Low CO2 blood test: what a low bicarbonate means and when it matters

The line called CO2, total CO2 or carbon dioxide on a metabolic panel measures bicarbonate, the base your blood uses to soak up acid. When the bicarbonate number falls, one of two things is usually happening: acid is building up or bicarbonate is being lost (metabolic acidosis, meaning the blood has become slightly too acidic), or you have been breathing fast for days and your kidneys have let bicarbonate drift down to match. A basic panel cannot tell those two apart on its own, but three other numbers on the same report get you most of the way: the anion gap, chloride and creatinine.

What CO2 on your report measures

Most of the total CO2 measured on a basic or comprehensive metabolic panel is bicarbonate (HCO3); a small remainder is dissolved carbon dioxide and carbonic acid ¹¹. That is why the same result can be printed as bicarbonate, HCO3, total CO2, CO2 or carbon dioxide, total. The total CO2 on a metabolic panel is a different number from the pCO2 on a blood gas, which measures the carbon dioxide gas your lungs control, in mmHg.

Most adult labs put the serum bicarbonate range at about 22 or 23 up to 29 mmol/L (MedlinePlus gives 23 to 29 ¹¹), and a few run to 30; most healthy adults sit in the mid-20s. Children's and especially infants' ranges run a little lower, so use the range printed on the child's report. For bicarbonate, mmol/L and mEq/L are the same number, so a 24 mEq/L on one report equals a 24 mmol/L on another (the lab units guide covers why). Your own lab's range, printed beside the result, is the one to read against; the reference ranges guide explains why ranges move a point or two between labs. For what the marker does in general, and for a high result, see the main bicarbonate (CO2) page.

How low is too low: what each number usually means

No guideline publishes severity bands for an outpatient CO2, so the table below is a rough working guide to the usual next step at each level. Symptoms override it in both directions: a 17 in someone breathing hard and vomiting is more urgent than a 15 in a stable person with known kidney disease.

Two different reasons CO2 falls

A low bicarbonate can mean acid is accumulating, or it can be the kidneys' response to over-breathing. Both print the same low CO2, and only a blood gas (a blood test, often from an artery in the wrist, that measures pH, how acidic the blood is, and pCO2, the carbon dioxide gas your breathing controls) separates them with certainty ¹. In acidosis the blood pH falls and the lungs try to breathe off acid, so pCO2 drops too. In chronic over-breathing the pCO2 drops first, the blood turns slightly alkaline, and over two to three days the kidneys shed bicarbonate to pull the pH back toward normal ¹.

Pregnancy is the most common harmless example. Progesterone drives mild over-breathing from early pregnancy, and the kidneys lower bicarbonate to roughly 18 to 22 mmol/L as a normal adjustment ⁷, so a CO2 around 20 in pregnancy is usually expected. If you are pregnant, tell whoever ordered the test so they read your CO2 against pregnancy ranges, and mention it to your midwife or doctor if it is lower or you feel unwell. A low CO2 in pregnancy with vomiting that will not stop, high blood sugar or ketones still needs same-day care, because ketoacidosis in pregnancy can happen at lower blood sugar levels. Living at or travelling to high altitude lowers CO2 for the same reason as pregnancy.

Use the anion gap to narrow it down

The anion gap estimates the acids your panel does not measure directly. The formula is sodium minus (chloride plus bicarbonate), and the typical range is roughly 4 to 12 mmol/L, depending on the lab's analyser ².

A high gap means unmeasured acid is present ²:

• Ketones, in diabetic, alcoholic or starvation ketoacidosis
• Lactate, in shock or severe infection
• Acids the kidneys fail to clear, in advanced kidney failure
• Toxic alcohols such as methanol or ethylene glycol (antifreeze)
• Salicylate, from an aspirin overdose

A normal gap with a high chloride means bicarbonate is being lost and chloride is taking its place ²:

• Diarrhoea
• Renal tubular acidosis (the kidney tubules fail to hold on to bicarbonate or clear acid)
• Early chronic kidney disease
• Carbonic anhydrase inhibitors (medicines that make the kidneys pass bicarbonate), such as acetazolamide and topiramate
• Large volumes of IV saline

Albumin needs a correction. Albumin is itself a negative charge in the gap, so each 1 g/dL drop in albumin below about 4 lowers the measured anion gap by about 2.5 mmol/L ³. In someone who is ill or malnourished with an albumin of 2.5, a gap that reads a normal 11 is really about 15, and a high-gap acidosis can hide behind it.

CalculatorAnion gap calculatorPut in your sodium, chloride, CO2 and albumin to get your anion gap, corrected for albumin.Work it out

If your gap is above your lab's range, the high anion gap page walks through the causes in more depth. If chloride is the number flagged high alongside the low CO2, the high chloride page covers that pattern.

Everyday causes of a low CO2

Most low CO2 results seen in routine bloodwork come from a short list of ordinary causes. The size of the drop is often the best clue to which one applies.

