Bicarbonate (Total CO2)
Bicarbonate, often printed as CO2 or total CO2, gives a clue about your body's acid-base balance. It is part of most routine blood panels.
AI-verified against cited sources. Awaiting expert reviewAlso called HCO3, HCO3-, TCO2LOINC 2028-9
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What it is
On a chemistry panel, "CO2" or "total CO2" usually means bicarbonate, not the gas you breathe out. Bicarbonate is a base your body uses to keep your blood from becoming too acidic or too alkaline. Your lungs and kidneys work together to keep it steady: the lungs control how much carbon dioxide you breathe out, and the kidneys control how much bicarbonate you keep. Most of the carbon dioxide in your blood is in the form of bicarbonate.
Why it matters
Bicarbonate is one of the main ways your body protects the acid level (pH) of your blood. A result outside the range can be a clue about how your kidneys, lungs, hydration or metabolism are working. It is read together with sodium, potassium and chloride, which are used to work out the anion gap. A small change is common, and sometimes it comes from how the blood tube was handled rather than from your body.
If it’s low
A lower result can mean your blood is more acidic than usual (metabolic acidosis). It can also happen when fast breathing lowers carbon dioxide and the kidneys adjust. A low result is also a common effect of sample handling, because carbon dioxide can escape from a tube that is left open or only partly filled. Many causes are mild and temporary, and your clinician can sort out which one fits.
Common causes
- A blood tube that was underfilled, left open or tested late, which lets carbon dioxide escape
- Diarrhea or other loss of bicarbonate from the gut
- Kidney disease, where the kidneys remove less acid
- Diabetes with ketones building up (diabetic ketoacidosis), or ketones from fasting, undernutrition or heavy alcohol use
- Lactic acid build-up during severe illness, shock or low oxygen
- Some medicines and toxins (your clinician can tell which ones matter)
- Fast or deep breathing (hyperventilation)
- Addison disease, a rare adrenal condition
What can help
- Ask whether the test should be repeated on a freshly drawn, completely filled tube
- If you have diarrhea or vomiting and can't drink enough, ask your clinician what to do about fluids
- Ask whether your kidney function, blood sugar and anion gap should be reviewed
If it’s high
A higher result is linked to blood that is less acidic than usual (metabolic alkalosis). It can also be the kidneys' way of making up for slower breathing, as in some long-term lung conditions. Many causes are everyday ones, such as losing fluid and stomach acid through vomiting, or taking water pills (diuretics). Your clinician can put your result in context.
Common causes
- Vomiting or other loss of stomach acid and fluid
- Dehydration or low body fluid volume
- Water pills (diuretics)
- Low potassium
- Lung conditions that make the body hold on to carbon dioxide, which the kidneys offset by keeping more bicarbonate
- Large amounts of calcium carbonate antacids or other alkali
- Hormone conditions that affect the adrenal glands (less common)
What can help
- Ask your clinician whether recent vomiting, fluid loss or a medicine could explain it
- Ask whether your potassium and kidney function should be checked
- Tell your clinician about any antacids or supplements you take
Usual ranges
What labs print as the reference range, by who was tested. Ranges differ a little between labs, so the range on your own report is the one your result is judged against.
The usual range depends on age. Choose an age to see the one that applies.
- 13+20–29 mmol/L
Sources: Labcorp, Mayo Clinic Laboratories
See Bicarbonate by age and sex, with the U.S. survey numbers where they exist.
If this applies to you
Bicarbonate means a little more for some people. Open the one that fits you.
Kidney disease or at risk
As kidney function falls, the kidneys make less bicarbonate, and the blood can slowly become more acidic. KDIGO 2024 notes that bicarbonate (often printed as CO2) starts to fall once eGFR drops below 60, and it is one of the CKD lab changes clinicians watch and may treat.
How common is your level?
NHANES describes the US civilian, non-institutionalized population, not a 'healthy' reference group. People with disease, on medication, fasting or not (except fasting-subsample markers), smokers and so on are all included. A percentile says where a value sits among Americans of that sex and age, not whether it is normal.
Use the left and right arrow keys to read each age. Shift moves five years.
Reference data derived from CDC/NCHS National Health and Nutrition Examination Survey (NHANES), cycles 2015-2016, 2017-March 2020 (pre-pandemic), August 2021-August 2023. Analysis by bloodtesttracker.com; not endorsed by CDC, NCHS or HHS.
Explore Bicarbonate by age and sex: every percentile as a table, with a download.
What can affect your result
- How the sample is handled: carbon dioxide escapes from open or underfilled tubes, and delays before the serum is separated can lower the result
- Age: children usually have slightly lower values than adults
- Fluid loss from vomiting, diarrhea or sweating
- Kidney and lung health
- Medicines, including water pills and antacids
- Fast breathing (hyperventilation) can lower it
- Carbon dioxide escapes from a tube that is left open, underfilled or kept a long time before testing, which makes the result read falsely low (Labcorp warns that this varies between labs).
