The CO2 test is almost never ordered by itself. It is usually ordered along with sodium, potassium, and chloride as part of an electrolyte panel. The electrolyte panel is used to detect, evaluate, and monitor electrolyte imbalances. It may be ordered as part of a routine exam or to help evaluate a chronic or acute illness. It may be ordered at intervals to help monitor conditions, such as kidney disease and hypertension, and to monitor the effectiveness of treatment for known imbalances.

When an acid-base imbalance is identified, the CO2 test (as part of the electrolyte panel) and blood gases may be ordered to evaluate the severity of the imbalance, 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) in nature, and monitor its treatment until the acid-base balance is restored.

Carbon dioxide testing may be ordered, usually as part of an electrolyte panel when:

• you are having a routine blood screen;
• your doctor suspects that you may be retaining water or are dehydrated, upsetting your electrolyte balance;
• your doctor wants to evaluate your body’s acid-base balance (pH);
• your doctor wants to monitor a condition or treatment that might cause an electrolyte imbalance.

When CO2 levels are higher or lower than normal, it suggests that your body is having trouble maintaining its acid-base balance or that you have upset your electrolyte balance, perhaps by losing or retaining fluid. Both of these imbalances may be due to a wide range of dysfunctions.

NOTE: The result of your CO2 test is measured by your doctor against a reference range for the test to determine whether the result is “normal” (it is within the range of numbers), high (it is above the high end of the range), or low (it is below the low end of the range). Because there can be many variables that affect the determination of the reference range, the reference range for this test is specific to the lab where your test sample is analyzed. For this reason, the lab is required to report your results with an accompanying reference range. Typically, your doctor will have sufficient familiarity with the lab and your medical history to interpret the results appropriately.

While there is no such thing as a “standard” reference range for CO2, most labs will report a similar, though maybe not exactly the same, set of numbers as that included in medical textbooks or found elsewhere online. For this reason, we recommend that you talk with your doctor about your lab results. For general guidance only, we are providing the reference range for this test from the classic medical text, Tietz Textbook of Clinical Chemistry and Molecular Diagnostics.

For more information on reference ranges, please read Reference Ranges and What They Mean.

Some drugs may increase blood carbon dioxide levels including: fludrocortisone, barbiturates, bicarbonates, hydrocortisone, loop diuretics, and steroids.

Drugs that may decrease blood carbon dioxide levels include methicillin, nitrofurantoin, tetracycline, thiazide diuretics, and triamterene.