Unit Converter
Bicarbonate (HCO3-)
(Serum / Plasma Total CO₂ – Major Blood Buffer – Acid–Base Homeostasis Marker)
Synonyms
- Bicarbonate
- HCO₃⁻
- CO₂ (total CO₂ on chemistry panel)
- Standard bicarbonate
- Carbonic acid–bicarbonate buffer component
- Alkali reserve
Units of Measurement
- µmol/L
- mmol/L
- mg/L
- mg/dL
- mg/100mL
- mg%
- µg/mL
- mEq/L (most commonly used in clinical labs)
(1 mmol of HCO₃⁻ = 1 mEq because valency = 1)
Description
Bicarbonate (HCO₃⁻) is the main extracellular buffer that maintains the body’s acid–base balance.
It represents:
- Kidney’s metabolic regulation of acid-base status
- The metabolic component of the Henderson–Hasselbalch equation
- ~95% of the total CO₂ content in blood
Measured in:
- Serum/plasma (as total CO₂, mostly HCO₃⁻)
- Arterial blood gas (ABG) as calculated bicarbonate
Bicarbonate is crucial in diagnosing:
- Metabolic acidosis
- Metabolic alkalosis
- Respiratory compensation
- Renal tubular disorders
- Diabetic ketoacidosis
- Shock & critical illness
Physiological Role
1. Buffering System (Primary Role)
Part of the CO₂ + H₂O ↔ H₂CO₃ ↔ H⁺ + HCO₃⁻ system.
2. Kidney Regulation
- Reabsorbed in proximal tubule
- New bicarbonate synthesized in distal nephron
- Excretion of acids as NH₄⁺ and H₂PO₄⁻
3. Lung Compensation
CO₂ exhalation influences pH and bicarbonate equilibrium.
Clinical Significance
Low Bicarbonate (Metabolic Acidosis)
Seen in:
- DKA
- Lactic acidosis
- Renal failure
- Renal tubular acidosis
- Diarrhea (bicarbonate loss)
- Toxins (methanol, ethylene glycol, salicylates)
- Shock, sepsis
Severity:
- Mild: 18–22 mEq/L
- Moderate: 10–18 mEq/L
- Severe: < 10 mEq/L
High Bicarbonate (Metabolic Alkalosis)
Seen in:
- Vomiting / gastric suction (acid loss)
- Diuretics
- Hyperaldosteronism
- Cushing syndrome
- Hypokalemia
- Excess bicarbonate intake
Often >30 mEq/L.
Reference Intervals
(Tietz 8E + ABG guidelines + Mayo/ARUP)
Serum Bicarbonate (Total CO₂)
- 22 – 28 mEq/L (mmol/L) = normal
- Children similar
- Critical: < 10 or > 40 mEq/L
Arterial Bicarbonate (ABG)
- 22 – 26 mEq/L
Total CO₂ in chemistry = HCO₃⁻ (~95%) + dissolved CO₂ (~5%).
Unit Meanings
| Unit | Meaning |
| mmol/L | millimole per liter |
| mEq/L | milliequivalent per liter |
| µmol/L | micromole per liter |
| mg/L | milligram per liter |
| mg/dL / mg% | milligram per deciliter |
| mg/100mL | mg% |
| µg/mL | microgram per milliliter |
Diagnostic Uses
1. Acid–Base Disorders
- Metabolic acidosis
- Metabolic alkalosis
- Compensation assessment
Use with arterial blood gases, anion gap, pH, pCO₂.
2. DKA Monitoring
Bicarbonate <18 mEq/L → metabolic acidosis.
<10 mEq/L → severe DKA.
3. Renal Tubular Acidosis
- Type 1 (distal): low HCO₃⁻, high urine pH
- Type 2 (proximal): low HCO₃⁻, normal anion gap
- Type 4: hyperkalemia + low bicarbonate
4. Critical Care
Low HCO₃⁻ in sepsis, shock, multi-organ failure.
5. Lung/Kidney Compensation
Critical to interpret mixed acid–base disorders.
Analytical Notes
- Measured by automated chemistry analyzers (total CO₂).
- ABG bicarbonate is calculated, not directly measured.
- Delayed sample processing → underestimated bicarbonate (CO₂ escapes).
- Avoid exposure of tubes to air.
- Hemolysis minimal impact.
Clinical Pearls
- Serum total CO₂ ≈ bicarbonate level in most cases.
- Always interpret bicarbonate with:
- pH
- pCO₂
- Anion gap
- Clinical context
- pH
- In DKA, β-hydroxybutyrate correlates better with severity than HCO₃⁻ - but HCO₃⁻ is essential for acid–base assessment.
- Low HCO₃⁻ with normal pH → mixed disorders.
- High bicarbonate + low chloride → suggests metabolic alkalosis.
Interesting Fact
Bicarbonate accounts for ~90% of CO₂ transport in the blood, acting as the body's most powerful natural buffer system - essential for life.
References
- Tietz Clinical Chemistry and Molecular Diagnostics, 8th Edition - Acid-Base Balance.
- IFCC Acid-Base Standardization.
- ADA/ISPAD DKA Guidelines.
- Mayo Clinic Laboratories - Bicarbonate (CO₂).
- ARUP Consult - Acid–Base Disorder Interpretation.
- MedlinePlus / NIH - Bicarbonate Test.
- ABG & Critical Care Texts - Henderson–Hasselbalch Physiology.