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Also known as: Total CO2; TCO2

At a Glance

Why Get Tested?

Usually as part of a renal profile (collection of tests which help investigate the kidney), to screen for an electrolyte or acid-base imbalance in conditions known to cause such disturbances or to monitor a known imbalance

When to Get Tested?

May be part of a routine blood test that includes electrolyte measurements or may be requested by your doctor if you have a medical condition or are experiencing symptoms that could indicate problems with the acid-base balance of your body

Sample Required?

A blood sample taken from a vein in the arm

Test Preparation Needed?


The Test Sample

What is being tested?

When you breathe, you bring oxygen (O2) into your lungs and release carbon dioxide (CO2). Carbon dioxide in your blood is present in three forms: carbonic acid (H2CO3), CO2 dissolved in blood, and bicarbonate (HCO3-), the predominant form. Bicarbonate is a negatively charged ion that is excreted and reabsorbed by your kidneys. Its main job is to help maintain the acid-base balance (pH) in your body and secondarily to work with sodium, potassium, and chloride to maintain electrical neutrality at the cellular level.

Measuring bicarbonate gives your doctor a rough estimation of acid-base balance. This is usually sufficient, but measurements of gasses dissolved in the blood may be done if more information is needed. Bicarbonate may be measured with sodium, potassium, and possibly chloride in an electrolyte profile as it is the balance of the three or four that gives your doctor the most information.

How is the sample collected for testing?

A blood sample is taken 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.

The Test

Common Questions

Ask a Laboratory Scientist

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NOTE: This article is based on research that utilizes the sources cited here as well as the collective experience of the Lab Tests Online Editorial Review Board. This article is periodically reviewed by the Editorial Board and may be updated as a result of the review. Any new sources cited will be added to the list and distinguished from the original sources used.