Lab result explainedMetabolic panelLast updated November 1, 2026

CO2 (bicarbonate) blood test: what high and low levels mean

Carbon dioxide (bicarbonate) (CO2) · Comprehensive metabolic panel (CMP) / Basic metabolic panel (BMP) / Electrolyte panel · reported in mEq/L (same as mmol/L)

CO2 on a blood test is not the gas you breathe out; it is 'total CO2', which is almost entirely bicarbonate, the main buffer that keeps your blood from becoming too acidic. The normal range on most US reports is about 22 to 29 mEq/L (some labs use 23 to 29 or 20 to 32). The two most common reasons it reads high are vomiting and diuretic medicines; the two most common reasons it reads low are diarrhea and reduced kidney function, with a sample that sat open to the air a close third.

Bicarbonate is made and reclaimed by the kidneys and is consumed whenever acid enters the blood, whether from metabolism (ketones, lactate), from the kidneys failing to excrete acid, or from losing bicarbonate through the gut. The lungs adjust it indirectly: when they retain carbon dioxide, the kidneys raise bicarbonate to compensate, and when you over-breathe, the kidneys lower it.

A CO2 of 20 or 31 in someone who feels well is usually noise, a sample artefact, or a diet or medicine effect, and is simply repeated. A CO2 below 18 or above 35 usually reflects a real acid-base disturbance that needs a cause found. This page explains the mechanisms, the common culprits, and what genuinely moves the number.

CO2 at a glance

What it is: total carbon dioxide in blood, about 95 percent of which is bicarbonate, the body's main acid buffer.
Normal: about 22 to 29 mEq/L (mmol/L); labs vary from 20–32.
High CO2 (metabolic alkalosis or compensated CO2 retention): vomiting, diuretics, excess antacids or baking soda, aldosterone or cortisol excess, COPD, sleep apnea.
Low CO2 (metabolic acidosis or compensated hyperventilation): diarrhea, kidney disease, diabetic ketoacidosis, keto diet, lactic acidosis, toxins, acetazolamide, anxiety or altitude breathing.
Lab artefact: an underfilled or uncapped tube loses CO2 and reads falsely low by 2 to 4 points.
Next step: repeat with a full panel and anion gap; a value under 18 or over 35, or any symptoms, needs prompt review.

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What CO2 measures

Carbon dioxide travels in blood in three forms: dissolved gas, carbonic acid and bicarbonate ion. Bicarbonate makes up roughly 95 percent of the total, so 'CO2' on a metabolic panel is effectively a bicarbonate measurement. The lab acidifies the sample to release all the CO2, measures it, and reports the total in mEq/L (mmol/L).

This is different from the pCO2 on an arterial blood gas, which measures the partial pressure of the dissolved gas and reflects breathing. The blood-gas machine also calculates a bicarbonate, which usually runs 1 to 3 points lower than the total CO2 on a chemistry panel. Total CO2 is the value used for everyday screening; a blood gas is added when the panel shows a significant abnormality.

Because CO2 escapes from blood exposed to air, tube handling matters. A tube that is underfilled, left uncapped, or processed late can read 2 to 4 mEq/L low. A mildly low CO2 with a normal anion gap and no symptoms is often just this.

Normal range for CO2

Reference ranges vary between labs and by age, sex and pregnancy. Use the range printed on your own report.

Group	Typical range (mEq/L (same as mmol/L))	Notes
Adults	22–29 mEq/L	Some labs use 23–29, 21–31 or 20–32.
Children	20–28 mEq/L
Infants	17–25 mEq/L	Lower normal values in the first year.
Pregnancy	18–22 mEq/L	Faster breathing lowers pCO2, so bicarbonate runs lower; this is normal.
High altitude residents	Often 18–22 mEq/L	Compensation for the lower pCO2 of altitude breathing.
Arterial blood gas bicarbonate	22–26 mEq/L	Calculated; runs 1–3 points below total CO2.

High CO2 (high bicarbonate)

A high CO2, above about 29 to 32 mEq/L, means the blood is holding more bicarbonate than usual. There are two broad reasons. Either the body has lost acid or gained alkali (a metabolic alkalosis), or the lungs are retaining carbon dioxide and the kidneys have raised bicarbonate to compensate (a compensated respiratory acidosis).

