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Low CO2 Symptoms: Causes, Signs & What to Do

Low CO2 on a metabolic panel most often reflects metabolic acidosis, but it can also come from over-breathing or from how the blood sample was handled. This page covers what a slightly low vs very low result means, the symptoms and causes, and what to check next.

Published

April 23, 2026

Last updated

November 3, 2026

Read time

8 min read

Medically reviewed by

Rhonda Collins, FNP-C

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Short answer: CO2 on a metabolic panel measures bicarbonate (HCO3), the main acid buffer in your blood. A low result most often reflects metabolic acidosis, a buildup of acid or a loss of bicarbonate. It can also be the body’s response to over-breathing (respiratory alkalosis), or a falsely low reading from an underfilled tube or a sample left open to air ^[1][3][4]. A slightly low result with no symptoms is usually rechecked rather than treated as an emergency. A very low result, or any low result with deep, rapid breathing or confusion, needs prompt care. See the Bicarbonate biomarker overview for how serum CO2 is measured.

What Low CO2 Means

Most of the carbon dioxide in your blood is carried as bicarbonate, which helps keep your blood’s pH in balance ^[1]. Bicarbonate falls for three broad reasons:

• Metabolic acidosis. The body is making extra acid (such as ketones or lactate), cannot clear acid through the kidneys, or is losing bicarbonate through the gut or kidneys ^[5][7]. The body compensates by breathing faster and deeper to blow off CO2 gas. In severe cases this is called Kussmaul breathing ^[6].
• Respiratory alkalosis. Breathing too fast or too deeply, called hyperventilation, makes the blood less acidic, and bicarbonate falls in response ^[1].
• A sample artifact. Bicarbonate reads lower when a blood tube is only partly filled or the serum is left open to air before it is measured, which can mimic metabolic acidosis ^[3][4].

When acidosis is the cause, the anion gap narrows it down: high anion gap metabolic acidosis (from unmeasured acids like ketones, lactate, or toxins) versus normal anion gap metabolic acidosis (from direct bicarbonate loss or failure to excrete acid) ^[5]. An out-of-range result does not always mean you have a condition that needs treatment, and follow-up tests are often used to confirm it ^[1].

Slightly Low vs Very Low CO2

How worried to be depends on how low the number is, whether it is repeated, and how you feel. Reference ranges differ between labs (Cleveland Clinic lists 20 to 29 mmol/L), so read your result against the range printed on your report ^[2]. For bicarbonate, mmol/L and mEq/L are the same number.

• Just below your lab’s range (about 18 to 22 mEq/L), no symptoms. Usually not urgent. A repeat metabolic panel is a sensible first step, because a single mildly low value can come from how the sample was collected or handled ^[1][3][4].
• Below 18 mEq/L. This is clearly low. Bicarbonate below 18 mmol/L is one of the criteria for diabetic ketoacidosis (DKA) in people with diabetes ^[6]. Get it evaluated promptly rather than waiting for a routine recheck.
• Below 10 mEq/L. This is very low. In DKA, bicarbonate below 10 mmol/L marks the most severe category, managed in intensive care ^[6]. This level needs emergency care.

Symptoms of Low CO2

Symptoms track how low bicarbonate is and what is causing it. A mildly low result often causes no symptoms at all.

Slightly low (18-22 mEq/L):

• Often asymptomatic or minimal symptoms
• Mild fatigue and weakness

Moderately low (10-18 mEq/L):

• Rapid or deep breathing (the body trying to blow off CO2 to compensate)
• Nausea, vomiting, and loss of appetite
• Fatigue and weakness
• Headache
• Confusion or difficulty concentrating

Very low (below 10 mEq/L):

• Kussmaul breathing: deep, labored, rapid breathing with a characteristic sighing quality
• Fruity breath odor (ketones, in DKA) ^[6]
• Severe confusion and altered consciousness
• Cardiac arrhythmias
• Shock and cardiovascular collapse (in lactic acidosis)

What Causes Low CO2

Severe diarrhea and kidney failure are the most common causes of metabolic acidosis ^[8]. Causes split by the anion gap ^[5].

High anion gap metabolic acidosis (acid production exceeds excretion):

• Diabetic ketoacidosis (DKA): a complication of type 1 and type 2 diabetes; ketones are unmeasured anions that widen the gap ^[1][6]
• Lactic acidosis: from sepsis, shock, tissue hypoxia, metformin toxicity in kidney failure, or ischemia
• Starvation ketosis (milder than DKA, bicarb usually above 18 mEq/L)
• Salicylate toxicity (aspirin poisoning, which causes a complex mixed picture)
• Toxic alcohol ingestion (methanol or ethylene glycol)
• Uremia from kidney failure (impaired acid excretion and organic acid accumulation)

Normal anion gap metabolic acidosis (bicarbonate loss or failure to excrete H+):

• Diarrhea: direct loss of bicarbonate from the gut (intestinal secretions are alkaline) ^[8]
• Renal tubular acidosis (RTA): kidneys fail to excrete H+ or reabsorb bicarbonate ^[8]
• Large volume saline infusion (dilutional acidosis)
• Adrenal insufficiency (low aldosterone reduces H+ excretion; Addison disease is a recognized cause of low CO2) ^[1]

Low CO2 without acidosis:

• Respiratory alkalosis from hyperventilation (rapid, deep breathing) ^[1]
• A falsely low reading from an underfilled tube or a sample exposed to air ^[3][4]

Normal CO2 (Bicarbonate) Levels

Labs set their own ranges, and some list a lower limit of 20 mmol/L ^[2]. Use the range on your own report.

