A low carbon dioxide level on a standard blood panel almost always refers to low bicarbonate, a form of CO₂ that your kidneys help regulate and that acts as the body’s main acid buffer. The normal range on most lab reports falls between about 22 and 30 mmol/L. A result below that range signals that your blood is more acidic than it should be, or that your body is compensating for another imbalance. The reasons span from relatively harmless situations like hyperventilation or being at high altitude to serious conditions like uncontrolled diabetes or kidney disease, so the number alone does not tell you much without context.
What “CO₂” Actually Measures on a Blood Test
When your doctor orders a basic metabolic panel, the line labeled “CO₂” or “total CO₂” is not measuring the carbon dioxide gas you breathe out. It is measuring bicarbonate dissolved in your blood, plus a small fraction of dissolved CO₂ gas. Bicarbonate makes up the vast majority of that measurement. Your body produces CO₂ constantly as cells burn fuel, and roughly three-quarters of it travels through the bloodstream inside red blood cells, where enzymes convert it into bicarbonate for transport to the lungs.1Oxford Academic (BJA Education). Carbon dioxide transport When the blood reaches the lungs, the process reverses: bicarbonate turns back into CO₂ gas, which you exhale.
The ratio between bicarbonate and dissolved CO₂ determines your blood pH. Your lungs control the CO₂ side by breathing faster or slower, while your kidneys control the bicarbonate side by retaining or dumping it into the urine.2PubMed. Metabolic aspects of the regulation of systemic pH A low reading on your lab report means the bicarbonate side of that balance has dropped, and there are two broad categories of reasons why.
The Two Main Pathways to Low Bicarbonate
Low CO₂ on a lab panel comes down to either your body using up bicarbonate to neutralize excess acid (metabolic acidosis) or your kidneys deliberately lowering bicarbonate to compensate for too much breathing (respiratory alkalosis). These are very different problems, and your doctor uses additional tests and your clinical picture to tell them apart.
In metabolic acidosis, something is producing acid faster than your body can handle. The bicarbonate gets consumed doing its job as a buffer. Common culprits include diabetic ketoacidosis, severe infections, kidney failure, prolonged diarrhea, and certain medications. In respiratory alkalosis, the lungs are blowing off too much CO₂ gas, which makes the blood too alkaline. Your kidneys respond by excreting extra bicarbonate to pull pH back toward normal, which shows up as a lower number on your lab work.3PubMed Central. Breathing and balance: Clinical insights and management strategies of respiratory acid‐base disorders The distinction matters because the treatment is completely different.
Metabolic Acidosis and Its Common Triggers
Metabolic acidosis is the more medically concerning of the two pathways. When your cells produce acids faster than your kidneys and lungs can clear them, bicarbonate gets used up as a chemical buffer, and your lab value drops. Several conditions do this.
Diabetic ketoacidosis is one of the most dramatic. When the body cannot use glucose for energy (because of insufficient insulin), it burns fat instead, producing acidic byproducts called ketones. These ketones flood the blood and consume bicarbonate rapidly, sometimes driving it well below the normal range.4PubMed Central. Is There Any Lower Limit of Serum Bicarbonate in Diabetic Ketoacidosis? This is a medical emergency that typically shows up with very high blood sugar, fruity-smelling breath, nausea, and deep rapid breathing.
Lactic acidosis works similarly but through a different acid. When tissues are not getting enough oxygen, whether from severe infection, shock, heart failure, or intense exercise beyond your body’s capacity, cells switch to a backup energy pathway that generates lactic acid. This form of acidosis is the most frequent cause of metabolic acidosis in intensive care settings.5Kidney International. L-lactic acidosis: pathophysiology, classification, and causes; emphasis on biochemical and metabolic basis
Chronic kidney disease is a slower route to the same destination. Healthy kidneys regenerate bicarbonate and excrete acid into the urine all day long. As kidney function declines, both processes weaken, and bicarbonate gradually drifts downward over months or years.6PubMed. Adverse Effects of the Metabolic Acidosis of Chronic Kidney Disease A less common kidney-related cause is renal tubular acidosis, where specific tubule defects block either the reabsorption of filtered bicarbonate or the excretion of acid, even though overall kidney function might be relatively preserved.7PubMed Central. Renal Tubular Acidosis and Management Strategies: A Narrative Review
Severe diarrhea is another everyday cause that people overlook. The intestinal fluid your body secretes during digestion is rich in bicarbonate. A bout of prolonged diarrhea can flush large amounts of it out of your system, leaving your blood level noticeably low. This is one of the most common reasons for a mildly low CO₂ in otherwise healthy people who show up at an emergency department dehydrated.