- Serum or plasma should be separated from the cells promptly. Labcorp asks for within 45 minutes and Mayo Clinic Laboratories within 2 hours.
- Badly hemolyzed samples can be rejected (Mayo Clinic Laboratories).
- Some prescription and over-the-counter medicines can raise or lower the result (MedlinePlus).
Breathing can shift acid-base balance within minutes to hours, and the kidneys adjust over hours to days.
When to talk to a clinician
Contact the clinician who ordered the test soon if your result is outside your lab's range, especially if you also have repeated vomiting or diarrhea, deep or fast breathing, confusion or unusual sleepiness. Some labs, including ARUP, treat a result below 10 mmol/L or above 40 mmol/L as urgent. Call 911 or your local emergency number for chest pain, severe trouble breathing, fainting or a seizure.
Questions you could ask
- Which of my medicines or health conditions could affect my bicarbonate?
- How often should we recheck it?
- Could vomiting, fluid loss or one of my medicines explain a higher result?
- Should we check my potassium and kidney function too?
- Could the way the blood was drawn or handled explain a low result, and should we repeat it?
- Should we look at my kidney function, blood sugar and anion gap?
Questions people ask
Why does my report say CO2 when it means bicarbonate?
Labs measure the total carbon dioxide in your blood. Most of it is bicarbonate, so the two names are used for the same result on a chemistry panel. It is not the same as the carbon dioxide pressure (pCO2) reported on a blood gas.
Can the way my blood was collected change this result?
Yes. Carbon dioxide can escape from a tube that is left open, underfilled or kept a long time before testing, and that makes the result read low. If a low result doesn't fit how you feel, your clinician may ask for a repeat.
Why do some labs list 20–29 and others 22–29?
Each lab sets its own range using its own method and patients. Labcorp lists 20–29 mmol/L for people over 12, and Mayo Clinic Laboratories lists 22–29 mmol/L for adults. Use the range printed on your own report first.
Is mEq/L the same as mmol/L for bicarbonate?
Yes. Bicarbonate carries a single charge, so the numbers are the same in both units. A result of 24 mmol/L is 24 mEq/L.
Does a low or high result mean my blood is too acidic or too alkaline?
It can be a clue, but bicarbonate alone doesn't give your blood pH. Your clinician looks at it with your other electrolytes, your symptoms and sometimes a blood gas test.
Show the science and sources
Units
- 1 mmol/L = 1 mmol/L
- 1 mmol/L = 1 mEq/L
Is a change real?
Bicarbonate naturally varies about 4.8% from test to test in the same healthy person. Between two results from the same lab, a change of roughly 15% or more is unlikely to be day-to-day noise or lab variation (95% reference change value). Source: Desirable Biological Variation Database specifications, 2014 update (Minchinela J, Ricos C, et al.; the 'Ricos database', a compilation of median published CVI/CVG).
For clinicians and researchers
Total CO2 in serum or plasma is bicarbonate plus dissolved CO2, carbonic acid and carbamino compounds. Labcorp notes that in practice 80 to 90% is bicarbonate. Enzymatic methods, typically PEP carboxylase with malate dehydrogenase, are standard. Pre-analytical loss of CO2 starts when the stopper is removed and varies between laboratories, so Labcorp advises cautious interpretation, and venous total CO2 on a chemistry panel is a different measurement from calculated bicarbonate on a blood gas. Acid-base status depends on the ratio of bicarbonate to PCO2 (Henderson-Hasselbalch), so pH and PCO2 are needed to name a disorder. The anion gap, Na - (Cl + HCO3), sorts metabolic acidosis into high-gap causes (ketoacids, lactate, renal failure, toxins) and normal-gap, hyperchloremic causes (gastrointestinal or renal bicarbonate loss). In chronic kidney disease, KDIGO 2012 suggested oral bicarbonate when serum bicarbonate is below 22 mmol/L, while the KDIGO 2024 guideline suggests considering treatment to prevent acidosis with clinical implications and gives below 18 mmol/L in adults as the example, noting that trials have not shown that correcting bicarbonate improves outcomes. Reference intervals differ by lab: Labcorp 20–29 mmol/L over age 12, Mayo Clinic Laboratories 22–29 mmol/L for adults. ARUP and Michigan Medicine Laboratories call results below 10 or above 40 mmol/L.
LOINC 2028-9 (primary), 1963-8, 20565-8. Specimen: serum-or-plasma.
Sources
- Carbon Dioxide, Total (test 001578), Labcorp
- Bicarbonate, Serum (test ID HCO3), Mayo Clinic Laboratories
- Critical Values List (CORP-APPEND-0104A, Rev. 46), ARUP Laboratories (2026)
- Critical Value Policy, Michigan Medicine Laboratories (MLabs)
- Quest Diagnostics Priority Result Reporting Policy (Client Synopsis), Quest Diagnostics (2025)
- Diagnostic Laboratories: Critical Values, Houston Methodist
AI-verified against cited sources. Awaiting expert review. Last reviewed 2026-10-03. How we build and check these pages.