Low Bicarbonate on Your Blood Test
What a low bicarbonate or low CO2 result means, why it is often nothing to worry about, and when it needs urgent attention — in plain English.
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· · Reviewed against the sources listed at the end of this page.
The Quick Answer
Bicarbonate (HCO3), also reported as CO2 or Total CO2 on many US metabolic panels, is a measure of your body's metabolic acid-base balance, not the carbon dioxide gas in your lungs. The typical adult reference range is roughly 22-29 mmol/L in Australia (or mEq/L in the US), though this varies a little between labs.
The single most common reason for an unexpectedly low result is a sample handling issue, where dissolved CO2 escapes from the tube before testing. After that, the most frequent real causes are over-breathing (hyperventilation or anxiety) and diarrhoea. Less often, a persistently low bicarbonate points to kidney disease, and a sharply low result alongside high blood glucose can signal diabetic ketoacidosis, which needs urgent care.
What Does Bicarbonate Actually Measure?
Your blood constantly balances two systems to keep its pH steady: a respiratory system, controlled by how fast and deep you breathe, and a metabolic system, controlled mainly by your kidneys and how much bicarbonate they hold onto or excrete. The bicarbonate (or CO2) result on a metabolic panel is a snapshot of the metabolic side of that balance, not a direct reading of the CO2 gas moving in and out of your lungs.
This distinction matters. Breathing fast can lower the CO2 gas dissolved in your blood within minutes, and your kidneys then adjust bicarbonate over hours to days to compensate. A blood test taken during or shortly after a period of over-breathing can therefore show a low bicarbonate even though your lungs and oxygen levels are completely normal.
Bicarbonate can be lowered in two broad ways: it can be used up buffering an acid your body is producing too much of, such as ketones in diabetic ketoacidosis or lactic acid after intense exercise, or it can be lost directly, most commonly through the gut during diarrhoea, or through the kidneys in some rarer conditions. Working out which of these is happening is the key step in interpreting a low result.
Causes of Low Bicarbonate
Causes are listed from the most common and usually benign explanations through to the less common ones that need closer attention. Most people with a mildly low result have one of the first few causes on this list.
Sample handling (pre-analytical artefact)
Dissolved CO2 slowly escapes from a blood sample once drawn, especially from an underfilled tube or after a delay before testing. This produces a falsely low reading that does not reflect your true acid-base status. Often the single most likely explanation when you feel well and every other result is normal.
Hyperventilation and anxiety
Breathing faster or deeper than the body needs blows off carbon dioxide from the lungs. Over hours to days the kidneys respond by excreting bicarbonate to rebalance blood pH, producing a mild, persistent drop.
Diarrhoea
The fluid lost from the lower gut during diarrhoea is naturally rich in bicarbonate, so prolonged or severe diarrhoea directly depletes the body’s bicarbonate stores. Usually corrects once the diarrhoea settles and fluids are replaced.
Heavy or prolonged exercise
Intense muscle activity produces lactic acid, which is buffered by bicarbonate and temporarily lowers it. Usually returns to normal within a few hours of rest and is rarely tested for at that time.
Chronic kidney disease
Healthy kidneys constantly regenerate bicarbonate and excrete acid. As kidney function declines, particularly once eGFR drops well below 60, this balancing act becomes harder, producing a slow, persistent fall in bicarbonate known as chronic metabolic acidosis of kidney disease.
Renal tubular acidosis (RTA)
A group of conditions where the kidney tubules fail to handle acid and bicarbonate normally even though overall kidney filtration looks fine on standard tests. Usually picked up when bicarbonate is unexpectedly low with normal or near-normal kidney function.
Diabetic ketoacidosis (DKA)
Occurs when there is too little insulin for the body to use glucose, so it burns fat instead and produces acidic ketones that consume bicarbonate. Usually comes with a high blood glucose, thirst, frequent urination and rapid breathing. This is a medical emergency.
