A low reading of carbon dioxide (Total \(\text{CO}_2\) or \(\text{TCO}_2\)) on a routine blood test, such as a Basic or Comprehensive Metabolic Panel, often concerns patients. This value does not measure the gaseous carbon dioxide you exhale. Instead, it measures the bicarbonate (\(\text{HCO}_3^-\)) concentration in your blood, which is the body’s primary chemical buffer. A low total \(\text{CO}_2\) reading signals a shortage of this buffer, indicating an acid-base imbalance called metabolic acidosis, where the blood has become too acidic.
What Low Carbon Dioxide Means in Blood Tests
When a laboratory report shows low carbon dioxide, it refers to the total amount of \(\text{CO}_2\) in the serum, 90 to 95% of which is bicarbonate. Bicarbonate is an electrolyte central to maintaining the body’s pH balance by neutralizing metabolic acids. The normal range typically falls between 22 and 29 milliequivalents per liter (mEq/L) for adults. A measurement below this range means the buffering capacity is diminished because bicarbonate has been consumed neutralizing excess acid.
This total \(\text{CO}_2\) value is distinct from the partial pressure of carbon dioxide (\(\text{PCO}_2\)), which is the gaseous form regulated by the lungs and measured with an Arterial Blood Gas (ABG). \(\text{PCO}_2\) reflects the respiratory component of the acid-base balance. Total \(\text{CO}_2\) reflects the metabolic component, which is controlled largely by the kidneys. Therefore, a low \(\text{CO}_2\) on a standard panel points toward a metabolic problem driving the body toward acidosis.
Primary Causes of Low Bicarbonate
Metabolic acidosis results from three main mechanisms: the overproduction of acid, the excessive loss of bicarbonate, or the impaired excretion of acid by the kidneys. Increased acid production often creates a high anion gap, meaning the body generates organic acids that consume the bicarbonate buffer. Diabetic Ketoacidosis (DKA) is a classic example, where a lack of insulin causes the body to break down fat for energy, leading to a buildup of acidic ketone bodies. Lactic acidosis is another cause, typically resulting from severe infection, shock, or tissue hypoxia where cells produce lactic acid faster than the liver can clear it.
Bicarbonate loss is primarily a gastrointestinal or renal issue leading to a normal anion gap acidosis. Severe, prolonged diarrhea causes the body to excrete large amounts of bicarbonate present in intestinal secretions. Certain kidney disorders, known as Renal Tubular Acidosis (RTA), cause the kidneys to fail to reabsorb filtered bicarbonate or fail to excrete sufficient hydrogen ions, resulting in a net loss of base.
Reduced acid excretion is common in Chronic Kidney Disease (CKD), especially as kidney function declines. Healthy kidneys generate new bicarbonate and remove acid waste products. When kidney function is impaired, these acid products accumulate in the bloodstream, continuously consuming bicarbonate and lowering the total \(\text{CO}_2\) level. This chronic acidosis is linked to the progression of kidney disease, bone, and muscle wasting.
Symptoms Associated with Low Bicarbonate
The signs and symptoms associated with low bicarbonate relate to the severity of the metabolic acidosis and the specific underlying cause. In mild or chronic cases, a person may experience no noticeable symptoms, making the lab finding an unexpected discovery. As acidosis worsens, general symptoms often appear, including persistent fatigue, weakness, and nausea with or without vomiting.
In severe acidosis, a change in the breathing pattern occurs. The body attempts to compensate for the acid buildup by increasing the rate and depth of respiration, known as Kussmaul breathing. This deep, labored pattern is the respiratory system’s attempt to rapidly expel gaseous \(\text{CO}_2\), which slightly raises the blood pH by shifting the acid-base balance. In the most severe cases, brain function can be impaired, leading to confusion, drowsiness, and altered mental status.
Diagnosis and Medical Management
When a low total \(\text{CO}_2\) is found, physicians order additional tests to confirm metabolic acidosis and determine the specific cause. The Anion Gap calculation is a key diagnostic tool. It compares measured electrolytes to determine if the low bicarbonate is due to the addition of new, unmeasured acids (high anion gap) or the loss of bicarbonate (normal anion gap). An Arterial Blood Gas (ABG) test is often performed next to precisely measure the blood’s pH and \(\text{PCO}_2\), confirming the degree of acidosis and assessing respiratory compensation.
Managing low bicarbonate requires treating the underlying condition causing the acid-base imbalance. For a patient with DKA, this means administering insulin and intravenous fluids to correct high blood sugar and stop ketone production. In cases of kidney failure, treatment is aimed at managing the kidney disease to improve acid excretion. In rare, acute, and life-threatening cases of severe acidosis, a physician may administer intravenous sodium bicarbonate to temporarily boost the blood’s buffer capacity and stabilize the patient. However, this is not a long-term solution; the focus remains on diagnosing and resolving the root cause of the bicarbonate deficit.