Normal Bicarbonate Value in Arterial Blood Gas
The normal bicarbonate (HCO3-) range in arterial blood gas analysis is 22-26 mmol/L, though some sources cite a slightly broader range up to 27 mmol/L. 1, 2, 3
Standard Reference Range
- The normal pH range of blood is 7.35-7.45, and bicarbonate plays a critical role in maintaining this acid-base balance 1
- Bicarbonate values of 22-26 mmol/L are considered normal for most patients, with values below 22 mmol/L indicating metabolic acidosis and values above 26-27 mmol/L suggesting metabolic alkalosis or chronic respiratory compensation 2, 3
Clinical Context and Thresholds
Screening Applications
- A serum bicarbonate threshold of 27 mmol/L is clinically significant for screening purposes, particularly in conditions like Obesity Hypoventilation Syndrome 1, 2
- Values <27 mmol/L have a very high negative predictive value (99.0%; 95% CI, 97.9-99.6%) for ruling out chronic hypercapnic conditions, potentially eliminating the need for arterial blood sampling 2
- Bicarbonate >27 mmol/L in obese patients should prompt arterial blood gas measurement to confirm hypercapnia 1, 2, 3
Therapeutic Targets
- For patients with chronic kidney disease or on maintenance dialysis, serum bicarbonate should be maintained at or above 22 mmol/L to prevent complications of metabolic acidosis 2, 3
- Low bicarbonate levels (<22 mmol/L) in these patients warrant oral bicarbonate supplementation 2, 3
Important Measurement Considerations
ABG vs. Serum Bicarbonate
- Bicarbonate on ABG is calculated from pH and pCO2, while serum bicarbonate on a Basic Metabolic Panel is measured as total CO2 content 2, 4
- Total CO2 includes bicarbonate (approximately 96% of total), dissolved CO2, and other minor carbon dioxide components 2
- Strong positive correlations exist between calculated ABG and measured serum bicarbonate values across various pH ranges 5
Common Pitfalls
- Sequential rather than simultaneous sampling can lead to discrepancies, especially in unstable patients 2, 3
- Some ABG analyzers have a maximum display limit of 60 mmol/L; values above this may not be displayed and require manual calculation 4
- Patients with chronic respiratory disease treated with furosemide or dexamethasone may have bicarbonate values exceeding the analyzer's display range 4
Interpretation Framework
Compensated vs. Uncompensated States
- Elevated bicarbonate (>27 mmol/L) with normal pH suggests chronic respiratory acidosis with full renal compensation, as kidneys respond to chronic hypercapnia by increasing bicarbonate retention 2, 6
- In acute respiratory acidosis, bicarbonate may be normal or only slightly elevated because renal compensation takes hours to days to develop 1
- The standard bicarbonate (HCO3 standard) represents bicarbonate concentration at a normal pCO2 of 40 mmHg, making it a better indicator of the metabolic component independent of respiratory influences 3