Carbon Dioxide (CO2)

Optimal Result: 20 - 29 mEq/L, or 20.00 - 29.00 mmol/L.

What Is Carbon Dioxide (CO2) in a Blood Test?

Carbon Dioxide (CO2) is a natural byproduct of your body’s metabolism. As your body breaks down nutrients for energy, CO2 is produced and transported through the bloodstream—mostly in the form of bicarbonate (HCO3)—to the lungs, where it is exhaled. In healthy individuals, CO2 levels in the blood remain within a stable, normal range.

CO2 plays a critical role in maintaining your body's acid-base balance (pH). It acts as a buffer to keep your blood from becoming too acidic or too alkaline. As an electrolyte, CO2 works alongside other key electrolytes like sodium, potassium, and chloride to maintain cellular stability and fluid balance.


What Does a CO2 Blood Test Measure?

A CO2 blood test (also called a bicarbonate test, total CO2 test, or CO2 content test) measures the amount of carbon dioxide in your blood, primarily in the form of bicarbonate. This test helps assess your acid-base balance and electrolyte levels, and may indicate whether there is an underlying condition affecting your lungs, kidneys, or metabolic function.

Alternate names for the CO2 blood test include:

  • Bicarbonate (HCO3) test

  • Total CO2 (TCO2)

  • CO2 content test

  • CO2 serum test

  • Carbon dioxide content


Why Is the CO2 Blood Test Important?

Maintaining the right level of carbon dioxide is essential for healthy body function. Abnormal CO2 levels—either too high or too low—can signal a range of health conditions such as:

  • Respiratory issues

  • Kidney dysfunction

  • Metabolic imbalances

  • Dehydration

  • Acidosis or alkalosis

The test is often part of a comprehensive metabolic panel (CMP) or electrolyte panel, which also checks sodium, potassium, and chloride levels.


When Is a CO2 Test Ordered?

Your healthcare provider may order a CO2 test if you’re experiencing symptoms of a pH imbalance or electrolyte disorder, including:

  • Shortness of breath

  • Nausea or vomiting

  • Fatigue

  • Breathing difficulties

CO2 levels may also be monitored regularly if you’re undergoing oxygen therapy, certain surgeries, or treatments for chronic lung or kidney conditions.


CO2 vs. Bicarbonate: What's the Difference?

While they’re often used interchangeably, there is a subtle difference:

  • Bicarbonate (HCO3) is the main form of carbon dioxide in your blood.

  • The CO2 blood test actually measures total bicarbonate levels.

This is why the terms CO2 test and bicarbonate test usually refer to the same lab measurement.


What Are Normal CO2 Levels?

Normal carbon dioxide levels in the blood typically range from:

20 – 29 milliequivalents per liter (mEq/L) for adults

However, what's considered "normal" can vary slightly by lab. A CO2 result outside the reference range does not always mean there’s a serious health issue—factors like dehydration, medications, or temporary illness may affect levels.


Additional Testing: Arterial Blood Gases (ABG)

In some cases, your doctor may order an arterial blood gas (ABG) test to get more precise measurements of CO2, bicarbonate, pH, and oxygen levels. Unlike a standard blood test taken from a vein, an ABG sample is drawn from an artery and is particularly useful in evaluating lung function and respiratory conditions.


What Conditions Are Linked to Abnormal CO2 or Bicarbonate Levels?

Too low or high bicarbonate levels can be associated with:

  • Metabolic acidosis (too much acid in the body)

  • Metabolic alkalosis (too much base)

  • Kidney disease

  • Liver failure

  • Diarrhea or vomiting

  • Chronic obstructive pulmonary disease (COPD)

  • Eating disorders like anorexia

Your healthcare provider will interpret your CO2 level alongside other test results and symptoms to identify the root cause.


Final Thoughts

The carbon dioxide (CO2) blood test is a quick and effective tool for assessing acid-base status and electrolyte balance in your body. It’s often part of broader metabolic testing and is especially important for people with respiratory, kidney, or metabolic concerns.

If your test results are outside the normal range, follow up with your doctor. The results must be interpreted in context, alongside other lab values and clinical symptoms.


References & Further Reading:

What does it mean if your Carbon Dioxide (CO2) result is too high?

What Does Elevated CO2 on a Blood Test Mean?

If your comprehensive metabolic panel (CMP) shows an elevated level of carbon dioxide (CO2), it could mean there's an imbalance in your body's pH (acid-base) levels. CO2 in this test refers to bicarbonate (HCO3-), which helps keep your blood's pH stable. When CO2 levels are higher than normal, it may indicate certain health issues. Here's what you need to know:

What Could Cause High CO2 Levels?