Alcohol-related or starvation ketoacidosis
Prolonged fasting, very low carbohydrate intake, or heavy drinking with poor food intake can all cause the body to produce ketones for energy, which lowers bicarbonate in a similar way to diabetic ketoacidosis, even without diabetes.
Metformin-associated lactic acidosis
A rare but serious build-up of lactic acid, almost always in the setting of significantly reduced kidney function, dehydration, heavy alcohol use, or shortly after some contrast dye scans. Routine metformin use with normal kidney function carries a very low risk.
Salicylate (aspirin) overdose
A large aspirin overdose disturbs acid-base balance through several mechanisms at once and can significantly lower bicarbonate. Always an emergency presentation, not something found on a routine test.
The Anion Gap: How Doctors Narrow Down the Cause
Most metabolic panels report sodium, chloride and bicarbonate together, which lets your doctor calculate the anion gap (roughly sodium minus the sum of chloride and bicarbonate). This single calculation is one of the most useful tools for working out why bicarbonate is low, because it splits the causes into two very different groups.
High anion gap acidosis
Diabetic ketoacidosis, alcohol or starvation ketoacidosis, lactic acidosis (including the rare metformin-associated form), kidney failure, and salicylate or other toxic ingestions. An unmeasured acid is building up in the blood.
Normal anion gap acidosis
Diarrhoea, renal tubular acidosis, and some medication effects. Bicarbonate is being lost directly rather than being consumed by an unmeasured acid, so chloride typically rises to compensate.
You do not need to calculate this yourself. It is mentioned here because if your results include sodium and chloride alongside a low bicarbonate, your doctor is very likely already using this exact calculation to guide what happens next.
Symptoms That Can Accompany Low Bicarbonate
Many people with a mildly low bicarbonate, especially from a sample handling issue or mild over-breathing, have no symptoms at all. Symptoms tend to appear as the underlying cause becomes more significant, particularly with diabetic ketoacidosis or advanced kidney disease.
Fatigue and general weakness
A common, non-specific symptom of most causes of metabolic acidosis, from a sluggish gut bug to more serious causes. Rarely helpful on its own for working out the cause.
Rapid, deep or laboured breathing
Known as Kussmaul breathing, this is the body’s attempt to blow off carbon dioxide and raise blood pH when bicarbonate is significantly low. A prominent finding in diabetic ketoacidosis and other severe acidosis.
Nausea, vomiting or loss of appetite
Common alongside diarrhoea-related causes and diabetic ketoacidosis. Vomiting itself can complicate the picture because it tends to raise, rather than lower, bicarbonate if it becomes the dominant problem.
Abdominal pain
A classic feature of diabetic ketoacidosis, sometimes severe enough to mimic a surgical abdomen. Also common with significant diarrhoeal illness.
Confusion or drowsiness
A red-flag symptom suggesting the acidosis is significant enough to affect brain function, or that dehydration and electrolyte shifts have become severe. Needs urgent medical assessment.
Excessive thirst and frequent urination
Suggests high blood glucose is part of the picture and raises the possibility of new or poorly controlled diabetes with ketoacidosis.
Muscle cramps or weakness
Can accompany the electrolyte shifts, particularly potassium changes, that often travel alongside a low bicarbonate on the same metabolic panel.
Unusual or fruity-smelling breath
A recognised sign of ketones building up in the blood, most often in diabetic ketoacidosis. Worth mentioning to a doctor even if it seems like an odd detail.
Red Flags — When to Seek Care Promptly
A mildly low bicarbonate found incidentally can usually wait for a routine follow-up. But some combinations of findings need same-day medical attention, or an emergency department:
Low bicarbonate with high blood glucose
Especially with thirst, frequent urination, nausea or abdominal pain. This combination raises concern for diabetic ketoacidosis and needs emergency assessment.
