Should bicarbonate be measured in an arterial blood gas (ABG) sample or in a venous serum chemistry sample?

Bicarbonate Measurement: Venous Serum Chemistry vs. Arterial Blood Gas

For routine clinical assessment of bicarbonate levels, venous serum chemistry (total CO2) is the preferred method, reserving arterial blood gas analysis for situations requiring simultaneous assessment of pH, PaCO2, or when evaluating critically ill patients with shock or severe acid-base disturbances. 1, 2

Understanding the Difference Between Methods

Serum bicarbonate measured on chemistry panels represents "total serum CO2," where bicarbonate comprises 96% of the total, with the remainder being dissolved CO2. 3 This measurement uses chemical methods that liberate all CO2 from the serum and provides a reliable assessment of bicarbonate status for most clinical purposes. 3

When to Use Venous Serum Chemistry

Venous serum bicarbonate from chemistry panels should be the default method for:

• Screening for chronic respiratory acidosis (e.g., obesity hypoventilation syndrome), where elevated bicarbonate suggests chronic CO2 retention 3
• Routine metabolic assessment in patients without critical illness 1
• Serial monitoring during diabetic ketoacidosis (DKA) management, as venous pH and bicarbonate show acceptably narrow limits of agreement with arterial values (mean difference 1.20 mmol/L, 95% limits -2.73 to +5.13 mmol/L) 4, 5
• Initial emergency department evaluation for most patients, where venous samples provide sufficient accuracy (95% limits of agreement: 4.3 to -5.8 mmol/L) 6

The American Thoracic Society specifically recommends using serum bicarbonate levels (cutoff 27 mmol/L) to stratify which patients with suspected obesity hypoventilation syndrome require arterial blood gas confirmation. 3

When Arterial Blood Gas is Required

ABG analysis becomes necessary when you need:

• Simultaneous assessment of oxygenation, ventilation, and acid-base status in critically ill patients 1, 2
• Patients in shock or with systolic blood pressure <90 mmHg, where the initial blood gas should be arterial 7
• Confirmation of hypercapnia when serum bicarbonate is elevated (>27 mmol/L) in suspected hypoventilation syndromes 3
• Severe acidosis (pH <7.0-7.15) where bicarbonate therapy might be considered 1, 8
• Critically ill patients requiring insulin infusion, where arterial sampling from indwelling catheters is preferred over finger-stick capillary samples 9

Critical Considerations for DKA Management

For diabetic ketoacidosis specifically, venous blood gas analysis is sufficiently reliable to assess pH, bicarbonate, and guide management decisions. 1, 4 The Diabetes Care guidelines note that venous pH is typically 0.03 units lower than arterial pH, and repeat arterial blood gases are generally unnecessary once initial assessment is complete. 1

Resolution criteria for DKA (glucose <200 mg/dL, bicarbonate ≥18 mEq/L, venous pH >7.3) can be monitored using venous samples rather than repeated arterial punctures. 1

Practical Clinical Algorithm

1. Start with venous serum chemistry for bicarbonate assessment in stable patients or routine screening 3
2. Obtain arterial blood gas if:
   • Patient is critically ill, in shock, or hypotensive 2, 7
   • Simultaneous PaCO2 and PaO2 measurement needed 1, 7
   • Severe acidosis suspected (pH <7.15) 1, 8
   • Serum bicarbonate >27 mmol/L in suspected hypoventilation 3
3. Use venous blood gas as acceptable alternative for pH and bicarbonate monitoring in DKA and most emergency situations 1, 4, 6

Important Caveats

While venous and arterial bicarbonate show good agreement in most situations, discordance can occur in 8-24% of paired samples when determining specific diagnostic thresholds (e.g., DKA diagnosis or resolution criteria). 10 However, this level of disagreement rarely changes clinical management in practice. 4, 5

Avoid capillary finger-stick samples in patients with shock, on vasopressors, or with severe peripheral edema, as these show unacceptable variability compared to arterial or venous samples. 9