Bicarbonate (HCO3) Blood Test: Levels & Meaning

When I review a patient’s blood gas results, the bicarbonate (HCO3) level is one of the first numbers I look at. It’s a direct window into how well the kidneys are regulating your body’s pH balance. In my clinical practice, I often see patients who are surprised that a simple blood test can reveal so much about their respiratory and kidney function. Let’s walk through what bicarbonate means, when it’s tested, and what the numbers tell us.

What Is Bicarbonate (HCO3)?

Bicarbonate is a negatively charged ion (anion) that acts as the body’s primary buffer against acid. It’s produced by the kidneys and helps maintain a stable pH — typically between 7.35 and 7.45. The HCO3 level is a key component of two standard lab panels: the Basic Metabolic Panel (BMP) and the Comprehensive Metabolic Panel (CMP), as well as Arterial Blood Gas (ABG) analysis.

The LOINC code for serum/plasma bicarbonate is 1963-8 (Bicarbonate [Moles/volume]). Laboratories may also report it as "CO2 content" or "total carbon dioxide."

Why Is HCO3 Measured?

Doctors order a bicarbonate test to evaluate acid-base disorders (metabolic acidosis or alkalosis), monitor kidney function, assess respiratory status, or check for electrolyte imbalances. Common scenarios include:

• Unexplained fatigue, confusion, or rapid breathing
• Chronic kidney disease or kidney failure
• Diabetic ketoacidosis (DKA)
• Severe vomiting or diarrhoea
• Respiratory conditions like COPD or hyperventilation

Reference Ranges for Bicarbonate (HCO3)

Reference values can vary slightly between laboratories, but the generally accepted normal range for adults is 22–29 mmol/L. Below is a detailed table with age- and gender-specific ranges based on commonly used laboratory standards.

Note: Always interpret results with your clinician. Gender differences are minimal except in pregnancy.

What Does a Low Bicarbonate Level Mean?

Metabolic Acidosis

A bicarbonate level below 22 mmol/L is called metabolic acidosis. This happens when the body produces too much acid or the kidneys fail to excrete enough acid. Common causes include:

• Diabetic ketoacidosis (high blood sugar and ketones)
• Lactic acidosis (from sepsis, shock, or intense exercise)
• Chronic kidney disease
• Severe diarrhoea (loss of bicarbonate)
• Salicylate (aspirin) poisoning

In my clinic, I often see patients with mild metabolic acidosis who complain of fatigue and deep, rapid breathing — the body’s attempt to blow off carbon dioxide.

Is Low Bicarbonate Dangerous?

Yes, a very low level (especially <15 mmol/L) can be a medical emergency because it means the blood is becoming dangerously acidic. Immediate evaluation and treatment (e.g., fluids, insulin for DKA, or dialysis) may be needed.

What Does a High Bicarbonate Level Mean?

Metabolic Alkalosis

A bicarbonate above 29 mmol/L is called metabolic alkalosis. It occurs when the body has too much bicarbonate or loses too much acid. Causes include:

• Prolonged vomiting (loss of stomach acid)
• Excessive use of diuretics (e.g., thiazides or loop diuretics)
• Hypokalemia (low potassium)
• Antacids in high doses
• Conn’s syndrome (hyperaldosteronism)

Patients with metabolic alkalosis often feel weak, confused, or have muscle twitching. In severe cases, breathing becomes slow and shallow.

High Bicarbonate and Kidney Disease

Paradoxically, some forms of kidney failure can raise bicarbonate if the kidneys fail to excrete bicarbonate properly. This is less common but important.

Bicarbonate During Pregnancy

Pregnancy naturally causes a mild increase in respiratory drive (due to progesterone), leading to a slight drop in carbon dioxide and a compensatory decrease in bicarbonate. Normal HCO3 in pregnancy is often 18–26 mmol/L. This is not harmful unless extreme. In my practice, I always check bicarbonate alongside electrolytes if a pregnant patient has nausea or vomiting.

How to Prepare for the Test

Bicarbonate measurement is part of a standard blood draw. You do not need to fast, but your doctor may advise avoiding strenuous exercise or certain medications beforehand. Tell your provider about any supplements or antacids you take.

When Should You Be Concerned?

Bicarbonate levels rarely go out of range without other symptoms. Seek medical advice if you experience:

• Rapid, deep breathing (Kussmaul breathing)
• Persistent nausea, vomiting, or diarrhoea
• Confusion or drowsiness
• Muscle cramps or weakness
• Swelling in legs or ankles (possible kidney involvement)

Key Takeaways

• Bicarbonate (HCO3) is the body’s main buffer against acidosis.
• Normal adult range: 22–29 mmol/L.
• Low levels suggest metabolic acidosis; high levels suggest metabolic alkalosis.
• Interpretation always requires a simultaneous pH and PCO2 measurement (ABG).
• Always discuss results with a healthcare professional — never treat based on numbers alone.