Clinical Significance of Serum CO₂ = 20 mmol/L in a Healthy 65-Year-Old
A serum total CO₂ of 20 mmol/L in an otherwise healthy 65-year-old represents mild metabolic acidosis that warrants further evaluation to identify the underlying cause, as this value falls below the recommended reference range of 23-30 mmol/L for healthy adults.
Understanding the Finding
The serum total CO₂ (TCO₂) measurement is a surrogate for bicarbonate (HCO₃⁻) concentration and serves as the initial assessment of acid-base status 1. Recent evidence suggests that the traditional reference range (22-29 mmol/L) is too wide and should be narrowed to 23-30 mmol/L for adults at sea level 1. In a large study of 28,480 healthy young adults (ages 18-40), 91% had values within this 23-30 mmol/L range, with a mean of 26.6 mmol/L in males and 25.0 mmol/L in females 1.
Your patient's value of 20 mmol/L falls below this range, indicating:
- Mild metabolic acidosis (TCO₂ < 23 mmol/L)
- Not severe enough to suggest immediate life-threatening pathology (values < 18 mmol/L are associated with significantly worse outcomes) 2, 3
- Requires investigation to determine the etiology
Clinical Approach
Immediate Assessment Needed:
Obtain venous blood gas to confirm the finding and assess:
- pH (to determine if true acidosis exists: pH < 7.35)
- Actual bicarbonate level
- Anion gap calculation (Na⁺ - [Cl⁻ + HCO₃⁻])
Review for common causes in "healthy" patients:
- Renal dysfunction (early chronic kidney disease can present with TCO₂ in the 18-22 range) 1
- Gastrointestinal bicarbonate loss (chronic diarrhea, ileostomy)
- Medications (carbonic anhydrase inhibitors, topiramate, metformin)
- Early diabetic ketoacidosis or other metabolic derangements
- Respiratory compensation for chronic respiratory alkalosis (uncommon)
Important Caveats:
Do not assume this is clinically insignificant simply because the patient appears "healthy." The BTS guidelines emphasize that metabolic acidosis can result from failure to excrete acid (renal failure) or increased acid production (diabetic ketoacidosis, lactic acidosis from tissue hypoxia) 4. A TCO₂ of 20 mmol/L suggests the body is buffering an acid load or losing bicarbonate.
Preanalytical factors matter: CO₂ loss can occur in improperly handled samples. If the tube was left uncapped, up to 24% decline can occur over 48 hours 5. However, this would falsely lower the value, so it doesn't explain away your finding—it might mean the true value is even lower.
Prognostic Significance
While not immediately life-threatening, TCO₂ values below the normal range carry prognostic implications:
- In sepsis patients, TCO₂ ≤ 20 mmol/L is associated with increased 28-day mortality (23.6% mortality for TCO₂ 15-20 mmol/L vs. 18.3% for TCO₂ > 20 mmol/L) 2
- In sepsis survivors, TCO₂ < 18 mmol/L predicts worse long-term outcomes, including increased all-cause mortality (HR 1.28), myocardial infarction (HR 1.83), and end-stage renal disease (HR 1.38) 3
- Values < 18 mmol/L generally suggest metabolic acidosis requiring urgent attention 6
Recommended Action Plan
- Repeat measurement with proper sample handling (capped tube, prompt analysis)
- Obtain complete metabolic panel including:
- Electrolytes (calculate anion gap)
- Creatinine and BUN (assess renal function)
- Glucose (rule out diabetic ketoacidosis)
- Perform venous blood gas to confirm acidosis and assess compensation
- Review medication list thoroughly
- Assess for symptoms of acidosis (though often absent in mild cases):
- Fatigue
- Confusion
- Dyspnea (compensatory hyperventilation)
The key message: A TCO₂ of 20 mmol/L in a "healthy" patient suggests they may not be as healthy as assumed. This finding requires investigation to identify treatable causes before progression to more severe acidosis or organ dysfunction occurs. The narrower reference range of 23-30 mmol/L exists precisely to catch these early metabolic derangements 1.