Blood Test for Carbon Monoxide Exposure Confirmation
Measure carboxyhemoglobin (COHb) level by laboratory co-oximetry using spectrophotometry on either arterial or venous blood—this is the definitive test to confirm carbon monoxide poisoning. 1
Recommended Testing Method
Laboratory-based co-oximetry via spectrophotometry is the gold standard for confirming CO exposure. 1 This method directly measures the concentrations of oxyhemoglobin, deoxyhemoglobin, carboxyhemoglobin, and methemoglobin by analyzing differential absorbance at various wavelengths. 1
Blood Sample Type
- Either arterial or venous blood may be used, as COHb levels are similar in both when CO body stores are in near equilibrium with alveolar CO. 1
- Venous sampling is often preferred for practical reasons (easier access, less painful) and provides equivalent diagnostic information. 1
Diagnostic Thresholds
Elevated COHb levels that confirm exposure beyond normal physiological ranges are: 2
Critical Testing Pitfalls to Avoid
Standard Pulse Oximetry Cannot Detect CO Poisoning
Standard two-wavelength pulse oximeters (660 and 990 nm) cannot differentiate carboxyhemoglobin from oxyhemoglobin. 1 COHb and oxyhemoglobin have similar absorbance at 660 nm, causing pulse oximeters to read COHb as if it were oxyhemoglobin. 1 Patients with COHb levels ≥25% routinely show falsely reassuring SpO2 readings >90%. 1
Older Blood Gas Analyzers May Report False Values
Blood gas machines without integrated co-oximeters calculate oxygen saturation from PaO2 and pH using algorithms, completely ignoring COHb presence. 1 A patient with 40% COHb and PaO2 100 mmHg would be reported as having 97-98% oxygen saturation when the true oxygen-carrying capacity is only 60%. 1
Timing Issues Can Produce False-Negative Results
COHb levels may appear low or normal if significant time has elapsed since exposure or if supplemental oxygen has already been administered. 1 A low measured COHb does NOT exclude clinically significant toxicity from the initial exposure. 1
Noninvasive Pulse CO-Oximetry Limitations
Fingertip pulse CO-oximetry (available since 2005) has insufficient sensitivity for definitive diagnosis. 1 In the highest-quality study, noninvasive pulse CO-oximetry demonstrated only 48% sensitivity (95% CI 27-69%) for detecting COHb ≥15%, with agreement exceeding acceptable ranges in 33% of patients. 1
If pulse CO-oximetry is used for initial screening, laboratory-based spectrophotometry confirmation is mandatory before making treatment decisions, especially regarding hyperbaric oxygen therapy. 1
Clinical Management Algorithm
Immediate Action (Before Test Results)
Administer 100% oxygen immediately to any patient with suspected CO poisoning—do not wait for COHb confirmation. 1 High-flow oxygen by mask or endotracheal tube is front-line treatment and should never be delayed. 1
Interpretation Framework
COHb levels serve only to document exposure and do NOT predict symptom severity or clinical outcome. 1 Treatment decisions must be guided by clinical presentation and exposure history rather than absolute COHb values. 1
The pathophysiology extends far beyond simple hemoglobin binding—CO causes mitochondrial dysfunction, lipid peroxidation, and immune-mediated injury that are not reflected in COHb measurements. 2
Why PaO2 Remains Normal
Arterial PaO2 stays normal in CO poisoning because it measures dissolved oxygen in plasma, which is unaffected by CO binding to hemoglobin. 2 This creates a dangerous clinical scenario where standard blood gas values appear reassuring despite severe tissue hypoxia. 2
Essential Safety Considerations
The CO source must be identified and eliminated before patient discharge to prevent re-exposure. 1 Ambient CO measurements by emergency personnel can confirm exposure even when patient COHb is low due to elapsed time or oxygen treatment. 1