Carbon Dioxide · low result
Low CO2 levels in a blood test: what it means, causes and what to check next
Short answer
A low CO2 on a metabolic panel — below about 22 to 23 mmol/L — means bicarbonate, the blood's main buffer, is being used up or lost: acid is accumulating (metabolic acidosis) or bicarbonate is leaving through the gut or kidneys. Uncontrolled diabetes, kidney disease, severe diarrhea, some medications and heavy alcohol are the medical causes; over-breathing from anxiety or pain lowers it as a normal adjustment. One common non-medical cause is the sample itself — a tube left open or under-filled loses CO2 to the air and reads low. The anion gap, glucose and kidney function decide how urgent a low value is.
Medically reviewed by Youmna DiStefano, MD · Last reviewed October 2026
When does this result need same-day care?
- A low CO2 with a high glucose, thirst, vomiting or abdominal pain in someone with diabetes — possible ketoacidosis, same-day emergency care
- Rapid deep breathing, drowsiness or confusion with a low CO2 — significant acidosis
- A CO2 under about 15 mmol/L, or any low value after a possible ingestion of antifreeze, methanol or a large aspirin dose
What is the normal range for Carbon Dioxide?
- age 18+: 22.0 – 29.0 mmol/L
Full reference table, citations and testing options: Carbon Dioxide biomarker page.
What are the common causes of low Carbon Dioxide?
- Diarrhea, laxative overuse or an ileostomy — bicarbonate is lost directly from the gut; chloride rises to fill the gap (a normal-anion-gap acidosis)
- Kidney disease — the kidneys cannot excrete the day's acid load; CO2 drifts down as the eGFR falls, and renal tubular acidosis does it with normal filtration
- Uncontrolled diabetes with ketoacidosis (DKA), starvation or alcoholic ketoacidosis — the anion gap is high and the glucose or ketones point the way
- Lactic acidosis — sepsis, shock, severe anemia, seizures or extreme exercise; rarely metformin in someone with poor kidney function
- Medications — acetazolamide and topiramate (carbonic anhydrase inhibitors), high-dose aspirin, and the early phase of large saline infusions
- Toxic ingestions — methanol, ethylene glycol (antifreeze), salicylate overdose: a high anion gap with a story to match
- Compensation for over-breathing (respiratory alkalosis) — anxiety or panic, pain, pregnancy, high altitude, liver disease, sepsis; the kidneys shed bicarbonate to balance a low carbon dioxide, and the low CO2 is expected
- A handling artefact — a blood tube left uncapped or under-filled before analysis loses CO2 to the air and can read 2–4 mmol/L low; the most common explanation for an isolated, unexpected low value in a well person
What symptoms go with low Carbon Dioxide?
- Usually none when mildly low — a value of 19–22 is commonly an incidental finding
- From acidosis when marked: deeper or faster breathing, fatigue, weakness, nausea and poor appetite, headache, confusion in severe cases
- From diabetic ketoacidosis: thirst, frequent urination, abdominal pain, fruity breath, vomiting — an emergency
- From the cause: diarrhea, the symptoms of kidney disease, the tingling and light-headedness of anxious over-breathing
What do clinicians usually check next?
- Calculate the anion gap from the same panel: sodium minus chloride minus CO2 (normal roughly 8–12). A high gap points to ketones, lactate, kidney failure or a toxin; a normal gap points to diarrhea, renal tubular acidosis or a medication — two different workups
- Glucose (and ketones if diabetic), creatinine and eGFR, potassium — these three decide urgency
- Repeat on a fresh, promptly processed sample if the value is mildly low, isolated and unexplained in someone well — a handling artefact is common
- A medication review: acetazolamide, topiramate, high-dose aspirin, metformin with reduced kidney function
- A venous or arterial blood gas when CO2 is under about 18, when breathing is abnormal, or when the primary problem could be respiratory rather than metabolic
- Lactate when there is fever, low blood pressure or severe illness; a urine pH and urine anion gap when renal tubular acidosis is suspected
- Kidney workup — urine albumin, trend of eGFR, blood pressure — when the low CO2 accompanies a reduced eGFR, since bicarbonate replacement is part of chronic kidney disease care
Follow-up tests available at TestWell: Glucose Test ($14.99).
How do you recheck Carbon Dioxide?
Prices are cash-pay; every order adds one $6 lab processing fee. Physician authorization is included; no insurance or referral needed.
| Test or panel | Type | Price | Fasting | Results |
|---|---|---|---|---|
| Basic Metabolic Panel | Test · 8 biomarkers | $17.99 | Yes | 1–3 business days |
Common questions
What does a low CO2 level in a blood test mean?
That bicarbonate is below the usual range — under about 22 to 23 mmol/L at most laboratories — because acid is building up in the blood or bicarbonate is being lost through the gut or kidneys. Common medical reasons are uncontrolled diabetes, kidney disease, severe diarrhea and certain medications; over-breathing from anxiety lowers it harmlessly; and a tube left open before testing lowers it artificially. Which of these applies is read from the glucose, kidney values and anion gap on the same panel.
Is a CO2 level of 19 or 20 dangerous?
On its own, usually not. Values of 19–22 are mildly low and are often a handling artefact, a result of over-breathing, or a mild chronic acidosis from kidney disease or a medication such as topiramate. The number becomes urgent in context: with a high glucose in someone with diabetes (possible ketoacidosis), with rapid breathing or confusion, with a sharply elevated anion gap, or when it is under about 15. A mildly low value in a well person is repeated on a fresh sample before anything else.
Can a low CO2 result be a lab error?
Yes, and it is the first thing to consider for an isolated low value in someone who feels fine. Bicarbonate escapes from blood as carbon dioxide gas when a tube is under-filled or left uncapped, or when the sample waits hours before analysis — enough to drop the reading by 2–4 mmol/L. Laboratories know this and many add a comment. If every other value on the panel is normal and there is no diarrhea, diabetes or kidney disease, a repeat on a promptly processed sample usually comes back normal.
What causes low bicarbonate in kidney disease?
Healthy kidneys excrete the acid the body produces each day and regenerate bicarbonate; as filtering capacity falls (usually once the eGFR is under about 30–40), acid accumulates and bicarbonate drifts down. A persistently low CO2 in chronic kidney disease matters because chronic acidosis accelerates bone and muscle loss and may speed the decline in kidney function; guidelines recommend keeping bicarbonate in the normal range, often with prescribed sodium bicarbonate. That is a decision for the treating clinician, not a supplement to start alone.
How do you raise a low CO2 level?
By treating the cause rather than the number. Diarrhea and dehydration are treated with fluids and the acidosis resolves; diabetic ketoacidosis is treated in hospital with insulin and fluids; a medication cause is stopped or changed with the prescriber; chronic kidney disease may be treated with prescribed bicarbonate or citrate. Baking soda taken on one's own is not a safe fix — it adds a large sodium load and can overshoot into alkalosis. If the low value was a handling artefact, a repeat shows there was nothing to raise.
What is the anion gap and why does it matter here?
It is sodium minus chloride minus CO2, all from the same panel, and it sorts a low CO2 into two groups. A high gap (above about 12–14) means an unmeasured acid is present — ketones in diabetes or starvation, lactate in severe illness, the acids of kidney failure, or a toxin such as methanol or antifreeze — and the workup is urgent. A normal gap means bicarbonate was lost rather than acid gained — diarrhea, renal tubular acidosis, acetazolamide — and chloride will be high to compensate. Our anion gap calculator does the arithmetic.
Educational content, not a diagnosis. Lab results are one input; a clinician interprets them with your history and exam.