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Blood Test Tracker

Bicarbonate (HCO3-), Blood Gas

The bicarbonate your blood gas analyzer calculates from pH and pCO2. It shows the kidney-controlled side of acid-base balance.

AI-verified against cited sources. Awaiting expert reviewAlso called cHCO3LOINC 1960-4

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What it is

Bicarbonate is the body's main acid buffer. It soaks up extra acid to keep blood pH steady. On a blood gas, the analyzer calculates it from the measured pH and pCO2 instead of measuring it directly. Your kidneys control bicarbonate over hours to days, while your lungs control carbon dioxide within minutes. It is not quite the same as the 'CO2' or 'bicarbonate' on a basic or comprehensive metabolic panel, which is a separate measurement on a vein sample.

Why it matters

Bicarbonate shows whether a pH change involves the body's chemistry, not just breathing. A low bicarbonate with a low pH points toward too much acid or lost bicarbonate. A high bicarbonate with a high pH points toward extra base. Bicarbonate also shifts slowly as the kidneys adapt to long-lasting breathing changes. That helps your clinician judge how long a problem has been going on.

If it’s low

A bicarbonate below the usual range means the blood has less base than expected. It can be the kidneys adapting to long-lasting fast breathing, as at high altitude, or it can reflect acid building up or bicarbonate being lost.

Common causes

  • The kidneys adapting to long-term fast breathing, for example at high altitude
  • Severe diarrhea, which causes bicarbonate loss
  • Kidney problems, since the kidneys help clear acid and recover bicarbonate
  • Diabetic ketoacidosis, or lactic acid building up when tissues are short of oxygen
  • Poisoning or an overdose

What can help

  • Ask your clinician what is driving the change, since the next steps depend on the cause
  • If you have diabetes or kidney disease, ask which warning signs should make you call right away
  • Ask whether electrolytes, lactate or kidney tests should be checked

If it’s high

A bicarbonate above the usual range means the blood has more base than expected. Often it is the kidneys adapting to long-lasting high carbon dioxide, as in some long-term lung conditions. It can also come from losing stomach acid or from certain medicines.

Common causes

  • The kidneys adapting to long-lasting high carbon dioxide, as in COPD
  • Vomiting, which removes stomach acid
  • Water pills (diuretics) and other medicines that change fluid and salt balance
  • Hormone conditions such as too much aldosterone

What can help

  • Ask your clinician whether this is the body adapting to a lung condition or something else
  • Review any water pills or other medicines with your clinician before changing anything
  • Ask whether electrolytes such as potassium and chloride should be checked

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.

For adults: 22–26 mmol/L. Source: MedlinePlus, National Library of Medicine, South African Family Practice, Medicine International, F1000Research, Northwell Health Laboratories, Oregon Health & Science University (OHSU) Laboratory.

What can affect your result

  • It is calculated, so any error in the pH or pCO2 carries into it, for example from air bubbles or delays
  • Sample type: venous values run about 1 mmol/L higher than arterial
  • Altitude: healthy people living high up have lower bicarbonate as the kidneys adjust
  • Long-lasting changes in breathing, which shift bicarbonate over days
  • Large amounts of intravenous fluid in hospital
  • It is calculated from pH and pCO2, so air bubbles, delayed analysis or a mislabeled venous sample that shift those values also shift bicarbonate.
  • A venous sample runs about 1 mmol/L higher than an arterial one.
  • Large amounts of intravenous fluid can dilute the blood and lower it.

Hours to days. The kidneys adjust bicarbonate more slowly than the lungs adjust carbon dioxide.

When to talk to a clinician

There is no widely published urgent limit for blood-gas bicarbonate. It is read together with pH, pCO2 and your symptoms. It is usually measured in a clinic or hospital, where your care team sees the result right away. If you are not in a care setting and feel very short of breath, confused, unusually drowsy or faint, or have chest pain, call 911 or your local emergency number.

Questions you could ask

  • Was this an arterial or a venous sample?
  • How does my bicarbonate fit with my pH and pCO2?
  • Could my bicarbonate be high because of a lung condition or a medicine I take?
  • Are my electrolytes worth checking?
  • Is my low bicarbonate more likely related to my kidneys, my metabolism or my breathing?
  • Should we check lactate, electrolytes or kidney function?

Questions people ask

Why is my blood gas bicarbonate different from the CO2 on my metabolic panel?

They are measured in different ways. The metabolic panel's 'CO2' is measured on a vein sample and includes a small amount of dissolved CO2. A blood gas calculates bicarbonate from the pH and pCO2. The two are usually close but not identical, and each has its own usual range.

Is the usual range different for a venous blood gas?

Slightly. In one review the venous range was 23 to 27 mmol/L, about 1 higher than the arterial range of 22 to 26. Your report should say which kind of sample was used.

Show the science and sources

Units

  • 1 mmol/L = 1 mmol/L
  • 1 mmol/L = 1 mEq/L

For clinicians and researchers

Blood-gas bicarbonate is calculated, not measured, from the Henderson-Hasselbalch equation: HCO3- = 0.03 x pCO2 x 10^(pH - 6.1), with HCO3- in mmol/L and pCO2 in mmHg. That differs from the total CO2 on a chemistry analyzer, which is measured on serum or plasma and equals bicarbonate plus dissolved CO2 (0.03 x pCO2, about 1.2 mmol/L at 40 mmHg); that measurement is catalogued as its own marker. Compensation rules from one review: in respiratory acidosis, HCO3- rises about 1 mmol/L per 10 mmHg of pCO2 above 40 when acute and about 4 mmol/L when chronic; in respiratory alkalosis, it falls about 2 mmol/L per 10 mmHg below 40 when acute and about 5 mmol/L when chronic. In metabolic acidosis, Winter's formula gives an expected PaCO2 of 1.5 x HCO3- + 8, plus or minus 2. If a pH recalculated from the reported HCO3- and pCO2 differs from the reported pH by more than about 0.05, an error is suspected. Standard bicarbonate, the value at a pCO2 of 40 mmHg, is a separate quantity. Venous bicarbonate is about 1 mmol/L higher.

LOINC 1960-4 (primary), 14627-4, 1959-6, 1961-2. Specimen: calculated.

Sources

  1. Arterial Blood Gas (ABG) Test, MedlinePlus, National Library of Medicine
  2. Mastering blood gas interpretation: A practical guide for primary care providers, South African Family Practice (2025)
  3. Blood gas analysis: Clinical applications, interpretation and future directions (Review), Medicine International (2026)
  4. Arterial blood analysis of healthy residents in Huamachuco, Peru (3,164 m): a cross-sectional study, F1000Research (2023)
  5. Blood Gas Arterial, Northwell Health Laboratories
  6. Blood Gases, Comprehensive, Oregon Health & Science University (OHSU) Laboratory

AI-verified against cited sources. Awaiting expert review. Last reviewed 2026-10-03. How we build and check these pages.

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