An unexpected low \(\text{CO}_2\) result on a blood test, often part of a routine metabolic panel, indicates an imbalance in the acid-base equilibrium of your blood. While the term \(\text{CO}_2\) suggests the gas you exhale, a low reading points to a systemic issue. Understanding what this low value represents is the first step your doctor will use to guide any necessary next steps.
What Does the \(\text{CO}_2\) Blood Test Measure?
The value labeled \(\text{CO}_2\) on a standard metabolic panel measures the total carbon dioxide content (\(\text{TCO}_2\)), not dissolved \(\text{CO}_2\) gas. This total measurement is primarily composed of bicarbonate (\(\text{HCO}_3^-\)) ions, which account for over 90% of the reported value. Bicarbonate is the body’s primary chemical buffer, neutralizing excess acids or bases to maintain the blood’s \(\text{pH}\) within the narrow range of 7.35 to 7.45.
A low \(\text{TCO}_2\) result, typically below 22 to 30 milliequivalents per liter (\(\text{mEq/L}\)), indicates metabolic acidosis. This condition means the concentration of bicarbonate is too low to adequately balance the acids in the bloodstream, causing the blood to become more acidic. The \(\text{TCO}_2\) test serves as an effective screening tool to assess acid-base status, signaling that the body is either losing too much base or accumulating too much acid.
Primary Reasons for Low Bicarbonate Levels
Metabolic acidosis results from either losing bicarbonate base or accumulating too much acid.
Loss of Base
Loss of base often occurs due to severe or chronic gastrointestinal fluid loss, such as persistent diarrhea. The digestive tract below the stomach is rich in bicarbonate, and excessive fluid loss depletes the body’s buffer reserves. This can lead to hyperchloremic acidosis.
Overproduction of Acid
The accumulation of metabolic acids is often a more acute issue. Diabetic ketoacidosis (DKA) is a recognized cause, occurring when the body burns fat for energy due to lack of insulin, producing acidic ketone bodies. Lactic acidosis develops when cells lack sufficient oxygen, often due to shock or severe infection, causing lactic acid to accumulate. These excess acids consume and neutralize the available bicarbonate buffer.
Kidney Dysfunction
Kidney function is primary in maintaining bicarbonate balance, as the kidneys regenerate and reabsorb bicarbonate while excreting excess acid. Impaired kidney function, such as chronic kidney disease, reduces the ability to excrete these acids, causing them to build up. Renal tubular acidosis (RTA) is a specific kidney disorder involving a defect in the tubules that prevents proper handling of acid and bicarbonate, leading to persistent base loss.
Medications and Toxins
Certain medications can contribute to lower bicarbonate levels, including specific diuretics used for blood pressure or fluid retention, which affect electrolyte balance. Additionally, an overdose of substances like aspirin (salicylate toxicity) or ethylene glycol can rapidly introduce large amounts of acid into the system, quickly overwhelming the bicarbonate buffer.
Recognizing Symptoms and Necessary Action
The seriousness of a low bicarbonate result depends on the degree of reduction and the underlying cause. Individuals with a mild, chronic reduction may experience no noticeable symptoms, as the body’s compensatory mechanisms, particularly the lungs, often adjust to manage the mild increase in acidity.
A severely low bicarbonate level indicates significant metabolic acidosis, which is a medical emergency. When the acid load is too high, the body compensates by increasing the respiratory rate to expel excess carbon dioxide. This results in a distinctive pattern of deep, rapid breathing, known as Kussmaul respiration.
Severe symptoms requiring immediate medical attention include:
- Confusion.
- Lethargy.
- Persistent nausea.
- Vomiting.
If you receive a low \(\text{CO}_2\) result and are experiencing any acute symptoms, seek emergency care immediately.
For any unexpected lab result, the appropriate action is to consult the physician who ordered the test. They will interpret the result within the context of your overall health, symptoms, and other blood markers. Self-diagnosing or attempting to self-treat is not recommended, as the correct approach depends on identifying the precise cause of the low bicarbonate level.