Metabolic alkalosis is by far the more common pattern in outpatients. Vomiting removes stomach acid, loop and thiazide diuretics make the kidney shed chloride and hydrogen, and baking soda or heavy antacid use adds alkali directly. Low potassium and low chloride usually accompany it. Aldosterone or cortisol excess is a less common cause that also raises blood pressure.

Compensated CO2 retention is the pattern to think about in someone with COPD, severe sleep apnea or obesity hypoventilation: the CO2 on the panel is high, the chloride is low, and daytime sleepiness or morning headaches may be present. A high CO2 in that setting is a prompt to measure a blood gas or arrange a sleep study.

Common causes

Loop and thiazide diuretics
Furosemide, bumetanide, hydrochlorothiazide and chlorthalidone cause the kidney to lose chloride, potassium and hydrogen while reclaiming bicarbonate. A CO2 of 30 to 34 is common on these drugs and is called a contraction alkalosis.
Vomiting or nasogastric suction
Each litre of gastric fluid lost removes about 100 mEq of hydrochloric acid. Bicarbonate generated in the stomach lining is left behind, and the kidney cannot excrete it while volume and chloride are low.
Excess antacids, baking soda or calcium carbonate
Direct alkali intake raises bicarbonate, particularly when kidney function is reduced. The milk-alkali pattern of high calcium, high bicarbonate and rising creatinine still occurs with heavy calcium carbonate use.
Chronic CO2 retention from lung disease or sleep apnea
In COPD, obesity hypoventilation and severe sleep apnea, carbon dioxide accumulates. The kidneys compensate over days by retaining bicarbonate, so the total CO2 on a chemistry panel climbs.
Low potassium
Potassium depletion drives hydrogen into cells and makes the kidney excrete acid and retain bicarbonate. It both causes and perpetuates alkalosis, and the two often need correcting together.
Hyperaldosteronism and Cushing's syndrome
Excess aldosterone or cortisol makes the kidney excrete potassium and acid and retain bicarbonate and sodium. The clue is high blood pressure with low potassium.
Severe dehydration
Loss of water concentrates bicarbonate and, by stimulating aldosterone, makes the kidney reabsorb more. Correcting fluids with saline usually fixes it.
Recovery from acidosis and citrate in transfusions
After treatment of ketoacidosis or after large blood transfusions (citrate is metabolised to bicarbonate), CO2 can overshoot high for a day or two.

Symptoms that can go with it

Often none when mild
Muscle cramps, twitching or tingling around the mouth and fingertips
Weakness, fatigue or confusion
Slow or shallow breathing
Irregular heartbeat, particularly when potassium is also low
Daytime sleepiness, morning headache or loud snoring if CO2 retention is the cause

Low CO2 (low bicarbonate)

A low CO2, below about 20 to 22 mEq/L, usually means bicarbonate has been used up buffering acid (a metabolic acidosis) or lost through the gut or kidney. Less often it means you are breathing faster than normal and the kidneys have lowered bicarbonate to compensate (a respiratory alkalosis), as in anxiety, pregnancy or high altitude.

The anion gap separates the metabolic causes. A high gap (above about 12) means an unmeasured acid is present: ketones in diabetic ketoacidosis, starvation or a strict ketogenic diet; lactate from shock, severe illness or metformin in kidney failure; retained acids in advanced kidney disease; or toxins such as aspirin, methanol or ethylene glycol. A normal gap means bicarbonate itself was lost: diarrhea, renal tubular acidosis, early CKD, or a drug like acetazolamide.

A CO2 of 19 to 21 in someone who feels well is very often an underfilled tube, a keto diet, altitude or mild kidney disease and is rechecked. A CO2 under 18 with symptoms, a high anion gap, a high glucose or a high creatinine needs same-day evaluation.

