A low carbon dioxide (CO2) result on a standard blood test often prompts concern, but this value must be understood in its proper context. The CO2 measurement reported on a metabolic panel is not a direct reading of the gaseous carbon dioxide you exhale. Instead, this test primarily reflects the level of bicarbonate (HCO3-) circulating in the blood. A low result signifies that the body’s primary chemical buffering system is diminished, suggesting an underlying imbalance that a healthcare provider must investigate to address the root cause.
Understanding the Measurement
The measurement labeled as “CO2” in a metabolic panel is the total carbon dioxide content in the blood plasma, with bicarbonate representing about 90 to 95% of that total. Bicarbonate is an electrolyte, a negatively charged ion dissolved in the blood that works with other electrolytes like sodium, potassium, and chloride. The result is typically reported in units of milliequivalents per liter (mEq/L) or millimoles per liter (mmol/L). For a healthy adult, the normal range for bicarbonate sits between 22 and 29 mEq/L. A low result indicates that the body is experiencing an accumulation of acid. This value is distinct from the partial pressure of carbon dioxide (pCO2), which is measured via an arterial blood gas test.
The Role of Bicarbonate in the Body
Bicarbonate functions as the primary component of the body’s acid-base balance system. It acts as a powerful buffer, neutralizing excess acids produced during metabolic processes. This buffering action is essential because the body must maintain a very narrow pH range, usually between 7.35 and 7.45, for enzymes and bodily functions to work correctly. The kidneys and lungs work together to regulate this balance. The kidneys control bicarbonate levels, and the lungs manage the excretion of carbon dioxide gas. When the bicarbonate level drops, the blood becomes too acidic, a condition known as metabolic acidosis. This state reflects a compromise in the body’s ability to manage its acid load.
Primary Causes of Low Bicarbonate
A low bicarbonate level is a direct sign of metabolic acidosis, which occurs through two primary mechanisms: the body either produces or takes in too much acid, or it loses too much bicarbonate base. Determining which mechanism is at play is central to the diagnostic process.
Acid Overload
This category involves the excessive production or retention of acid, which rapidly consumes the available bicarbonate. Diabetic Ketoacidosis (DKA) is a frequent cause, occurring when uncontrolled diabetes leads to the body breaking down fat for fuel, producing acidic ketone bodies that overwhelm the buffering system. Lactic acidosis is another common type, resulting from the buildup of lactic acid, often seen during severe shock, infection (sepsis), or prolonged lack of oxygen. Additionally, the ingestion of certain toxic substances, such as methanol or ethylene glycol, can lead to an acute acid load that depletes bicarbonate stores.
Base Loss
The second mechanism involves the direct loss of bicarbonate from the body, leading to a low serum level. Severe, chronic diarrhea is the most common cause, as the contents lost from the lower gastrointestinal tract are rich in bicarbonate. Certain kidney disorders can also cause this loss, specifically Renal Tubular Acidosis (RTA). In RTA, the kidneys fail to reabsorb bicarbonate from the filtered blood or cannot excrete hydrogen ions, resulting in a continuous drain of the base from the body.
Symptoms and When to Seek Care
The symptoms associated with metabolic acidosis can range from subtle to severe, depending on the speed and extent of the drop. In milder cases, a person may experience fatigue, nausea, or a general feeling of weakness. As the condition worsens, the body attempts to compensate for the increasing acidity through the respiratory system. This compensation manifests as deep, rapid breathing, sometimes referred to as Kussmaul breathing. This is a reflex action to expel more carbon dioxide and thus reduce the acid content in the blood. Severe acidosis can affect the central nervous system, leading to confusion, lethargy, or vomiting. If a person with known risk factors, like diabetes or kidney disease, develops sudden, marked symptoms or rapid breathing, immediate medical attention is necessary.
How Low Bicarbonate is Managed
The management of a low bicarbonate level focuses on identifying and treating the underlying medical condition. Simply raising the bicarbonate level with medication is typically a supportive measure, not a cure for the root problem. If the cause is DKA, treatment involves administering insulin and intravenous fluids to manage blood sugar and ketone production. In cases of severe base loss from chronic diarrhea, replacing fluids and electrolytes is the immediate goal. For chronic conditions like kidney disease or certain types of Renal Tubular Acidosis, long-term oral supplementation with sodium bicarbonate or similar alkali agents may be prescribed. This therapy is intended to maintain the bicarbonate level above a certain threshold, often 22 mEq/L, to slow the progression of chronic kidney issues and protect bone and muscle health. In life-threatening situations where the acid level is dangerously high, intravenous bicarbonate may be administered to quickly buffer the blood while the underlying cause is addressed.