Rapid, deep or laboured breathing
Known as Kussmaul breathing, this can be the body compensating for a significant acidosis and warrants urgent review, particularly alongside any of the other flags here.
Confusion, drowsiness or difficulty waking
Suggests the acidosis, dehydration or an electrolyte shift is significant enough to affect brain function. This needs emergency assessment, not a routine appointment.
Severe or persistent vomiting or diarrhoea with dehydration
Signs include a dry mouth, dizziness on standing, reduced urination, or not being able to keep fluids down. Needs prompt medical attention, particularly in children, older adults, or anyone with existing kidney disease.
On metformin with vomiting, muscle pain or unusual tiredness
Rare, but this combination should prompt an urgent call to your doctor to check for the very uncommon complication of metformin-associated lactic acidosis, especially if kidney function has recently changed.
Known kidney disease with a new or worsening low bicarbonate
Worth a prompt call to your GP or kidney specialist rather than waiting for the next scheduled review, since untreated chronic acidosis can affect bone and muscle health over time.
What Your Doctor Will Do Next — The Workup
Doctors generally follow a fairly logical sequence when investigating a low bicarbonate. Knowing the order helps you understand why each next step is being suggested.
Repeat the test properly
If the result was unexpected and you feel well, the first step is often simply repeating the blood draw with careful technique, a properly filled tube, and prompt processing to rule out a sample handling artefact.
Calculate the anion gap
Using sodium, chloride and bicarbonate from the same panel, your doctor can work out the anion gap, which narrows down whether an unmeasured acid is building up (high anion gap) or bicarbonate is simply being lost directly (normal anion gap).
Check glucose and ketones
If diabetes is a possibility, blood glucose and either blood or urine ketones are checked promptly, since diabetic ketoacidosis needs urgent treatment and cannot wait for routine follow-up.
Assess kidney function
eGFR, creatinine, and sometimes a urine test are used to see whether the kidneys are managing acid and bicarbonate balance normally, and whether chronic kidney disease could explain a persistent low reading.
Review medications and history
Metformin, topiramate, acetazolamide, and some other medications can affect acid-base balance. A history of diarrhoea, heavy exercise, alcohol use, fasting, or breathing pattern is also reviewed carefully.
Check lactate if unwell
If you are acutely unwell, breathless, or your doctor suspects lactic acidosis, a blood lactate level helps confirm whether lactic acid build-up is contributing to the low bicarbonate.
Arterial or venous blood gas if acute
In an acute or more severe presentation, a blood gas sample gives a fuller picture of pH, bicarbonate, and the respiratory side of the equation together, rather than relying on the metabolic panel alone.
Treatment — What Happens Once the Cause Is Known
Sample handling artefact
No treatment is needed. A carefully repeated test, drawn without delay and sent to the lab promptly, is usually enough to confirm the true result and close the loop.
Over-breathing (hyperventilation or anxiety)
Bicarbonate normalises on its own once the breathing pattern settles. If anxiety or panic is driving it, your GP may discuss breathing techniques, counselling, or other supports depending on how much it is affecting you.
Diarrhoea-related loss
Rehydration with oral fluids, or intravenous fluids if severe, along with treating the underlying cause of the diarrhoea. Bicarbonate typically recovers as the gut settles and fluid balance is restored.
Chronic kidney disease
Oral sodium bicarbonate supplements are sometimes prescribed to correct persistent acidosis, alongside dietary advice and regular monitoring under a GP or kidney specialist. The goal is to protect bone and muscle health over the long term.
Diabetic ketoacidosis and other acute causes
Diabetic ketoacidosis, severe lactic acidosis, and salicylate overdose all need urgent hospital treatment, typically with intravenous fluids, careful monitoring of potassium and other electrolytes, and treatment aimed directly at the underlying cause. These are not managed at home.
Sources and reference ranges
Adult reference ranges on this page follow the AACB and RCPA harmonised reference intervals unless stated otherwise; the range printed on your own report always takes precedence because laboratories differ.
