How are oxyhemoglobin, carboxyhemoglobin, deoxyhemoglobin, and methemoglobin differentiated on extended blood gases (Arterial Blood Gases, ABG)?

Differentiation of Hemoglobin Species on Extended Blood Gases

Oxyhemoglobin, carboxyhemoglobin, deoxyhemoglobin, and methemoglobin are differentiated on extended blood gases by absorbance at specific wavelengths. 1

Principles of Hemoglobin Species Differentiation

Modern blood gas analyzers use co-oximetry, which is a spectrophotometric technique that differentiates hemoglobin species based on their unique light absorption characteristics:

• Each hemoglobin species (oxyhemoglobin, deoxyhemoglobin, carboxyhemoglobin, and methemoglobin) has a distinct absorption spectrum in the visible light range
• Co-oximeters use multiple wavelengths of light (typically in the 500-600 nm region) to measure the absorbance patterns 2, 3
• The analyzer then uses these absorbance measurements to calculate the percentage of each hemoglobin species present

Technical Aspects of Co-oximetry

• Co-oximeters employ narrow bands of light at discrete wavelengths to detect the small differences in light absorption between hemoglobin derivatives 3
• The measurement is based on the Lambert-Beer law, which relates the absorption of light to the properties of the material through which the light is traveling 3
• Modern analyzers can measure total hemoglobin concentration and percentages of each hemoglobin species simultaneously within about 60 seconds 4

Clinical Significance and Limitations

Importance in Critical Care

• Regular pulse oximetry cannot differentiate between oxyhemoglobin and carboxyhemoglobin, which can lead to falsely elevated oxygen saturation readings 5
• This limitation is particularly important in carbon monoxide poisoning, where a normal or high oximetry reading should be disregarded 5
• As noted in the BTS guideline: "A normal or high oximetry reading should be disregarded because saturation monitors cannot differentiate between carboxyhaemoglobin and oxyhaemoglobin, owing to their similar absorbances." 5

Diagnostic Value

• Co-oximetry is essential for diagnosing methemoglobinemia, where measurement of methemoglobin levels using blood gas measurement is a key test 5
• In cases of suspected methemoglobinemia, enzyme activity can be measured using ferricyanide as a substrate and monitoring oxidation of NADH at 340 nm 5
• The technique allows for rapid clinical decisions in critical situations by providing simultaneous information about acid-base status, ventilation, oxygenation, and abnormal hemoglobin species 1

Quality Control Considerations

• Daily calibration of blood gas analyzers with co-oximetry capability is essential to ensure accurate and reproducible measurements 1
• Meticulous attention to blood sampling technique is required to avoid pre-analytical errors 1
• For forensic applications, this technique can help determine causes of death in cases of asphyxia or exposure to toxic gases 6

Common Pitfalls

• Failure to use co-oximetry when abnormal hemoglobin species are suspected can lead to misdiagnosis
• Regular pulse oximetry is inadequate for detecting carboxyhemoglobin or methemoglobin 5
• In carbon monoxide poisoning, blood gas PO₂ will appear normal despite tissue hypoxia, making co-oximetry essential for diagnosis 5
• Poor calibration of equipment can lead to inaccurate results and misdiagnosis of critical conditions

In conclusion, the differentiation of hemoglobin species on extended blood gases relies on the principle of absorbance at specific wavelengths through co-oximetry, which is the only reliable method to distinguish between these clinically important hemoglobin derivatives.