  1. Respiratory Issues
    Problems with breathing can affect CO2 levels in your blood. For example:

    • Slow or shallow breathing (hypoventilation) may cause CO2 to build up in your blood.

    • Chronic respiratory conditions like COPD can also make it harder for your lungs to remove CO2.

  2. Metabolic Alkalosis
    This happens when there’s too much bicarbonate in your blood. Causes include:

    • Excessive vomiting (which removes stomach acid).

    • Dehydration or losing fluids for a long time.

    • Overuse of diuretics (medications that help remove excess water from your body).

  3. Kidney Problems
    Your kidneys play a big role in balancing bicarbonate and CO2 levels. If they aren’t working properly—like in chronic kidney disease—they might not remove enough bicarbonate, leading to higher CO2 levels.

  4. Cushing's Syndrome
    This condition occurs when your body produces too much cortisol, a hormone that can affect your CO2 levels.

  5. Chronic Lung Conditions
    Diseases like chronic obstructive pulmonary disease (COPD) or other lung issues can cause difficulty in removing CO2 from your blood.

What Should You Do?

If you have high CO2 levels, don’t panic! Elevated CO2 can be caused by many factors, and it’s important to work with your healthcare provider to figure out what’s going on. They might recommend additional tests or ask about symptoms like breathing problems, dehydration, or recent illnesses.

Here are some questions you can ask your doctor:

  • What could be causing my high CO2 levels?

  • Do I need more tests?

  • Are there lifestyle changes or treatments that can help?

Takeaway

An elevated carbon dioxide (CO2) level on a comprehensive metabolic panel (CMP) can indicate an imbalance in the body’s acid-base regulation, potentially pointing to conditions like respiratory or metabolic alkalosis, kidney dysfunction/failure, or respiratory disorders like COPD. The presence of high CO2 levels suggests that the body may be retaining too much bicarbonate, which can disrupt normal pH levels and impact overall health. Next steps should include further investigation by a healthcare provider to determine the root cause. This may involve additional tests such as arterial blood gas analysis, kidney function tests, or respiratory assessments. Treatment will depend on the underlying condition, ranging from managing respiratory function to addressing kidney health or electrolyte imbalances. It’s important to follow up with a healthcare professional to tailor an appropriate treatment plan.

What does it mean if your Carbon Dioxide (CO2) result is too low?

What Causes Low Carbon Dioxide (CO2) or Bicarbonate Levels?

A low level of carbon dioxide (CO2) in the blood—typically referring to a low bicarbonate (HCO3) concentration—can result from various medical conditions and physiological imbalances. This can signal disruptions in your body's acid-base balance, often pointing to metabolic acidosis or respiratory alkalosis, depending on the pH level of your blood.


Common Causes of Low CO2 (Bicarbonate) Levels

Several health conditions and situations can cause CO2 levels to drop:

  • Hyperventilation (rapid breathing that lowers CO2)

  • Aspirin (salicylate) or alcohol overdose

  • Diarrhea, dehydration, or severe malnutrition

  • Liver disease or kidney dysfunction

  • Heart attack (especially a large or massive one)

  • Hyperthyroidism (overactive thyroid)

  • Uncontrolled diabetes

  • Sepsis (a severe body-wide infection)


Understanding Low CO2 and Blood pH Levels

The meaning of low bicarbonate levels can vary based on your blood pH:

1. Metabolic Acidosis

When both bicarbonate and blood pH are low (pH < 7.4), the condition is known as metabolic acidosis. It occurs when your body produces too much acid or the kidneys fail to remove enough acid. Common causes include:

  • Chronic kidney failure

  • Liver failure

  • Severe or prolonged diarrhea

  • Lactic acidosis (from intense exercise, shock, or certain diseases)

  • Diabetic ketoacidosis

  • Seizures

  • Cancer

  • Prolonged lack of oxygen (hypoxia)

2. Respiratory Alkalosis

When bicarbonate is low but blood pH is elevated (pH > 7.4), it may indicate respiratory alkalosis, which is often due to excessive loss of CO2 through rapid breathing. Common triggers include:

  • Hyperventilation

  • Fever

  • Severe pain

  • Anxiety or panic attacks


Medications That Can Lower Bicarbonate Levels

Certain medications are known to reduce blood bicarbonate levels, either directly or as a side effect of how they affect acid-base regulation:

  • Methicillin (antibiotic)

  • Nitrofurantoin (used for urinary tract infections)

  • Tetracycline (broad-spectrum antibiotic)

  • Thiazide diuretics (used to manage blood pressure or fluid retention)

  • Triamterene (a potassium-sparing diuretic)


Final Note

If your CO2 or bicarbonate levels are low, your doctor will evaluate them alongside other tests such as electrolyte panels, blood pH, and clinical symptoms to determine the root cause and appropriate treatment.

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