What is the Carboxyhaemoglobin Test?

The Carboxyhaemoglobin (COHb) test measures the percentage of haemoglobin in the blood that is bound to carbon monoxide instead of oxygen. Carbon monoxide binds to haemoglobin with much higher affinity than oxygen, significantly reducing the blood’s ability to transport oxygen to tissues.

Carboxyhaemoglobin forms after inhalation of carbon monoxide from sources such as smoke, faulty heating systems, vehicle exhausts or environmental exposure. Even low-level exposure can cause symptoms, while higher levels may be life-threatening.

This test provides an objective measure of carbon monoxide exposure and is essential in both acute poisoning and chronic low-level exposure assessment.

Why do I need a test for Carboxyhaemoglobin?

You may need this test if carbon monoxide exposure is suspected, or if you have unexplained symptoms such as headaches, dizziness or fatigue. Because symptoms are often non-specific, carbon monoxide poisoning can be overlooked without blood testing.

The test is also commonly used to assess smoking exposure, as smokers typically have higher baseline carboxyhaemoglobin levels. It is valuable in occupational health, emergency assessment and environmental exposure investigations.

Symptoms and scenarios where testing is recommended:

• Persistent headaches
• Dizziness or light-headedness
• Nausea or vomiting
• Shortness of breath
• Fatigue or confusion
• Suspected carbon monoxide exposure
• Faulty boilers or heating systems
• Fire or smoke inhalation
• Occupational exposure (drivers, mechanics)
• Smoking exposure assessment

Benefits of the Carboxyhaemoglobin Test

• Confirms carbon monoxide exposure
  Provides objective biochemical evidence of exposure.
• Assesses oxygen transport impairment
  Identifies reduced oxygen delivery to tissues.
• Supports diagnosis of carbon monoxide poisoning
  Essential in emergency and clinical settings.
• Useful for smoking exposure assessment
  Differentiates smoker and non-smoker baseline levels.
• Detects chronic low-level exposure
  Helps identify environmental or workplace risks.
• Guides urgent medical intervention
  High levels may require immediate treatment.
• Supports occupational and environmental investigations
  Useful in workplace safety and housing assessments.
• Simple and rapid blood test
  Requires only a venous blood sample.

Step-by-Step: How Testing Works

Choose your preferred testing method

You can attend one of our UK partner clinics or arrange a home or hotel visit with a mobile phlebotomist.

Option 1 – In-Clinic Blood Draw

1. Select your nearest UK clinic and book an appointment.
2. A clinician or phlebotomist collects a venous blood sample.
3. The sample is analysed for carboxyhaemoglobin percentage.
4. Results are delivered securely online.

Option 2 – Home or Hotel Phlebotomy (+£60)

1. Book a UK-wide home or hotel visit at a convenient time.
2. A trained phlebotomist collects your blood sample on-site.
3. The sample is packaged and transported promptly to the laboratory.
4. Results are uploaded digitally without the need to travel.

Access your report

You can download and share your results with your GP, occupational health clinician or emergency provider.

Your Results Explained

Low Levels

Low carboxyhaemoglobin levels indicate minimal or no carbon monoxide exposure.
Typical UK reference range:

• Non-smokers: < 2%
• Smokers: up to 5%

Normal / Expected Levels

Expected results reflect baseline exposure appropriate for smoking status.
This suggests no clinically significant carbon monoxide exposure.

High Levels

Elevated carboxyhaemoglobin indicates recent or ongoing carbon monoxide exposure.
Levels above 10% may cause symptoms, while higher levels increase risk of serious toxicity.
Urgent medical assessment may be required depending on severity.