The good news is that one other number on the same page usually narrows it down considerably. Most articles on this topic never mention it. This one starts there.
- Low CO2 on a blood test usually means metabolic acidosis — but the anion gap, calculated from the same panel, is what actually narrows down the cause.
- A wide anion gap points to ketones, lactate, or a toxin accumulating and tends to be urgent; a normal gap points to bicarbonate loss and is usually more chronic.
- The same bicarbonate number can mean very different things — urgency depends on the trend, the cause, and how you feel, not the number alone.
- Never self-dose baking soda to raise bicarbonate — it has caused serious electrolyte disturbances and doesn’t treat the underlying cause.
First: When a Low CO2 Needs Attention Today
Go to an emergency department now if a low CO2 comes with any of these: you have diabetes and are vomiting, very thirsty, passing a lot of urine, or breathing deeply and rapidly (this pattern suggests diabetic ketoacidosis); fever, shivering, a racing heart, or feeling profoundly unwell (lactic acidosis from sepsis presents this way); confusion, drowsiness, or difficulty staying awake; deep, labored, sighing breathing you can’t control; chest pain, or a heart rate over 120 at rest; vomiting or diarrhea severe enough that you can’t keep fluids down; or you’ve taken a large amount of aspirin or any overdose.
Contact your doctor the same day if you have known kidney disease, take metformin or an SGLT2 inhibitor, or feel markedly worse than usual alongside the result.
If you feel well and found this on a routine panel, it still needs following up — but it’s a phone call, not an emergency.
What the Test Actually Measures
The “CO2” on a metabolic panel isn’t the gas you breathe out. It’s total carbon dioxide, and roughly 95% of it is bicarbonate — the buffer your body uses to keep blood acidity in a very narrow range. It’s the same compound as sodium bicarbonate, which your pancreas produces and which also happens to be sold as baking soda.
MedlinePlus gives the normal range as 23 to 29 mEq/L, and notes that ranges vary slightly between laboratories — so compare your result against the range printed on your own report, not one you found online.
Two organs control this. Your lungs blow off carbon dioxide gas; your kidneys reclaim or discard bicarbonate. When bicarbonate falls, it’s because acid is being produced faster than you can buffer it, or because bicarbonate is being lost — usually through the gut or the kidneys.
The Anion Gap: The Number That Actually Narrows It Down
This is the part worth understanding, because it’s how clinicians split a long list of causes into two much shorter ones.
The anion gap is calculated from numbers already on your panel: sodium minus the sum of chloride and bicarbonate. Normal is roughly 8 to 12 mEq/L. It estimates the acids present in your blood that the panel doesn’t measure directly.
A wide anion gap means something unmeasured is accumulating — ketones, lactate, or a toxin. These are the causes that tend to be acute and serious.
A normal anion gap means you’re simply losing bicarbonate, or your kidneys aren’t excreting acid properly. These causes are usually more chronic and less immediately dangerous.
Your lab may calculate this for you. If it doesn’t, it’s a reasonable thing to ask about — “was my anion gap normal?” is a more useful question than “is 20 bad?”
Common Causes of Low Carbon Dioxide on Blood Test Results
Wide anion gap — acid is accumulating
- Diabetic ketoacidosis. Without enough insulin, the body burns fat and produces ketones, which are acids. This is a medical emergency and the commonest serious cause in anyone with diabetes.
- Lactic acidosis. Lactate builds up when tissues aren’t getting enough oxygen — sepsis, shock, severe illness. Rarely, metformin contributes, mainly in people whose kidney function has declined.
- Poisoning and overdose, including large amounts of aspirin.
- Advanced kidney failure, where acids that should be excreted accumulate instead.
Normal anion gap — bicarbonate is being lost
- Diarrhea. Intestinal fluid is alkaline, so prolonged diarrhea flushes bicarbonate out. This is a common and entirely benign explanation for a mildly low result.
- Renal tubular acidosis. The kidney tubules fail to reabsorb bicarbonate or excrete acid. Several types exist, some inherited, some drug-induced.
- Chronic kidney disease. As filtration declines, acid handling declines with it. Metabolic acidosis becomes more common at later stages.
Breathing, not metabolism
Sustained overbreathing — from anxiety, pain, fever, or a lung condition — blows off carbon dioxide gas. The kidneys respond over days by letting bicarbonate go, so the panel shows a low CO2 even though nothing is wrong with your metabolism. COPD usually raises CO2 rather than lowering it, but some patients hyperventilate to maintain oxygen and show the opposite.
Medications worth mentioning to your doctor
Diuretics, particularly carbonic anhydrase inhibitors like acetazolamide. Metformin and SGLT2 inhibitors, both of which raise the risk of an acidosis in specific circumstances. High-dose aspirin. Some antibiotics that affect kidney tubules. Take your full list — prescriptions, over-the-counter drugs and supplements — to the appointment.
What the Number Alone Can’t Tell You
You’ll find charts online matching each CO2 value to a severity level and a recommended action. Treat those with suspicion. Urgency depends on the cause and how fast it developed, not on the number by itself.