Common causes

Diarrhea
Intestinal and pancreatic secretions are rich in bicarbonate. Prolonged or severe diarrhea removes it faster than the kidney can replace it, producing a normal anion gap acidosis with a rising chloride.
Chronic kidney disease
As eGFR falls below about 40, the kidney excretes less acid and makes less new bicarbonate. A CO2 under 22 is common in stage 3b to 5 CKD and accelerates bone loss, muscle wasting and kidney decline if untreated.
Diabetic ketoacidosis
Without enough insulin, fat breaks down to ketone acids that consume bicarbonate. CO2 can fall below 10 with a high glucose, a high anion gap and fruity breath; it is a medical emergency.
Ketogenic diet, fasting or heavy alcohol use
Nutritional ketosis lowers CO2 by 2 to 4 points in many people and more in alcoholic ketoacidosis. The effect is reversible but worth mentioning to your clinician before a test.
Lactic acidosis
Shock, sepsis, severe heart failure, seizures, extreme exercise or metformin accumulation in kidney failure raise lactate, which consumes bicarbonate. Intense exercise just before a draw can lower CO2 for an hour or two.
Renal tubular acidosis
Inherited or acquired defects (Sjogren's disease, lithium, amphotericin, topiramate) stop the tubules from excreting acid or reclaiming bicarbonate. Kidney stones and low potassium are common companions.
Hyperventilation (compensated respiratory alkalosis)
Anxiety, pain, pregnancy, fever, sepsis, liver disease, lung clots and high altitude all increase breathing. Over hours to days the kidney lowers bicarbonate to compensate, so total CO2 reads low with a normal or high blood pH.
Medicines
Acetazolamide and topiramate block bicarbonate reabsorption; high-dose aspirin, metformin in kidney failure, and some HIV drugs cause acidosis; large volumes of normal saline cause a hyperchloremic acidosis.
Toxic alcohols and aspirin overdose
Methanol, ethylene glycol (antifreeze) and salicylate poisoning produce a severe high anion gap acidosis and need emergency treatment.
Lab artefact: underfilled or uncapped tube
CO2 escapes from blood exposed to air. Partially filled tubes, tubes left open, and samples processed hours later read 2 to 4 mEq/L low, which explains many isolated mild results.

Symptoms that can go with it

Often none when mild
Rapid, deep breathing (the body blowing off acid)
Fatigue, weakness or loss of appetite
Nausea, vomiting or abdominal pain
Headache, confusion or drowsiness when severe
Fruity breath, extreme thirst and frequent urination in ketoacidosis
Bone pain, kidney stones or muscle loss in long-standing acidosis from kidney disease

Read it alongside

Anion gap — sodium minus chloride plus bicarbonate; a high gap points to ketones, lactate, kidney failure or toxins, a normal gap to diarrhea, the kidney tubules or saline.
Chloride — rises as bicarbonate falls in normal gap acidosis and falls as bicarbonate rises in alkalosis; the two are read as a pair.
Potassium — low potassium accompanies and perpetuates alkalosis; high potassium with low CO2 suggests kidney disease or a type 4 renal tubular acidosis.
Creatinine and eGFR — a low CO2 with an eGFR below 45 is usually CKD acidosis and is a treatment target.
Glucose — a high glucose with a low CO2 raises concern for diabetic ketoacidosis; check ketones.
Arterial or venous blood gas — the pH and pCO2 confirm whether the primary problem is metabolic or respiratory when the panel is clearly abnormal.
Lactate and ketones — ordered when the anion gap is high to identify the acid.

What usually happens next

An isolated CO2 of 19 to 21 or 30 to 32 in someone who feels well is usually repeated with a properly filled tube, fasting, off a keto diet for a few days, and without heavy exercise beforehand.
Your clinician will calculate the anion gap from the same report and review diuretics, antacids, baking soda, acetazolamide, topiramate, metformin, aspirin and alcohol.
A low CO2 with a high anion gap leads to urgent glucose, ketones, lactate and kidney tests, and often a blood gas.
A low CO2 with a normal gap and a low eGFR is treated as CKD acidosis; guidelines suggest keeping bicarbonate at 22 or above with diet or sodium bicarbonate.
A high CO2 with low potassium and chloride leads to a review of vomiting and diuretics, a urine chloride, and occasionally aldosterone and cortisol tests if blood pressure is high.
A high CO2 in someone with COPD, obesity or loud snoring is a reason for a blood gas and a sleep study rather than any change in diet.

