How to Get Your Bicarbonate Level Checked
Bicarbonate levels are routinely obtained through a standard venous blood draw for a Basic Metabolic Panel (BMP) or Comprehensive Metabolic Panel (CMP), which measures total CO2 content (of which bicarbonate comprises approximately 96%). 1, 2
Primary Method: Serum Bicarbonate via Basic Metabolic Panel
Order a Basic Metabolic Panel (BMP) or Comprehensive Metabolic Panel (CMP) through standard venous blood draw—this is the most practical and least invasive method for obtaining bicarbonate levels 1, 2
The value reported as "serum bicarbonate" on these panels is actually total CO2 content, measured using chemical methods that capture all CO2 liberated from the serum 1, 2
Venous sampling is sufficient and preferred over arterial sampling for bicarbonate measurement in most clinical scenarios, as it avoids the risks of arterial puncture (thrombosis, arteriovenous fistula, pseudoaneurysms, hematoma) 3
Alternative Method: Arterial Blood Gas (ABG)
Bicarbonate can also be obtained from an arterial blood gas analysis, though this requires arterial puncture and is typically reserved for situations requiring simultaneous assessment of oxygenation and ventilation 4
Venous and arterial bicarbonate values show close agreement (average difference 1.20 mmol/L with 95% limits of agreement -2.73 to +5.13 mmol/L), making venous measurement an acceptable substitute for arterial in most cases 5
Clinical Context for Testing
For screening purposes (such as evaluating for Obesity Hypoventilation Syndrome), serum bicarbonate from a BMP is more reliable and practical than ABG 1
The American Thoracic Society recommends using a serum bicarbonate threshold of 27 mmol/L to determine whether arterial blood gas measurement is needed in patients with suspected respiratory conditions 4
A serum bicarbonate <27 mmol/L has a 99% negative predictive value for ruling out conditions like Obesity Hypoventilation Syndrome, potentially eliminating the need for arterial sampling 1
Important Caveats
Sequential rather than simultaneous sampling can lead to differences between serum and ABG bicarbonate values, especially in unstable patients—verify timing of sample collection if discrepancies arise 1
While total CO2 and calculated bicarbonate from ABG correlate well (r = 0.977), they may not be interchangeable at extreme values, particularly at lower bicarbonate levels 6
Serum bicarbonate should be monitored monthly in patients with chronic kidney disease or on maintenance dialysis 1