What does a serum CO2 (carbon dioxide) level of 43 indicate?

Serum CO2 of 43 mEq/L Indicates Metabolic Alkalosis or Chronic Compensated Respiratory Acidosis

A serum total CO2 (bicarbonate) level of 43 mEq/L is significantly elevated and indicates either primary metabolic alkalosis or chronic compensated respiratory acidosis with renal bicarbonate retention. This value is well above the normal range of 23-30 mEq/L at sea level and requires immediate clinical correlation with arterial blood gas analysis to determine the underlying acid-base disorder 1.

Clinical Interpretation

Understanding the Measurement

• Serum total CO2 (TCO2) measured on basic metabolic panels is primarily bicarbonate (HCO3-) plus a small amount of dissolved CO2 1
• Normal range is 23-30 mEq/L at sea level; values outside this range indicate acid-base disturbance 1
• A level of 43 mEq/L represents marked elevation requiring urgent evaluation 2

Two Primary Diagnostic Possibilities

1. Chronic Compensated Respiratory Acidosis (Most Common)

• Seen in patients with chronic CO2 retention, particularly COPD 2
• The kidneys retain bicarbonate over hours to days to buffer chronic elevation in PaCO2 2
• Results in high PaCO2, high bicarbonate, and near-normal pH ("compensated respiratory acidosis") 2
• In COPD patients, 47% have PaCO2 >45 mmHg, and chronic retention leads to compensatory bicarbonate elevation 2

2. Primary Metabolic Alkalosis

• Less common with this degree of elevation
• Would show elevated pH, elevated bicarbonate, and compensatory mild respiratory acidosis 2
• Causes include volume depletion, diuretic use, vomiting, or mineralocorticoid excess

Immediate Clinical Actions

Obtain Arterial Blood Gas Analysis

This is the critical next step to differentiate the underlying disorder 2:

• If pH is normal or mildly acidotic (7.35-7.40) with elevated PaCO2 (>46 mmHg): chronic compensated respiratory acidosis 2
• If pH is elevated (>7.45) with normal or mildly elevated PaCO2: primary metabolic alkalosis 2
• If pH is low (<7.35) with very high PaCO2: acute-on-chronic respiratory acidosis requiring urgent intervention 2

Assess for COPD or Chronic Respiratory Disease

• Patients >50 years, long-term smokers with chronic breathlessness on exertion should be assumed to have COPD 2
• These patients are at high risk for CO2 retention and may have chronic compensated respiratory acidosis 2
• Check for history of chronic lung disease, neuromuscular disorders, or obesity hypoventilation syndrome 3

Critical Oxygen Management Considerations

If the patient is receiving supplemental oxygen and has COPD or suspected chronic CO2 retention:

• Target oxygen saturation of 88-92% rather than normal ranges 2
• Excessive oxygen (PaO2 >75 mmHg or 10 kPa) in CO2 retainers increases risk of worsening respiratory acidosis 2
• Use controlled oxygen delivery via Venturi mask at 28% or nasal cannula at 1-2 L/min 2
• Never abruptly discontinue oxygen if acidosis is present—step down gradually as oxygen falls faster than CO2 corrects 2

Common Pitfalls to Avoid

Do Not Rely on "Normal Range" from Many Clinical Laboratories

• Many laboratories report inappropriately wide normal ranges (as low as 18-20 mEq/L, as high as 33-35 mEq/L) 1
• This leads to missed "hidden" acid-base disorders 1
• A value of 43 mEq/L is abnormal regardless of laboratory reference range 1

Do Not Assume Adequate Oxygenation Without Blood Gas

• Pulse oximetry may appear normal even with significant hypercapnia 2
• Type 2 respiratory failure (hypercapnia) can exist with normal oxygen saturation 2
• Hypercapnia (PaCO2 >46 mmHg) is actually more common than pure hypoxemia in hospitalized patients 2

Recognize Chronic Kidney Disease Context

• In CKD stages 3-5, serum TCO2 should be maintained >22 mEq/L to prevent metabolic acidosis complications 2
• However, a value of 43 mEq/L far exceeds this target and represents a different pathologic process 2
• Chronic metabolic acidosis in CKD contributes to bone disease and requires alkali supplementation, but this patient has the opposite problem 2

Risk Stratification

High-Risk Features Requiring Urgent Intervention

• Altered mental status, confusion, or decreased consciousness (suggests severe hypercapnia or acidosis) 2
• Respiratory rate <12 or >30 breaths/minute 3
• Use of accessory muscles or paradoxical breathing 3
• If arterial pH <7.25 with elevated PaCO2: severe respiratory acidosis requiring possible ventilatory support 2

Moderate Risk

• Known COPD with acute exacerbation symptoms 2
• Recent increase in oxygen requirements 2
• New or worsening dyspnea 3