What to Check Next

If your CO2 came back low, these tests sort out the likely cause:

• A repeat metabolic panel. Confirms whether the low value is real and whether it is changing. An underfilled tube or a sample left open to air can lower bicarbonate on its own ^[3][4].
• Anion gap. Calculated from sodium, chloride, and bicarbonate on the same panel. A normal gap is about 8 to 12 mEq/L. A high gap points toward ketones, lactate, kidney failure, or toxins; a normal gap points toward bicarbonate loss, such as from diarrhea or renal tubular acidosis ^[5].
• Albumin. Albumin is the main unmeasured anion in the gap, so low albumin lowers the anion gap and can hide a high-gap acidosis ^[5].
• Glucose and ketones. High glucose with ketones suggests DKA. DKA can also occur with glucose below 200 mg/dL, most often in people taking SGLT2 inhibitor diabetes medicines ^[6]. Ketones need a separate blood or urine test.
• Kidney function: BUN and creatinine. Kidney failure is one of the most common causes of metabolic acidosis ^[8].

Mito’s Comprehensive Metabolic Panel (CMP) reports bicarbonate, sodium, and chloride for the anion gap, plus glucose, BUN, creatinine, and albumin. The Basic Metabolic Panel (BMP) is the narrower option: it covers the same electrolytes, glucose, and kidney markers, but not albumin. Neither panel measures ketones.

When to See Your Care Team

Talk with a clinician if your CO2 stays below your lab’s range on a repeat test, or if it is below 18 mEq/L. Go to emergency care if a low result comes with deep, rapid breathing, confusion, vomiting, or severe weakness, or if you have diabetes and feel unwell, since DKA can develop over hours to days ^[6]. Every Mito result is reviewed by a clinician before your analysis is finalized, and a 1:1 clinician consult is available as an add-on if you want to talk the result through.

Frequently Asked Questions

What is the anion gap and why does it matter when CO2 is low?

The anion gap = sodium - (chloride + bicarbonate). Normal is about 8 to 12 mEq/L ^[5]. When extra acid (unmeasured anions like ketones or lactate) accumulates, the gap widens because these acids consume bicarbonate while the sodium stays the same. High anion gap acidosis suggests DKA, lactic acidosis, uremia, or toxins. Normal anion gap acidosis suggests diarrhea, renal tubular acidosis, or saline infusion. The distinction quickly narrows the cause.

What is Kussmaul breathing?

Kussmaul breathing is a deep, labored, and rapid breathing pattern the body uses to compensate for severe metabolic acidosis. By breathing faster and deeper, the body exhales more CO2 gas, which reduces carbonic acid in the blood and partially corrects the pH. It is a visible sign of severe metabolic acidosis and is classically associated with DKA ^[6].

Can diarrhea cause low CO2?

Yes. Unlike stomach acid (which is acidic), intestinal secretions are rich in bicarbonate. Severe or prolonged diarrhea directly removes bicarbonate from the body, producing normal anion gap metabolic acidosis ^[8]. This is especially significant in children and people with cholera or secretory diarrhea, where bicarbonate losses can be massive.

Does metformin cause low CO2?

Not under normal conditions. However, metformin inhibits hepatic lactate clearance, and in people with kidney failure (where metformin accumulates to toxic levels) it can cause severe lactic acidosis. This is the reason metformin is contraindicated with an eGFR below 30 mL/min/1.73m2 and used cautiously below 45 mL/min/1.73m2.

References

1. MedlinePlus: Carbon dioxide (CO2) in blood
2. Cleveland Clinic: CO2 blood test
3. Herr RD, Swanson T. Pseudometabolic acidosis caused by underfill of Vacutainer tubes. Ann Emerg Med. 1992
4. Šupak-Smolčić V, et al. Serum bicarbonate stability study at room temperature: influence of time to centrifugation and air exposure. Clin Chem Lab Med. 2023
5. StatPearls: Anion gap and non-anion gap metabolic acidosis
6. Umpierrez GE, et al. Hyperglycemic crises in adults with diabetes: a consensus report. Diabetes Care. 2024
7. StatPearls: Metabolic acidosis
8. Cleveland Clinic: Metabolic acidosis

Related Resources

• Bicarbonate: Biomarker Overview
• High CO2 Symptoms: Causes, Signs & What to Do
• How to Improve Your Bicarbonate Naturally