How do you interpret a difference in bicarbonate (HCO3) levels between arterial blood gas (ABG) and serum electrolyte panel results?

Interpreting Differences Between Bicarbonate Levels in ABG and Serum

When there is a difference between bicarbonate levels measured in arterial blood gas (ABG) and serum electrolyte panels, the serum bicarbonate value should be considered more reliable for screening purposes, with a threshold of >27 mmol/L indicating the need for ABG confirmation of hypercapnia. 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 2

• ABG bicarbonate values are typically calculated rather than directly measured, using the Henderson-Hasselbach equation based on pH and PCO2 measurements, which can introduce calculation errors 3

• Significant differences between serum total CO2 and calculated ABG bicarbonate have been documented, with human calculation error identified as a potential source of these discrepancies 3

Clinical Implications and Recommendations

When to Trust Serum Bicarbonate vs. ABG Values

• For screening purposes (such as for Obesity Hypoventilation Syndrome), serum bicarbonate levels are more reliable, with a threshold of >27 mmol/L indicating the need for ABG confirmation 1

• A serum bicarbonate <27 mmol/L has a very high negative predictive value (99.0%; 95% CI, 97.9–99.6%) for ruling out conditions like OHS, potentially eliminating the need for arterial blood sampling 1

• When critical decisions need to be made about acid-base status, ABG values should be used as they provide direct measurement of pH and PCO2, allowing for more accurate assessment of the primary disorder and compensation 4

Common Pitfalls to Avoid

• Do not rely solely on ABG sodium values for clinical decision-making, as significant differences have been documented between ABG and auto-analyzer sodium measurements 5, 6

• Potassium values from ABG and auto-analyzer measurements show better correlation and can be more reliably used for clinical decisions 6

• Time delays in processing samples can significantly affect measurements - blood samples should be analyzed within 15 minutes of collection to minimize pre-analytical errors 7

• Temperature affects sample stability - samples stored at room temperature show faster and greater changes in pH, PaO2, PaCO2, and electrolytes compared to samples stored on ice 7

Algorithm for Interpreting Bicarbonate Discrepancies

1. If screening for conditions like OHS:

   • Use serum bicarbonate with a threshold of 27 mmol/L 1
   • If serum bicarbonate <27 mmol/L, OHS is highly unlikely 1
   • If serum bicarbonate >27 mmol/L, proceed with ABG to confirm hypercapnia 1

2. If assessing acute acid-base disorders:

   • Rely primarily on ABG values for pH, PCO2, and calculated HCO3- 4
   • If large discrepancies exist between serum and ABG bicarbonate:
     • Consider recalculating the ABG bicarbonate using the Henderson-Hasselbach equation 3
     • Check sample collection timing and storage conditions 7
     • Consider the possibility of laboratory error 3

3. For electrolyte management:

   • Trust potassium values from either ABG or serum measurements 6
   • Rely on serum sodium rather than ABG sodium measurements 5, 6

By following this approach, clinicians can appropriately interpret differences between bicarbonate measurements and make sound clinical decisions based on the most reliable values for the specific clinical context.