. These symptoms are not specific to bicarbonate alone, so the result needs to be discussed with a doctor.
›Is special preparation needed before a bicarbonate test?
Usually no special preparation is needed, but it is important to tell your doctor about every medicine you take, because many of them can affect the result. If other tests are done at the same time, you may need to avoid eating before the blood draw.
›Which other tests are bicarbonate results assessed with?
Bicarbonate is most often tested together with potassium, sodium and chloride as a general electrolyte panel. If a specific cause is suspected, a doctor may additionally order creatinine or blood gas tests.
Sources
Related markers
This site is not an emergency service. If severe chest or head pain, breathlessness, confusion, difficulty speaking, heavy bleeding or another acute problem has come on suddenly — do not wait for tests, call 112.
This information is not a diagnosis and does not replace a consultation with a doctor. Reference ranges depend on the laboratory, your age and sex — always compare your result with the range printed on your own report.