Dyshemoglobins and Pulse Oximetry: Understanding COHb and MetHb Effects in Modern Device Validation

Dyshemoglobins and Pulse Oximetry: How COHb and MetHb Influence Measurement Accuracy 

Pulse oximetry has become central to physiological monitoring across wearables, medical devices, and clinical research studies. Yet even the most advanced sensor architectures face fundamental challenges when hemoglobin exists in altered forms—known as dyshemoglobins. Carboxyhemoglobin (COHb) and methemoglobin (MetHb) change light absorption in ways that directly influence SpO₂ readings and algorithm behavior. 

For engineering teams preparing for FDA and CE mark pathways, understanding the impact of dyshemoglobins is critical for protocol development, accuracy expectations, and regulator-ready data strategies. 

This post breaks down how COHb and MetHb interact with pulse oximetry signals and how PRL incorporates this knowledge into robust physiological monitoring research and medical device validation studies. 

What Are Dyshemoglobins? 

Hemoglobin normally carries oxygen in the form of oxyhemoglobin (HbO₂) and deoxygenated hemoglobin (HHb). Dyshemoglobins are modified forms of hemoglobin that cannot bind oxygen normally and absorb light differently, which complicates SpO₂ measurement.  

Common Dyshemoglobins 

• Carboxyhemoglobin (COHb): Formed when carbon monoxide binds to hemoglobin. 

• Methemoglobin (MetHb): Hemoglobin with iron in the ferric (Fe³⁺) state, reducing its ability to carry oxygen. 

  
  

Although dyshemoglobins occur in specific physiological scenarios, their optical signatures are essential considerations when designing or validating pulse oximetry systems in a hypoxia laboratory. These different species of hemoglobin can be accurately analyzed by co-oximetry using arterial blood gas samples. 

How Dyshemoglobins Challenge Pulse Oximetry Technology 

Pulse oximeters estimate oxygen saturation using two wavelengths of light: red and infrared. Dyshemoglobins absorb these wavelengths differently than standard hemoglobin species. 

Effect on Light Absorption 

• COHb absorbs light similarly to HbO₂, which may cause a device to overestimate oxygen saturation. 

• MetHb absorbs red and infrared light more evenly, which often causes readings to move toward a mid-range value, regardless of true arterial oxygenation. 

  
  

  

These spectral properties reduce the ability of the device to differentiate hemoglobin species, creating challenges for pulse oximetry systems. 

Impact on SpO₂ Accuracy Measurements 

Pulse oximetry algorithms assume that hemoglobin exists primarily as HbO₂ and HHb. When COHb or MetHb are present in meaningful proportions: 

• The ratio-of-ratios calculation becomes less reliable 

• The relationship between optical signals and arterial oxygen saturation (SaO₂) degrades 

• The potential for systematic bias increases 

  
  

These behaviors are discussed extensively in the literature and community initiatives, such as the Open Oximetry Project, which aim to improve transparency and strengthen validation practices across the field. 

Why Dyshemoglobin Awareness Matters in Device Validation 

Even when a device is not intended to measure dyshemoglobins, understanding their influence is essential for: 

1. Robust Algorithm Development 

Engineering teams must develop a robust empirical calibration curve (R-value) of their pulse oximetry systems. Development can be performed in a hypoxia lab before the final or pivotal medical device validation study. 

2. Study Protocol Alignment 

Regulator-ready clinical research studies require protocols that define expected physiological ranges and testing limitations. PRL supports trial protocol alignment that reflects consensus scientific understanding, such as suggested by the Open Oximetry Project, GCP compliance, and ISO 14155 requirements. 

3. Accurate Interpretation of Controlled Desaturation Data 

In a controlled desaturation or hypoxia lab environment, dyshemoglobins may influence SpO₂ trends, especially when evaluating device responsiveness to changing SaO₂ values. Following ISO 80601-2-61, COHb or MetHb are reported utilizing co-oximetry. Data may be excluded from the validation analysis if their concentration exceeds the predefined limits.  

4. Risk Communication in FDA Submissions 

FDA clinical trial strategy often includes demonstrating awareness of known performance limitations and ensuring pulse oximetry data is transparent and well-documented. 

How PRL Supports Sponsors Navigating Dyshemoglobin Complexity 

PRL does not manipulate dyshemoglobin levels, but our team provides clinical and scientific expertise that helps sponsors: 

• Strengthen pulse oximetry development and study methodologies 

• Evaluate algorithm performance across clinically relevant ranges (100-70% SaO₂) 

• CRO transparency, monitoring, and risk mitigation 

• Build regulator-ready clinical endpoints aligned with modern guidelines 

  
  

Our pulse oximetry studies lab integrates inclusive participant recruitment using the Monk scale, controlled desaturation workflows, and ISO-aligned operational rigor to generate high-quality physiological monitoring data. 

Conclusion: Understanding Dyshemoglobins Leads to Better Pulse Oximetry Design and Validation 

COHb and MetHb introduce predictable but significant challenges to optical measurement. For CTOs, R&D engineers, and regulatory leads, incorporating dyshemoglobin awareness into design reviews, validation planning, and submission strategies strengthens both device performance and regulatory confidence. 

Parameters Research Laboratory helps MedTech teams navigate these medical device validation complexities with scientific clarity, protocol expertise, and regulator-ready data generation. 

If your team is preparing a pulse oximetry study or needs support refining a validation strategy, PRL can assist. Contact us today!

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