What does a discrepancy between serum bicarbonate and arterial blood gas bicarbonate levels indicate?

Interpreting Discrepancy Between Serum Bicarbonate and Blood Gas Bicarbonate Levels

A significant discrepancy between serum bicarbonate (35 mmol/L) and arterial blood gas bicarbonate (45 mmol/L) likely indicates a measurement error or calculation issue rather than a true physiological difference, and recalculation using the Henderson-Hasselbalch equation should be performed to verify values. 1

Understanding the Measurement Differences

• Serum bicarbonate on a Basic Metabolic Panel (BMP) is actually measured as total CO2 content, which includes bicarbonate (HCO3-), dissolved CO2, and other minor carbon dioxide components, with bicarbonate representing approximately 96% of the total CO2 2, 3
• The term "serum bicarbonate" is not strictly accurate because the chemical methods measure all CO2 liberated from the serum, with some laboratories using the more correct term "total serum CO2" 4
• Normal discrepancies between arterial blood gas bicarbonate and serum bicarbonate are typically small (average difference of 1.20 mmol/L with 95% limits of agreement being -2.73 to +5.13 mmol/L) 5

Interpreting the Large Discrepancy

• A difference as large as 10 mmol/L between serum bicarbonate (35 mmol/L) and arterial blood gas bicarbonate (45 mmol/L) is highly unusual and suggests:
  • Potential human error in calculation of blood gas HCO3- 1
  • Possible laboratory error in one of the measurements 1
  • Samples may have been drawn at significantly different times 5

Recommended Approach to Resolving the Discrepancy

1. First step: Recalculate the blood gas bicarbonate using the Henderson-Hasselbalch equation to verify if the reported value is accurate 1

   • This has been identified as the most important initial step when encountering large discrepancies 1

2. Verify timing of sample collection:

   • Ensure both samples were drawn simultaneously or very close in time 5
   • Sequential rather than simultaneous sampling can lead to differences, especially in unstable patients 1

3. Consider clinical context:

   • Both values are significantly elevated above normal range (normal: 22-26 mmol/L), suggesting metabolic alkalosis or compensation for chronic respiratory acidosis 4, 6
   • The elevated bicarbonate levels may indicate conditions such as:
     • Chronic respiratory acidosis with renal compensation 6
     • Possible Obesity Hypoventilation Syndrome if patient is obese 4
     • Primary metabolic alkalosis 6

Clinical Implications

• Both values indicate significant acid-base disturbance requiring further evaluation 6
• If verified, the elevated bicarbonate suggests chronic respiratory acidosis with renal compensation, as the kidneys respond to chronic respiratory acidosis by increasing serum bicarbonate level 4
• A serum bicarbonate >27 mmol/L in obese patients should prompt consideration of arterial blood gas measurement to confirm hypercapnia 4, 2

Common Pitfalls to Avoid

• Do not assume the arterial blood gas value is always more accurate; both measurements can have errors 1
• Avoid making clinical decisions based on discrepant values without verification 1
• Remember that the RoMe technique (Respiratory opposite, Metabolic equal) can help interpret acid-base status once accurate values are obtained 7
• Don't overlook the possibility of mixed acid-base disorders which can complicate interpretation 6