When interpreting an arterial blood gas, should I use the calculated bicarbonate from the ABG or the standard serum bicarbonate?

Standard vs Calculated HCO3 on ABG: Which to Use

Use the calculated bicarbonate from the ABG for acute acid-base assessment and immediate clinical decision-making, but recognize that serum bicarbonate (total CO2) from a basic metabolic panel is more reliable for screening chronic conditions and has superior negative predictive value for ruling out metabolic disorders. 1

Understanding the Key Differences

The two measurements are fundamentally different:

• ABG bicarbonate is calculated from pH and PaCO2 using the Henderson-Hasselbalch equation, representing the bicarbonate concentration at the exact moment of arterial sampling 2, 3

• Serum bicarbonate (reported as total CO2 on BMP) is directly measured and includes bicarbonate (96% of total), dissolved CO2, and other minor carbon dioxide components 1, 4

• Standard bicarbonate represents the bicarbonate concentration normalized to a PaCO2 of 40 mmHg, making it a better indicator of the metabolic component independent of respiratory influences 4

Clinical Decision Algorithm

For Acute Acid-Base Disorders (ICU/Emergency Settings)

Use ABG calculated bicarbonate when:

• Managing critically ill patients requiring immediate assessment of pH, PaCO2, and metabolic status 5, 6
• Evaluating patients with shock or hypotension (systolic BP <90 mmHg) 5
• Titrating oxygen therapy in patients at risk for hypercapnic respiratory failure 5
• Assessing response to acute interventions or ventilator changes 6

The ABG provides real-time, simultaneous assessment of all three critical parameters (pH, PaCO2, HCO3-) from a single sample 7. Research demonstrates strong correlation (r = 0.91, p < 0.0001) between calculated ABG bicarbonate and measured serum bicarbonate 8.

For Screening and Chronic Conditions

Use serum bicarbonate (total CO2 from BMP) when:

• Screening obese patients with sleep-disordered breathing for obesity hypoventilation syndrome—a threshold of >27 mmol/L indicates need for ABG confirmation 5, 1
• A serum bicarbonate <27 mmol/L has 99.0% negative predictive value (95% CI: 97.9-99.6%) for ruling out OHS, eliminating the need for arterial sampling 1
• Monitoring chronic kidney disease patients—serum bicarbonate should be checked monthly and maintained at 22-26 mmol/L 1, 4
• Evaluating stable outpatients where arterial puncture is not practical 1

Important Clinical Caveats

When Values Diverge

Sequential rather than simultaneous sampling is the most common cause of discrepancy, especially in unstable patients 1. If you observe a significant difference:

• Verify the timing of sample collection 1
• In unstable patients, prioritize the ABG value for immediate management 7
• Consider that elevated serum bicarbonate (>27 mmol/L) with normal ABG bicarbonate may indicate chronic respiratory acidosis with renal compensation 1

Treatment Thresholds

Research shows clinicians are more likely to treat acidemia based on different thresholds:

• pH >7.1: Bicarbonate treatment less likely regardless of which bicarbonate value is used 7
• pH 6.9-7.0: Bicarbonate treatment more likely, with stronger correlation between ABG and serum values in this range 7

Sodium bicarbonate administration should be limited to severe acidosis (pH <7.1 and base deficit <-10) or special circumstances like hyperkalemia or tricyclic antidepressant overdose 1, 4. The best method of reversing acidosis in cardiac arrest is restoring spontaneous circulation, not administering buffers 1.

Common Pitfalls to Avoid

• Don't rely on pulse oximetry alone: Normal SpO2 does not exclude significant acid-base disturbances or abnormal bicarbonate levels 5, 4, 6
• Don't ignore the clinical context: A patient with chronic respiratory disease on furosemide or dexamethasone may have bicarbonate values exceeding the analyzer's display range (>60 mmol/L) 3
• Don't use ABG bicarbonate for long-term monitoring: Serum bicarbonate is more practical and reliable for tracking chronic conditions 1, 4

Special Populations

COPD and Chronic Respiratory Disease

• Patients with baseline hypercapnia should have ABG monitoring after each oxygen titration to detect respiratory acidosis and worsening hypercapnia 5
• These patients may develop compensatory elevation in serum bicarbonate that is clinically appropriate 1

Obesity Hypoventilation Syndrome

• Use serum bicarbonate >27 mmol/L as a screening threshold to determine who needs ABG confirmation 5
• This approach eliminates unnecessary arterial punctures in 64-74% of obese patients with OSA 5