Type of test Blood

 Normal findings

Adult/elderly: 23-30 mEq/L or 23-30 mmol/L (SI units)

Child: 20-28 mEq/L

Infant: 20-28 mEq/L

Newborn: 13-22 mEq/L

Possible critical values < 6 mEq/L

Test explanation and related physiology

The CO2 content is a measure of CO2 in the blood. In the peripheral venous blood, this assists in evaluation of the pH status of the patient and in evaluation of electrolytes. The serum CO2 test is usually included with other assessments of electrolytes. It is usually done with a multiphasic testing machine that also measures sodium, potassium, chloride, blood urea nitrogen (BUN), and creatinine.

It is important not to get this test confused with Pco2 . This CO2 content measures the H2 CO3 , the dissolved CO2 , and the bicarbonate ion (HCO3 ) that exists in the serum. Because the amounts of H2 CO3 and dissolved CO2 in the blood are so small, CO2 content is an indirect measure of the HCO3 anion. The HCO3 anion is second in importance to the chloride ion in electrical neutrality (negative charge) of extracellular and intracellular fluid; its major role is in acid–base balance.

Levels of HCO3 are regulated by the kidneys. Increases cause alkalosis, and decreases cause acidosis. See further discussion of this test as it is performed on arterial blood (p. 104). When CO2 content is measured in the laboratory with other serum electrolytes, air affects the specimen, and the CO2 partial pressure can be altered. Therefore venous blood specimens are not very accurate for true CO2 content or HCO3 determination. This test is used mostly as a rough guide to the patient’s acid–base balance.

Interfering factors

 • Underfilling the tube with blood allows CO2 to escape from the serum specimen and may significantly decrease HCO3 values.

* Drugs that may cause increased serum CO2 and HCO3 levels include aldosterone, barbiturates, bicarbonates, ethacrynic acid, hydrocortisone, loop diuretics, mercurial diuretics, and steroids.

* Drugs that may cause decreased levels include methicillin, nitrofurantoin (Furadantin), paraldehyde, phenformin hydro chloride, tetracycline, thiazide diuretics, and triamterene.

 Procedure and patient care

 • See inside front cover for Routine Blood Testing.

 • Fasting: no

 • Blood tube commonly used: red or green

Abnormal findings

 Increased levels

- Severe diarrhea

- Starvation

- Severe vomiting

- Aldosteronism

- Emphysema

- Metabolic alkalosis

- Gastric suction

Decreased levels

- Renal failure

- Salicylate toxicity

- Diabetic ketoacidosis

- Metabolic acidosis

- Shock

- Starvation

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