Diet, lifestyle and natural levers with evidence

What can move CO2 once the cause is known. Nothing here replaces finding the cause.

Fruit and vegetables for low bicarbonate in kidney disease

Moderate evidence

In randomised trials in CKD stage 3 and 4, adding 2 to 4 cups of fruit and vegetables daily raised bicarbonate as effectively as oral sodium bicarbonate over 1 to 3 years, with lower blood pressure, less weight gain and slower eGFR decline. Plant foods are metabolised to bicarbonate, while meat, cheese and grains yield acid.

With an eGFR below 30, potassium needs monitoring; coordinate with a renal dietitian.

Sodium bicarbonate only under supervision

Moderate evidence

In a randomised trial of 134 people with CKD stage 4 to 5 and low bicarbonate, oral sodium bicarbonate slowed the fall in creatinine clearance and reduced progression to dialysis over 2 years. It is a prescription-level intervention: the sodium load can worsen blood pressure and swelling, and it is harmful if the CO2 is already high. Never self-treat a lab value with baking soda.

Baking soda taken for heartburn is a common cause of a high CO2 and should be stopped if the level is raised.

Fluid and electrolyte replacement after vomiting or diarrhea

Strong evidence

Vomiting raises CO2 and diarrhea lowers it, and both respond to replacing what was lost. Oral rehydration solution containing sodium, chloride, potassium and glucose restores volume and lets the kidney correct the acid-base balance within a day or two; plain water alone does not supply the chloride needed to reverse an alkalosis.

Potassium-rich foods when on diuretics

Moderate evidence

Potassium depletion sustains metabolic alkalosis, and loop and thiazide diuretics waste potassium. Trials of potassium supplementation or potassium-rich diets in diuretic users lower blood pressure and correct the low-potassium, high-bicarbonate pattern. Bananas, potatoes, beans, leafy greens and yoghurt are practical sources.

Do not add potassium with an eGFR below 30, on ACE inhibitors or spironolactone, or if potassium is already above 5 without checking with your clinician.

Reconsider a strict ketogenic diet before testing

Preliminary evidence

Nutritional ketosis lowers serum bicarbonate by roughly 2 to 4 mEq/L in small studies and case series, and topiramate or acetazolamide taken alongside can push it lower. The effect reverses within days of adding carbohydrate. If your CO2 is borderline and you are on a keto diet, tell your clinician so the result is interpreted correctly rather than investigated.

Treat sleep apnea and quit smoking

Moderate evidence

A persistently high CO2 with a low chloride in someone who snores, is sleepy or has COPD points to carbon dioxide retention. CPAP lowers daytime CO2 in obesity hypoventilation syndrome in randomised trials, and smoking cessation slows COPD progression. These address the lung cause behind the kidney's compensation, which no diet change will.

When to see a clinician promptly

A CO2 below 18 or above 35 mEq/L, or any abnormal value with symptoms, should be reviewed the same day.
Rapid deep breathing, drowsiness, confusion, fruity breath or vomiting with a low CO2, especially with diabetes: this may be ketoacidosis and is an emergency.
A low CO2 after taking a large amount of aspirin, or after possible exposure to antifreeze or methanol.
Muscle twitching, tingling, an irregular heartbeat or severe weakness with a high CO2, particularly on diuretics or after prolonged vomiting.
A low CO2 with an eGFR below 45, which is treatable acidosis that protects bone, muscle and kidney if corrected.
A high CO2 with daytime sleepiness, morning headaches or known COPD, which can signal dangerous carbon dioxide retention.

Frequently asked questions

What is CO2 on a blood test?

CO2 on a metabolic panel is total carbon dioxide, which is about 95 percent bicarbonate. Bicarbonate is the main buffer that keeps blood pH stable, and the kidneys and lungs adjust it constantly. It is not a measure of how much carbon dioxide you are breathing; that is the pCO2 on a blood gas test.

What is a normal CO2 level in blood?