A bicarbonate of 18 in someone with long-standing renal tubular acidosis may be stable and unchanged for years. The same 18 in someone with diabetes who started vomiting this morning is an emergency. Identical numbers, completely different situations.
What actually matters is the trend across previous panels, the anion gap, your glucose and kidney numbers, what medications you take, and how you feel. That’s why the result needs a clinician rather than a chart.
One practical caveat: a slightly low value on a single panel is sometimes nothing at all. Blood sitting too long before processing, a tight tourniquet, or a recent hard workout can nudge the number down. A repeat test is often the first thing your doctor orders.
What Your Doctor Will Likely Do
Expect a repeat metabolic panel to confirm the finding, and a look at the anion gap alongside your kidney function, glucose and electrolytes.
Depending on those, they may check ketones, lactate, or urine acidity, or order an arterial blood gas — which measures pH and dissolved CO2 directly and distinguishes a metabolic problem from a breathing one.
Treatment targets the cause, not the number. Ketoacidosis needs insulin and fluids in hospital. Kidney disease needs nephrology input. Hyperventilation from anxiety responds to breathing retraining and treating the anxiety.
Bicarbonate supplements are sometimes prescribed, particularly in chronic kidney disease. That’s a prescribing decision based on your kidney function and electrolytes — not something to attempt with baking soda from the cupboard, which has caused serious harm in people who tried it. Our guide to what sodium bicarbonate is and when it turns dangerous sets out why.
What You Can Usefully Do
There’s no food that raises blood bicarbonate, and the “alkaline diet” idea — that what you eat meaningfully shifts blood pH — doesn’t hold up. Your kidneys and lungs defend blood pH far too tightly for diet to move it.
What diet does influence is the acid load your kidneys have to clear, which matters if kidney function is already reduced. In that situation more fruit and vegetables and less animal protein can genuinely help, and it’s worth discussing with your doctor or a renal dietitian rather than self-prescribing. If you eat a lot of protein, our guide to structuring a higher-protein diet covers doing it in a balanced way.
Beyond that: stay reasonably hydrated, especially during illness with vomiting or diarrhea — water-rich foods like cucumbers and a sensible spread of potassium-rich foods contribute alongside fluids. Keep blood pressure controlled, since hypertension is a leading driver of kidney damage; our guide to what a normal blood pressure reading looks like is a starting point. And don’t stop any prescribed medication on your own because you suspect it’s responsible.
Frequently Asked Questions
What does low carbon dioxide on blood test results mean?
It can also appear when sustained overbreathing has led your kidneys to shed bicarbonate in response. The result alone doesn’t identify which, which is why it needs interpreting alongside your other numbers.
Is a CO2 of 20 dangerous?
Ask about your anion gap and whether previous panels showed the same. A stable, mildly low value with a normal gap is a follow-up conversation. A falling value, a wide gap, or feeling unwell changes the picture entirely.
What is the anion gap and why does it matter here?
A wide gap points toward ketones, lactate or a toxin — the causes that tend to be urgent. A normal gap points toward bicarbonate loss from the gut or kidneys, which is usually more chronic.
Can anxiety cause a low CO2 result?
This is worth confirming rather than assuming, since fever, pain and lung conditions cause the same breathing pattern. Breathing retraining and treating the anxiety usually resolves it.
Should I take baking soda to raise my bicarbonate?
Self-dosing with household baking soda has caused serious electrolyte disturbances and fluid overload, and in rare cases far worse — our guide to sodium bicarbonate safety covers the reported harms. It also does nothing about whatever is causing the acidosis.
Which foods raise CO2 levels?
Diet does affect how much acid your kidneys must clear, which matters when kidney function is reduced. In that specific situation more plant foods and less animal protein can help — a conversation for your doctor or a renal dietitian.
Can a low result be a lab error?
This is why a repeat test is often the first step for a mildly low value in someone who feels well.
SOURCESOur Editorial Standards
HealthMagHub is built on primary sources: peer-reviewed research, clinical practice guidelines, and health agencies such as the NIH, CDC, and FDA. Read our editorial standards to see how we research, cite, and update our articles.
- MedlinePlus Medical Encyclopedia. CO2 blood test. medlineplus.gov
- MedlinePlus Medical Tests. Anion Gap Blood Test. medlineplus.gov
- Burger M, Schaller DJ. Metabolic Acidosis. StatPearls, NCBI Bookshelf. ncbi.nlm.nih.gov
- Cleveland Clinic. Chronic Obstructive Pulmonary Disease (COPD). my.clevelandclinic.org
Disclaimer: This article is general information, not medical advice, diagnosis or treatment. It cannot interpret your specific result, and laboratory reference ranges vary between facilities — use the range printed on your own report. A low CO2 always needs interpreting by a clinician alongside your other results, medications and symptoms. Seek emergency care for confusion, deep labored breathing, chest pain, or vomiting you can't keep fluids down — and urgently if you have diabetes and feel unwell, since diabetic ketoacidosis is time-critical.