Low Bicarbonate — Frequently Asked Questions
What does it mean if my bicarbonate is low?
A low bicarbonate (HCO3) usually points to one of a handful of common causes: the blood sample was exposed to air or delayed before testing, you have been breathing fast or deeply for a while, you have had diarrhoea, or your kidneys are not managing acid balance as well as usual. Less commonly it can signal diabetic ketoacidosis or another metabolic problem, which is why your doctor will look at it alongside your other results rather than in isolation.
Why do some reports say CO2 instead of bicarbonate?
Bicarbonate and total CO2 (sometimes labelled TCO2) are essentially measuring the same thing on a standard blood chemistry panel. Australian pathology reports typically use the label Bicarbonate or HCO3, while many US labs report the same value as CO2 or Total CO2 on a basic or comprehensive metabolic panel (BMP or CMP). Neither is measuring the CO2 gas in your lungs directly.
Can a blood test error cause a falsely low bicarbonate result?
Yes, and it is one of the most common reasons for an unexpected low reading. Bicarbonate is partly derived from dissolved CO2, which can slowly escape from a blood sample once it is drawn, especially if the tube is underfilled, left standing at room temperature, or there is a delay before it reaches the lab. This produces a falsely low number that does not reflect your true acid-base status. If you feel completely well, your doctor will often simply repeat the test.
Is low bicarbonate the same as low blood oxygen?
No. Bicarbonate on a metabolic panel reflects your body’s metabolic acid-base balance, not how much oxygen or carbon dioxide is in your lungs at that moment. Oxygen levels are measured separately, usually with pulse oximetry or an arterial blood gas. It is possible to have completely normal oxygen levels with an abnormal bicarbonate, and vice versa.
Can anxiety or breathing fast cause low bicarbonate?
Yes. Breathing faster or more deeply than your body needs, which often happens with anxiety or panic, blows off carbon dioxide from your lungs. Over hours to days, your kidneys respond by excreting bicarbonate to try to rebalance your blood pH, which can show up as a mildly low bicarbonate on a blood test. This is usually mild and resolves once the breathing pattern settles.
What is diabetic ketoacidosis and how does it relate to low bicarbonate?
Diabetic ketoacidosis (DKA) happens when there is too little insulin for the body to use glucose properly, so it breaks down fat for energy instead and produces acidic by-products called ketones. Those ketones consume bicarbonate as the body tries to buffer the acid, so bicarbonate drops, often sharply, alongside a high blood glucose. DKA is a medical emergency that needs hospital treatment, usually in someone with known or newly diagnosed diabetes.
Can metformin cause low bicarbonate?
Rarely. Metformin is a very widely used and generally safe diabetes medication, but in rare circumstances, usually when kidney function is significantly reduced, or with dehydration, heavy alcohol use, or after certain contrast dye scans, it can be associated with a build-up of lactic acid called metformin-associated lactic acidosis. This lowers bicarbonate and is a medical emergency. Routine, well-monitored metformin use in someone with normal kidney function carries a very low risk.
How is low bicarbonate treated?
Treatment depends entirely on the cause, not the number itself. A repeat sample may show it was a handling artefact needing no treatment at all. Diarrhoea-related low bicarbonate usually improves with rehydration and treating the underlying gut cause. Kidney-related low bicarbonate may need oral bicarbonate supplements and dietary changes under a specialist. Diabetic ketoacidosis needs urgent hospital treatment with fluids, insulin, and close monitoring. Your GP will treat the cause, not just the lab value.
Related Reading
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This page provides general educational information about low bicarbonate and low CO2 results. It is not a substitute for professional medical advice, diagnosis, or treatment. Always consult your doctor about abnormal blood test results — they have access to your full medical history and can interpret your results in context. SmarterBlood does not provide medical care and is not affiliated with any pathology lab, hospital, or patient portal mentioned here.