Most adult reference ranges are 22 to 29 mEq/L (mmol/L); some labs print 23 to 29 or 20 to 32. Children run slightly lower, and pregnant women and people living at altitude normally sit at 18 to 22 because they breathe faster. A blood gas bicarbonate typically reads 1 to 3 points lower than the panel value.

What causes high CO2 in blood?

The common causes are diuretics, vomiting, heavy use of antacids or baking soda, low potassium and dehydration, all of which raise bicarbonate. Chronic carbon dioxide retention from COPD, severe sleep apnea or obesity hypoventilation also raises it as the kidneys compensate. Aldosterone or cortisol excess is a rarer cause, usually with high blood pressure.

What causes low CO2 in blood?

Diarrhea, chronic kidney disease, diabetic ketoacidosis, a ketogenic diet, lactic acidosis, renal tubular acidosis and drugs such as acetazolamide, topiramate and high-dose aspirin all lower bicarbonate. Breathing fast from anxiety, pregnancy or altitude lowers it as a compensation. An underfilled or uncapped tube is a frequent cause of a mildly low result.

Is a CO2 of 20 bad?

A CO2 of 20 is only slightly below most ranges. If your anion gap, glucose and creatinine are normal and you feel well, it is usually a sample artefact, a keto diet, altitude or faster breathing, and is simply rechecked. It matters more if your eGFR is under 45, your anion gap is high, or it falls further on repeat.

Is a CO2 of 32 bad?

A CO2 of 32 is mildly high and most often reflects a diuretic, recent vomiting, antacid or baking soda use, or dehydration. If potassium and chloride are low it fits a mild metabolic alkalosis that usually corrects with fluids and potassium. If you have COPD, snore heavily or are very sleepy in the day, your clinician may check a blood gas for CO2 retention.

Can a keto diet or fasting lower bicarbonate?

Yes. Ketone bodies are acids, and nutritional ketosis typically lowers serum bicarbonate by 2 to 4 mEq/L. In most healthy people this is harmless and reversible, but combined with dehydration, heavy alcohol use or drugs like topiramate it can dip lower. Mention your diet when your blood is drawn so a borderline result is read in context.

Does low CO2 mean kidney disease?

Sometimes. As kidney function falls below an eGFR of about 40, the kidneys excrete less acid and bicarbonate drifts down. A low CO2 with a raised creatinine, a normal anion gap and a high chloride is typical of CKD acidosis, which is treatable. But diarrhea, ketosis, hyperventilation and tube handling cause far more low results than kidney disease does, so the whole panel is read together.

References

Carbon Dioxide (CO2) in Blood. MedlinePlus, US National Library of MedicineView source
Metabolic Acidosis. Merck Manual Professional Version
Metabolic Alkalosis. Merck Manual Professional Version
Metabolic Acidosis. National Kidney FoundationView source
KDIGO 2024 Clinical Practice Guideline for the Evaluation and Management of Chronic Kidney Disease. Kidney International (2024)View source
de Brito-Ashurst I, Varagunam M, Raftery MJ, Yaqoob MM. Bicarbonate Supplementation Slows Progression of CKD and Improves Nutritional Status. Journal of the American Society of Nephrology (2009)
Goraya N, Simoni J, Jo CH, Wesson DE. Treatment of metabolic acidosis in patients with stage 3 chronic kidney disease with fruits and vegetables or oral bicarbonate reduces urine angiotensinogen and preserves glomerular filtration rate. Kidney International (2014)
Goraya N, Simoni J, Jo CH, Wesson DE. A comparison of treating metabolic acidosis in CKD stage 4 hypertensive kidney disease with fruits and vegetables or sodium bicarbonate. Clinical Journal of the American Society of Nephrology (2013)
Diabetic ketoacidosis. Mayo ClinicView source
Semler MW, Self WH, Wanderer JP, et al.. Balanced Crystalloids versus Saline in Critically Ill Adults. New England Journal of Medicine (2018)View source
Acid-Base Disorders: Metabolic Acidosis and Alkalosis. StatPearls, National Center for Biotechnology Information (2023)

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This page is educational and is not medical advice or a diagnosis. Lab results have to be interpreted together with your history, symptoms and other results by a clinician who knows you. Do not start or stop a supplement to change a lab value before the cause is known.