Also known as:Total CO2; TCO2; Carbon Dioxide Content; CO2 Content; Bicarb; HCO3-
Related tests:Electrolytes; Sodium; Potassium; Chloride; Comprehensive Metabolic Panel; Basic Metabolic Panel; Blood Gases
All content on Lab Tests Online has been reviewed and approved by our Editorial Review Board.
Total CO2 or bicarbonate is different than the partial pressure of CO2 (PCO2). Learn about PCO2
Why Get Tested?
As part of an electrolyte panel to identify or monitor an electrolyte imbalance or acid-base (pH) imbalance
When to Get Tested?
During a routine physical or as recommended by your healthcare practitioner if you are experiencing symptoms such as weakness, confusion, prolonged vomiting, or breathing problems that could indicate an electrolyte imbalance or an acid-base imbalance (commonly described as acidosis or alkalosis)
A blood sample drawn from a vein in your arm
Test Preparation Needed?
How is it used?
The bicarbonate (or total CO2) test is usually ordered along with sodium, potassium, and chloride as part of an electrolyte panel. The electrolyte panel is used to help detect, evaluate, and monitor electrolyte imbalances and/or acid-base (pH) imbalances (acidosis or alkalosis). It may be ordered as part of a routine exam or to help evaluate a variety of chronic or acute illnesses.
An electrolyte panel may be used to help monitor conditions, such as kidney disease, lung disorders, and high blood pressure (hypertension). When acidosis or alkalosis is identified, bicarbonate (as part of the electrolyte panel) and Blood Gases may be ordered to evaluate the severity of the pH imbalance. These tests help determine whether it is primarily respiratory (due to an imbalance between the amount of oxygen coming in and CO2 being released) or metabolic (due to increased or decreased amounts of bicarbonate in the blood). They also help monitor treatment until acid-base balance is restored.
When is it ordered?
This testing may be ordered when acidosis or alkalosis is suspected or when someone has an acute condition with symptoms that may include the following:
- Prolonged vomiting and/or diarrhea
- Weakness, fatigue
- Difficulty breathing (respiratory distress)
Electrolytes may be ordered at regular intervals when a person has a disease or condition or is taking a medication that can cause an electrolyte imbalance. Electrolyte panels or basic metabolic panels are commonly used to monitor treatment of certain problems, including high blood pressure (hypertension), heart failure, and liver and kidney disease.
What does the test result mean?
When bicarbonate levels are higher or lower than normal, it suggests that the body is having trouble maintaining its acid-base balance, either by failing to remove carbon dioxide through the lungs or the kidneys or perhaps because of an electrolyte imbalance, particularly a deficiency of potassium. Both of these imbalances may be due to a wide range of conditions.
Examples of causes of a low bicarbonate level include:
- Addison disease
- chronic diarrhea
- Diabetic ketoacidosis
- Metabolic acidosis
- Respiratory alkalosis, which can be caused by hyperventilation
- kidney disease
- Ethylene glycol or methanol poisoning
- Salicylate (aspirin) overdose
Examples of causes of a high bicarbonate level include:
- Severe, prolonged vomiting and/or diarrhea
- Lung diseases, including COPD
- Cushing syndrome
- Conn syndrome
- Metabolic alkalosis
Is there anything else I should know?
Some drugs may increase bicarbonate levels including fludrocortisone, barbiturates, bicarbonates, hydrocortisone, loop diuretics, and steroids.
Drugs that may decrease bicarbonate levels include methicillin, nitrofurantoin, tetracycline, thiazide diuretics, and triamterene.
What is being tested?
Bicarbonate is an electrolyte, a negatively charged ion that is used by the body to help maintain the body's acid-base (pH) balance. It also works with the other electrolytes (sodium, potassium, and chloride) to maintain electrical neutrality at the cellular level. This test measures the total amount of carbon dioxide (CO2) in the blood, which occurs mostly in the form of bicarbonate (HCO3-). The CO2 is mainly a by-product of various metabolic processes.
Measuring bicarbonate as part of an electrolyte or metabolic panel may help diagnose an electrolyte imbalance or acidosis or alkalosis. Acidosis and alkalosis describe the abnormal conditions that result from an imbalance in the pH of the blood caused by an excess of acid or alkali (base). This imbalance is typically caused by some underlying condition or disease.
The lungs and kidneys are the major organs involved in regulating blood pH through the removal of excess bicarbonate.
- The lungs flush acid out of the body by exhaling CO2. Raising and lowering the respiratory rate alters the amount of CO2 that is breathed out, and this can affect blood pH within minutes.
- The kidneys eliminate acids in the urine and they regulate the concentration of bicarbonate (HCO3-, a base) in blood. Acid-base changes due to increases or decreases in HCO3- concentration occur more slowly than changes in CO2, taking hours or days.
Any disease or condition that affects the lungs, kidneys, metabolism, or breathing has the potential to cause acidosis or alkalosis.
The bicarbonate test gives a healthcare practitioner a rough estimate of a patient's acid-base balance. This is usually sufficient, but measurements of gases dissolved in the blood (blood gases) may be done if more information is needed. Bicarbonate is typically measured along with sodium, potassium, and possibly chloride in an electrolyte panel as it is the balance of these molecules that gives the healthcare practitioner the most information.
How is the sample collected for testing?
A blood sample is drawn by needle from a vein in the arm.
NOTE: If undergoing medical tests makes you or someone you care for anxious, embarrassed, or even difficult to manage, you might consider reading one or more of the following articles: Coping with Test Pain, Discomfort, and Anxiety, Tips on Blood Testing, Tips to Help Children through Their Medical Tests, and Tips to Help the Elderly through Their Medical Tests.
Another article, Follow That Sample, provides a glimpse at the collection and processing of a blood sample and throat culture.
Is any test preparation needed to ensure the quality of the sample?
No test preparation is needed.
- If I've had a bicarbonate (total CO2) test, why does my doctor want to test my blood gases?
Blood gas tests, in which blood is drawn from an artery instead of a vein, can give your healthcare practitioner more information about your acid-base balance. They can tell your provider whether your lungs are working properly to keep oxygen and carbon dioxide at healthy levels.
- If bicarbonate levels are too high or low, what treatments can help?
If your bicarbonate is high or low, your healthcare practitioner will identify and treat the underlying cause. For example, high bicarbonate may be caused by emphysema, which may be treated with oxygen therapy and medications, or by severe diarrhea or vomiting, which would be addressed by treating the cause of the diarrhea or vomiting. Low bicarbonate may be caused by diabetic ketoacidosis, for example, which can be addressed in part by fluid and electrolyte replacement and insulin therapy.
© 2017 American Association for Clinical Chemistry, republished from Lab Tests Online.*
Descriptions of clinical laboratory tests were originally prepared for use on Lab Tests Online, an award-winning patient education website on clinical laboratory testing. Lab Tests Online is produced by the American Association for Clinical Chemistry (AACC), a global scientific and medical professional organization dedicated to clinical laboratory science and its application to healthcare. The Lab Tests Online website is developed in collaboration with other laboratory professional societies and is funded in part through corporate sponsorships.