What Causes Low Bicarbonate Levels?

A low bicarbonate level on a blood test usually means the body is losing bicarbonate, producing too much acid, or breathing off too much carbon dioxide. Bicarbonate is the body’s main chemical buffer — it keeps blood from becoming too acidic. When it drops, the cause is almost always one of three things: a metabolic problem, a respiratory problem, or a lab or sampling issue. Finding the real cause matters because low bicarbonate can signal anything from mild dehydration to kidney failure.

What Is Bicarbonate and Why Does the Body Care About It?

Bicarbonate (written as HCO3) is a chemical that neutralizes acid. Your blood needs to stay in a very narrow pH range — roughly 7.35 to 7.45 — for enzymes and organs to work. Bicarbonate is one of the main tools the body uses to hold that range.

The kidneys and lungs work as a team here. The lungs remove carbon dioxide when you exhale. The kidneys either hold onto bicarbonate or release it in urine. When either system slips, bicarbonate levels shift.

On a standard metabolic panel, normal bicarbonate usually falls between 22 and 29 milliequivalents per liter (mEq/L). A result below 22 mEq/L is generally considered low. Some labs use slightly different ranges, so always compare your number to the range printed on your own report.

One clarification worth knowing: bicarbonate and carbon dioxide (CO2) are often used almost interchangeably on lab reports. The “CO2” on a basic metabolic panel is actually a measure of bicarbonate. They are not the same molecule, but in this context the number reflects bicarbonate.

What Causes Low Bicarbonate Levels?

Low bicarbonate almost always reflects one of two broad problems: a buildup of acid in the body (metabolic acidosis) or a breathing pattern that blows off too much carbon dioxide (respiratory alkalosis). Sometimes both are happening at once.

Metabolic causes include:

• Diabetic ketoacidosis — when insulin is too low, the body makes acidic ketones
• Kidney disease — damaged kidneys cannot properly reclaim bicarbonate
• Severe diarrhea — stool is rich in bicarbonate, so losing it lowers blood levels
• Lactic acidosis — acid builds up when tissues do not get enough oxygen, such as in sepsis or shock
• Poisoning — certain toxins, including salicylates and methanol, drive acid levels up
• Addison’s disease — low adrenal hormones can impair acid-base balance

Respiratory causes center on breathing too fast or too deeply. Hyperventilation from anxiety, pain, fever, or a lung problem can push carbon dioxide out faster than the body makes it. The result is a rise in blood pH and a fall in bicarbonate as the kidneys compensate.

There are also situations where the number looks low but the body is fine. A blood sample that sits too long before testing, or one drawn from a vein with a tourniquet held too long, can read falsely low. This is not common, but it happens.

How Do Diarrhea and Kidney Problems Lower Bicarbonate?

Diarrhea and kidney disease are two of the most common and best-understood causes of low bicarbonate. They work through different mechanisms, but both drain the body’s buffer supply.

Stool contains a lot of bicarbonate. In normal digestion, the pancreas and intestines release bicarbonate to neutralize stomach acid. When diarrhea is heavy or prolonged, the body loses that bicarbonate faster than it can replace it. This is why severe or chronic diarrhea — from infection, inflammatory bowel disease, or other causes — can produce a condition called normal anion gap metabolic acidosis.

The kidneys handle bicarbonate differently. Healthy kidneys filter blood and reabsorb most of the bicarbonate before it leaves the body. They also generate new bicarbonate to replace what is used up. When kidney function drops — as in chronic kidney disease — this reabsorption and generation slow down. Acid accumulates, and bicarbonate falls. In advanced kidney disease, low bicarbonate is common and is one reason clinicians monitor it closely.

These two causes are not the only ones, but they are among the most frequent. A clinician looks at the full picture — symptoms, other lab values, and medical history — to sort out which is at play.

What Symptoms Come With Low Bicarbonate?

Low bicarbonate itself does not always cause obvious symptoms. Mild drops are often found on routine blood work with no complaints at all. Symptoms, when they appear, usually come from the underlying condition or from the acid-base imbalance it creates.

Possible signs include:

• Fast breathing or shortness of breath
• Fatigue or confusion
• Headache
• Nausea or vomiting
• Irregular heartbeat in severe cases

Severe acidosis — when pH drops well below normal — can affect the heart, brain, and blood pressure. This is a medical emergency and needs immediate care. But most low bicarbonate results are not emergencies. They are clues that point toward a cause that needs attention.

It is also worth noting that symptoms alone rarely identify the cause. Someone with low bicarbonate from diarrhea and someone with low bicarbonate from kidney disease may feel similar. The lab value is a starting point, not a diagnosis.

How Do Doctors Figure Out the Cause?

Doctors do not treat a low bicarbonate number in isolation. They use it alongside other tests to find the source.

The first step is usually an arterial or venous blood gas, which measures pH and carbon dioxide directly. This tells the clinician whether the problem is metabolic or respiratory. If pH is low and bicarbonate is low, it points to metabolic acidosis. If pH is high and bicarbonate is low, it points toward respiratory alkalosis.

Next comes the anion gap. This is a calculated value from sodium, chloride, and bicarbonate levels. A high anion gap suggests acids are accumulating from a source like ketones, lactate, or toxins. A normal anion gap points more toward bicarbonate loss through the gut or kidneys.

Additional tests may include:

• Blood glucose and ketones (to check for diabetic ketoacidosis)
• Kidney function tests (creatinine and BUN)
• Lactate level (to check for lactic acidosis)
• Urine pH and electrolytes
• Toxin screening if poisoning is suspected

This step-by-step approach helps narrow down dozens of possible causes to the most likely one. It is standard clinical practice and is well established.

When Should You Be Concerned About a Low Bicarbonate Result?

A single low bicarbonate reading is not automatically alarming. Mildly low results — say, 20 or 21 mEq/L — are sometimes temporary and correct themselves. Dehydration, a recent illness, or even a lab variation can nudge the number down.

What matters more is the pattern. A bicarbonate level that stays low on repeat testing, or one that drops further over time, deserves a closer look. So does a low result paired with symptoms like rapid breathing, confusion, or weakness.

Certain people should take low bicarbonate more seriously:

• Anyone with known kidney disease
• People with diabetes, especially type 1
• Those with chronic diarrhea or digestive conditions
• People taking medications that affect acid-base balance

No clinical guidelines currently exist for how often to recheck a mildly low bicarbonate in a person who feels well. That decision is best made with a clinician who knows your history.

Can Low Bicarbonate Be Prevented?

You cannot always prevent low bicarbonate because many causes — kidney disease, diabetic ketoacidosis, sepsis — are not within your control. But some steps can reduce risk.

Staying hydrated helps the kidneys work properly. Managing diabetes carefully keeps ketones from building up. Treating diarrhea early, especially in children and older adults, can prevent bicarbonate loss from becoming severe. Avoiding excessive alcohol and certain toxins lowers the risk of acid buildup.

For people with chronic kidney disease, clinicians sometimes prescribe sodium bicarbonate to help maintain normal levels. Whether this improves long-term outcomes is still an area of active research, and it is not a routine recommendation for everyone. It is a decision made case by case.

There is no diet or supplement proven to raise bicarbonate in people whose levels are low for medical reasons. Marketing claims about “alkaline” products do not change the underlying cause.