Respiratory Alkalosis and Compensatory Low Bicarbonate
If you have been hyperventilating, whether from anxiety, pain, fever, or a lung condition, you blow off more CO₂ gas than normal. That makes your blood temporarily alkaline. Over hours to days, your kidneys compensate by dumping bicarbonate into the urine, which lowers your serum CO₂ reading. Research on sustained hyperventilation found that for each small drop in blood CO₂ gas, serum bicarbonate fell in a predictable and proportional way.8PubMed. Chronic respiratory alkalosis. The effect of sustained hyperventilation on renal regulation of acid-base equilibrium
This kind of low bicarbonate is not necessarily dangerous in itself. It is your body’s smart attempt to keep blood pH in the safe zone. But it does mean something is driving you to breathe faster or deeper than usual, and that underlying cause still needs attention. Anxiety-related hyperventilation is the most benign version. On the more serious end, liver failure, sepsis, and central nervous system problems can all drive chronic hyperventilation that leads to persistently low bicarbonate.
Medications That Can Lower Your CO₂
Certain prescription drugs lower serum bicarbonate as a side effect, and this catches some people off guard when they get routine bloodwork. Topiramate, a medication used for seizures and migraine prevention, is the best-documented offender. It inhibits an enzyme called carbonic anhydrase in the kidneys, which impairs both the reabsorption of bicarbonate and the excretion of acid. The result is a form of renal tubular acidosis caused by the drug itself.9PubMed Central. Effect of topiramate on acid-base balance: extent, mechanism and effects
In one study, average serum CO₂ levels fell from about 25 mmol/L before starting topiramate to roughly 21 mmol/L three months later, pushing many patients below the standard normal range.10PubMed. Predisposition to metabolic acidosis induced by topiramate Other carbonic anhydrase inhibitors, including acetazolamide (used for glaucoma and altitude sickness), work through the same mechanism. If you are taking one of these medications and your lab comes back with low CO₂, the drug is likely the explanation, but your doctor will want to check the severity and may adjust your dose.
How Low Bicarbonate Feels
Mild drops in bicarbonate often produce no symptoms at all and are caught only because a lab panel happened to be drawn. As the imbalance becomes more significant, your body’s main visible response is to breathe faster and deeper, trying to blow off more CO₂ gas and shift the acid-base balance back. In severe metabolic acidosis, such as during diabetic ketoacidosis, this can progress to deep, labored breathing known as Kussmaul respiration.11PubMed Central. Effects of diabetic ketoacidosis in the respiratory system
Other symptoms tend to reflect the underlying cause rather than the low bicarbonate per se. Someone with lactic acidosis from sepsis will look and feel like someone with a severe infection. Someone with ketoacidosis will have nausea, vomiting, and confusion. Fatigue and weakness are common across most causes, but they are vague enough that they rarely point to low bicarbonate on their own. This is part of why the finding usually comes from a blood test rather than from a patient reporting specific symptoms.