• Diarrhoea or a recent stomach bug: stool is rich in bicarbonate, so a few days of loose stools can pull CO2 down by several points with a normal gap and high chloride ². It recovers within days to a couple of weeks.
• Chronic kidney disease: damaged kidneys make less bicarbonate and clear less acid, which gives a CO2 that drifts down slowly and stays down (see the kidney section below).
• Fasting, keto or very low-carb eating: mild ketosis usually lowers CO2 only slightly; a level in the teens on a diet is not expected and needs checking.
• Hard exercise shortly before the draw: lactate from intense effort lowers bicarbonate briefly, and it clears with rest.
• Heavy drinking: alcoholic ketoacidosis after a binge with little food can push CO2 well down, with a high gap.
• Medicines: topiramate and acetazolamide lower bicarbonate in many people who take them, and topiramate's US label recommends checking bicarbonate at baseline and during treatment ⁸; SGLT2 inhibitors can cause ketoacidosis; metformin causes lactic acidosis rarely, mostly with poor kidney function; high-dose aspirin can lower it. If you take one of these and your CO2 is low, do not stop or change the dose yourself; tell the doctor who prescribes it.
• Uncontrolled diabetes: diabetic ketoacidosis is the high-gap cause most people have heard of, and it comes with high glucose and ketones.

When the blood tube lowers the reading

Carbon dioxide escapes from blood into air. When a vacuum tube is only part filled, CO2 diffuses into the empty space above the sample and the measured total CO2 can read several mmol/L lower than the true value; emergency physicians described this as pseudometabolic acidosis in 1992 ⁴. Long delays before the sample is processed and tubes left uncapped have the same effect. A lone CO2 of 20 or 21 with a normal anion gap, normal chloride and normal kidney numbers is often this kind of artefact. A clean repeat, drawn without a long wait and processed promptly, usually settles it; the blood test preparation guide covers the rest.

Symptoms that can come with low bicarbonate

A CO2 in the high teens or low 20s usually causes no symptoms at all; if you feel unwell, the cause of the low CO2 is more likely to be the reason than the number itself. A true metabolic acidosis can cause:

• Deep, fast breathing that feels hard to control (Kussmaul breathing)
• Tiredness and weakness (see fatigue)
• Nausea, vomiting or tummy pain
• Headache, confusion or drowsiness
• With diabetic ketoacidosis: thirst, passing a lot of urine, and breath that smells fruity

Any of these with a low CO2 falls under the urgent box above. Over years, the mild acidosis of kidney disease causes no obvious symptoms but slowly draws calcium from bone and breaks down muscle.

Low CO2 and your kidneys

Healthy kidneys reclaim nearly all the bicarbonate filtered each day and make new bicarbonate to replace what the body's acid load uses up. As kidney function falls, bicarbonate falls with it, which makes chronic kidney disease the most common reason for a CO2 that stays low on repeated tests. The low bicarbonate also appears to matter for the kidneys themselves. In the CRIC study of 3,939 people with chronic kidney disease, each 1 mmol/L higher bicarbonate was linked to about a 3% lower risk of kidney failure or a halving of kidney function, and people under 22 had the highest risk ⁵.

The KDIGO 2024 guideline also advises that treatment should not push bicarbonate above the upper limit of normal ⁶. Treatment means bicarbonate tablets, which slowed kidney decline over two years in a randomised trial ⁹, or more fruit and vegetables, which protected kidney function about as well as tablets in another ¹⁰. Both are prescribed and monitored by a doctor, because the sodium in bicarbonate tablets can raise blood pressure and fluid, and fruit and vegetables can raise potassium in advanced kidney disease. Taking baking soda yourself carries the same risks with none of the monitoring. The chronic kidney disease page covers staging, the kidney function guide explains how creatinine and eGFR are read, and a raised creatinine has its own page at high creatinine.

Read it with these results

Glucose belongs on this list too: a low CO2 with a high fasting glucose and a high gap raises the question of diabetic ketoacidosis. If a lactate was measured, a high value explains a high gap, and ketones can be checked in urine (see urine ketones). A raised potassium alongside a low CO2 is covered on the high potassium page.

What to do with a low result

1. 1Size the dropNote how far under the range the CO2 is, using the band table above. A 21 and a 15 lead to different next steps.
2. 2Work out the anion gap and look at chlorideA normal gap with high chloride points to bicarbonate loss; a high gap points to added acid and needs prompt review.
3. 3List what was going on before the drawRecent diarrhoea or vomiting, fasting or keto, a hard workout, heavy drinking, pregnancy, a tube that was not filled fully or a sample that waited a long time before processing, and any new medicine, especially topiramate, acetazolamide, an SGLT2 inhibitor or metformin.
4. 4Repeat it cleanly if it is borderlineFor an isolated 20 to 21 with everything else normal, a repeat in a few weeks, rested, well hydrated and at the same lab, is the usual next step. Ask the doctor or service that ordered the test to arrange it.
5. 5Book a doctor for anything persistent or lowerA CO2 under 20 on repeat, a high gap, or abnormal kidney numbers is worth a routine appointment. Bring the full panel and your medicine list, and ask whether you need a blood gas, ketones or lactate, or a repeat kidney panel, to show which kind of low CO2 it is. Under about 15, or any warning sign from the urgent box above, is same-day care.

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One low CO2 is a snapshot. Two or three results a few weeks apart show whether the number is drifting or settling, which says more than any single value; the trends guide explains how to read a run of results, the metabolic panel guide puts CO2 in context with the rest of the panel, and the electrolytes guide shows how sodium, potassium, chloride and CO2 read together.

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