Serum CO2 of 38: Interpretation and Management
A serum CO2 of 38 mEq/L indicates metabolic alkalosis, as this value is significantly elevated above the normal range of 22-26 mEq/L (or 23-30 mEq/L by more stringent criteria). 1, 2
Understanding the Measurement
- The "CO2" on a basic metabolic panel actually measures total serum CO2, which consists of 96% bicarbonate and only 4% dissolved CO2, making it essentially a bicarbonate measurement 1
- Normal serum CO2 range is 22-26 mEq/L according to standard guidelines, though more recent evidence suggests tightening this to 23-30 mEq/L to avoid missing acid-base disorders 3, 1, 2
- A value of 38 mEq/L represents a substantial elevation of 12-16 mEq/L above the upper limit of normal 1, 2
Primary Differential Diagnosis
The elevated CO2 of 38 indicates one of two scenarios:
1. Primary Metabolic Alkalosis (Most Common)
- Contraction alkalosis from diuretic use is the most frequent cause in hospitalized patients, resulting from chloride depletion and volume contraction 1, 4
- Vomiting or nasogastric suction causing hydrogen ion loss 4, 5
- Excessive alkali administration 4, 5
2. Compensatory Response to Chronic Respiratory Acidosis
- In chronic respiratory acidosis (from COPD, obesity hypoventilation syndrome, neuromuscular disease, or chest wall deformities), the kidneys retain bicarbonate as compensation for chronically elevated CO2 3, 1, 6
- Critical distinction: In compensatory metabolic alkalosis, the elevated bicarbonate is secondary to the primary respiratory problem, not the primary disorder itself 6, 7
Diagnostic Algorithm
To differentiate between these two scenarios, assess the following in order:
Check arterial blood gas (ABG) to determine:
Evaluate clinical context:
Assess volume status:
- Signs of volume depletion (orthostatic hypotension, decreased skin turgor, elevated BUN/creatinine ratio) support diuretic-induced contraction alkalosis 1
Management Approach
For Primary Metabolic Alkalosis:
- Reduce or temporarily hold diuretics if bicarbonate rises significantly above 30 mEq/L and patient is volume depleted 1
- Replete chloride and volume with normal saline to restore volume and provide chloride, which allows the kidneys to excrete excess bicarbonate 1, 4
- Monitor electrolytes, particularly potassium, as hypokalemia often coexists with metabolic alkalosis 4
For Compensatory Metabolic Alkalosis (Secondary to Chronic Respiratory Acidosis):
- Do NOT attempt to correct the elevated bicarbonate, as it is a necessary compensatory mechanism maintaining near-normal pH 6
- Focus on managing the underlying respiratory disorder (optimize bronchodilators, consider non-invasive ventilation, treat obesity hypoventilation) 3, 1
- If supplemental oxygen is needed, target SpO2 of 88-92% to avoid worsening hypercapnia 1, 6
- Avoid excessive oxygen therapy that could disrupt the compensatory mechanism 6
Critical Pitfalls to Avoid
- Do not assume the elevated CO2 is "normal" for the patient without proper evaluation, as this may represent a new or worsening acid-base disorder 2
- Do not treat compensatory metabolic alkalosis in patients with chronic respiratory acidosis, as lowering bicarbonate will worsen acidemia 6
- Do not overlook "hidden" acid-base disorders: Many laboratories report inappropriately wide normal ranges for CO2 (as low as 18-20 mEq/L), which can delay diagnosis of clinically significant disorders 2
- In patients receiving diuretics with rising CO2, do not continue aggressive diuresis without addressing the developing contraction alkalosis 1
When to Obtain Arterial Blood Gas
Order an ABG if: 1
- Patient has respiratory symptoms (dyspnea, altered mental status)
- Bicarbonate rises above 35 mEq/L during diuresis
- Known COPD, obesity hypoventilation syndrome, or neuromuscular disease affecting respiration
- Clinical uncertainty about whether this represents primary metabolic alkalosis versus compensatory response