What Happens If It Stays Low for a Long Time
A persistently low bicarbonate level is not just a number to monitor. Even mild chronic acidosis can gradually harm several body systems. Bone health takes a hit because even a slight acid shift promotes mineral loss from bone and favors the cells that break bone down over the ones that build it up, raising the risk of osteoporosis and kidney stones.12PubMed Central. Low-grade metabolic acidosis as a driver of chronic disease: a 21st century public health crisis
In people with chronic kidney disease, the consequences are especially well documented. Uncorrected metabolic acidosis has been linked to muscle wasting, protein malnutrition, worsening of bone disease, increased inflammation, and faster progression of the kidney disease itself. Some of these effects have been observed even when bicarbonate is only slightly below normal. Encouraging evidence suggests that correcting the acidosis with base supplements can slow muscle loss, improve bone health, and potentially slow the decline in kidney function.6PubMed. Adverse Effects of the Metabolic Acidosis of Chronic Kidney Disease
Treatment Depends Entirely on the Cause
There is no single fix for low CO₂ because the treatment is the treatment of whatever is driving it down. In diabetic ketoacidosis, the answer is insulin and fluids. In lactic acidosis from sepsis, the answer is treating the infection and supporting blood flow to tissues. In medication-induced acidosis, adjusting or discontinuing the drug may be all that is needed.
Sodium bicarbonate supplements, either by mouth or intravenously, are the most direct way to push the number back up, and they work well in certain situations. Replacing bicarbonate lost through chronic diarrhea or through proximal renal tubular acidosis is clearly helpful. But giving bicarbonate for acute conditions like diabetic ketoacidosis or cardiac arrest has not been convincingly shown to improve survival or clinical outcomes.13PubMed Central. Sodium bicarbonate therapy in patients with metabolic acidosis In chronic kidney disease, oral bicarbonate supplements are more promising and are sometimes prescribed long-term, though the research is still building.
Diet plays a role too, particularly for people with kidney disease or kidney transplants. Diets heavy in animal protein (meat, cheese, fish) and light on fruits and vegetables tend to generate more acid in the body. One study in kidney transplant recipients found that people with high animal-protein intake and low fruit and vegetable consumption had measurably lower serum bicarbonate and blood pH.14PubMed Central. Dietary acid load and metabolic acidosis in renal transplant recipients Increasing fruit and vegetable intake can shift the body’s acid load in a more alkaline direction, though this is a complement to medical treatment rather than a replacement for it.
The Anion Gap and How Doctors Narrow Down the Cause
When your CO₂ comes back low, your doctor does not just look at that single number. The rest of the metabolic panel, particularly sodium, chloride, and potassium, provides context. One of the first things clinicians calculate is the anion gap, a simple arithmetic check using sodium, chloride, and bicarbonate levels. An elevated anion gap suggests that the low bicarbonate is being driven by excess unmeasured acids in the blood, pointing toward diagnoses like diabetic ketoacidosis, lactic acidosis, or certain toxic ingestions.15Clinical Chemistry. Value of the anion gap in clinical diagnosis and laboratory evaluation
If the anion gap is normal but bicarbonate is still low, the explanation is usually a direct loss of bicarbonate, whether through the gut (diarrhea) or through the kidneys (renal tubular acidosis). An arterial blood gas, which measures blood pH and dissolved CO₂ gas directly, rounds out the picture and helps determine whether the primary problem is metabolic or respiratory. You rarely need to know these details as a patient, but understanding that one low number triggers a logical workup can help you appreciate why your doctor might want additional tests rather than immediately prescribing a treatment.
Could It Be a Lab Error?
Yes, sometimes. Bicarbonate is one of the more finicky lab values because the dissolved CO₂ in the sample can escape into the air. If a blood tube sits open or is improperly sealed, CO₂ gas evaporates, dragging the measured bicarbonate down and producing an artificially low result. Research has shown that bicarbonate is prone to evaporation after just two hours of air exposure, and labs are advised to test it promptly after opening sample containers.16Clinical Chemistry. B-062 Impact of prolonged air exposure on stability of Carbon Dioxide (CO2), Chloride (Cl), Potassium (K) and Sodium (Na) assessed by Roche Cobas® c702 Analyzer
That said, for arterial blood gas samples, values stay clinically reliable for at least 30 minutes after collection, so a brief transport delay is unlikely to produce a meaningful error.17PubMed. Effects of Analysis Delay on pH, pO(2), pCO(2), Bicarbonate, Glucose, Lactate, Electrolytes, and Other Arterial Blood Gas Variables If your CO₂ is only slightly below normal and everything else on the panel looks fine, a repeat draw is a reasonable first step before assuming something is wrong.
Low CO₂ During Pregnancy
Pregnant women routinely run lower CO₂ levels than they did before pregnancy, and this is normal physiology, not a sign of illness. Progesterone stimulates the respiratory center in the brain, causing pregnant women to breathe more deeply throughout the day. This chronic mild hyperventilation lowers arterial CO₂ gas from a typical non-pregnant value of about 40 mmHg down to roughly 32 to 34 mmHg. The kidneys compensate by excreting bicarbonate, and serum levels settle into a range of about 15 to 20 mmol/L, well below the standard reference range printed on most lab reports.18Immunology and Allergy Clinics of North America. Respiratory Physiologic Changes in Pregnancy The decline becomes even more pronounced during labor.19PubMed. Maternal carbon dioxide level during labor and its possible effect on fetal cerebral oxygenation
This means that a pregnant woman with a serum CO₂ of 18 or 19 mmol/L might be perfectly healthy, while the same number in a non-pregnant adult would prompt urgent investigation. If you are pregnant and see a flagged-low CO₂ on your lab work, do not panic. Your care team should interpret it in the context of pregnancy-adjusted reference ranges rather than the standard ones.
High Altitude and Low Bicarbonate
Traveling to high altitude triggers the same compensatory mechanism seen in pregnancy but for a different reason. The thinner air at elevation contains less oxygen, which makes you breathe faster and deeper. That hyperventilation blows off CO₂ gas, creating a respiratory alkalosis, and over the next day or two the kidneys begin dumping bicarbonate to bring pH back in line. In one study tracking climbers during ascent, mean bicarbonate fell from about 24 mmol/L at baseline to around 18 mmol/L at 5,160 meters.20PubMed Central. Renal reactivity: acid‐base compensation during incremental ascent to high altitude
The kidney response is not instant. At moderate altitudes around 3,100 meters, only a slight reduction in bicarbonate was detectable within the first 24 hours, with a more distinct drop by about 44 hours.21PubMed Central. Early acclimatization to high altitude: Acid-base and fluid balance dynamics during the first 2 days at 3100 m Indigenous populations living permanently at elevation also carry lower bicarbonate levels than sea-level residents, reflecting lifelong adaptation.22PubMed. Acid-base balance at high altitude in lowlanders and indigenous highlanders If you had bloodwork done during a mountain trip, this explains a low CO₂ without any underlying disease.
Children and Normal Ranges That Shift With Age
Pediatric reference ranges for serum bicarbonate are not the same as adult ranges, and applying adult cutoffs to a child’s lab work can create unnecessary alarm. In infants and toddlers, bicarbonate tends to be lower than in older children, and it gradually rises as respiratory rates slow with age.23Pathology. Trends and physiology of common serum biochemistries in children aged 0-18 years A value of 19 or 20 mmol/L in a two-year-old may be perfectly normal, while the same number in a teenager warrants a closer look. Pediatricians use age-adjusted reference ranges for exactly this reason, but if you are reading your child’s lab results through a patient portal that only prints the adult normal range, it can look falsely alarming.
When to Worry and When to Wait
A mildly low CO₂ on one lab draw, especially if you are otherwise feeling fine, is often worth rechecking before pursuing an extensive workup. Lab artifacts, recent vigorous exercise, dehydration, or even a bout of anxiety-related fast breathing on the way to the blood draw can nudge the number below the reference range temporarily. If the repeat is normal and nothing else on the panel looks off, the first result was likely noise.
On the other hand, a CO₂ that is significantly below normal, is trending downward over multiple tests, or appears alongside other abnormalities like elevated blood sugar, high potassium, elevated creatinine, or an elevated anion gap deserves prompt evaluation. The combination of findings is what tells the story. A single slightly low bicarbonate is a conversation starter, not a crisis.