What is the cut-off level for diagnosis of Carbon Monoxide (CO) poisoning?

From the Guidelines

The cut-off level for diagnosis of carbon monoxide (CO) poisoning is typically considered to be a carboxyhemoglobin (COHb) level of at least 3-4% in non-smokers and at least 10% in smokers. This is based on the most recent and highest quality study available, which suggests that these levels are outside the expected physiological range 1. It's essential to note that clinical symptoms, rather than absolute COHb levels, should guide treatment decisions. Mild CO poisoning may present with headache, dizziness, and nausea at levels of 10-30%, while moderate poisoning (30-40%) can cause confusion, visual disturbances, and tachycardia. Severe poisoning (>40%) may lead to coma, seizures, and death.

Some key points to consider when diagnosing CO poisoning include:

• COHb levels may not correlate perfectly with clinical severity, especially if there's been a delay between exposure and testing
• Blood samples should be drawn as soon as possible after exposure, as COHb levels decrease once the patient is removed from the CO source and given oxygen
• Treatment should begin immediately with 100% oxygen for patients with suspected CO poisoning, regardless of their COHb level, and continue until levels fall below 5% and symptoms resolve
• Hyperbaric oxygen therapy may be considered for severe cases, pregnant patients, or those with neurological symptoms, typically when COHb levels exceed 25% 1

It's also important to note that noninvasive carboxyhemoglobin measurement can be used to accurately diagnose carbon monoxide toxicity, and cardiac testing can be used to predict morbidity or mortality in patients with acute CO poisoning 1. However, the most critical factor in diagnosis and treatment is a high index of suspicion, particularly during cold weather, in patients with acute coronary syndrome and arrhythmias, and in those with a history of recent CO exposure 1.

From the Research

Cut Off Level for Diagnosis of CO Poisoning

• The cut off level for diagnosis of CO poisoning is not explicitly stated in the provided studies, but carboxyhemoglobin (COHb) levels are used as an indicator of CO exposure.
• A study published in 1989 2 reported a mean peak COHb level of 38 mg% in patients who received hyperbaric oxygen therapy, with 88% of patients having a peak COHb level greater than 25 mg%.
• Another study published in 2005 3 found that the severity of CO poisoning varied among trials, but did not provide a specific cut off level for diagnosis.
• A case report published in 2017 4 described a patient with CO poisoning who had an initial carboxyhaemoglobin level of 2.5%, which is considered normal, but still had clinical and radiological evidence of neurological damage.

Hyperbaric Oxygen Therapy and COHb Levels

• Hyperbaric oxygen therapy is often used to treat CO poisoning, and has been shown to reduce COHb levels and improve outcomes.
• A study published in 2006 5 reported a decrease in COHb level from 26.7% to 17.2% within 1 hour of treatment with hyperbaric oxygen therapy.
• Another study published in 2007 6 found that hyperbaric oxygen therapy benefits the brain more than normobaric oxygen by improving energy metabolism, preventing lipid peroxidation, and decreasing neutrophil adherence.

Clinical Considerations

• The decision to treat CO poisoning with hyperbaric oxygen therapy is often based on clinical judgment, taking into account factors such as the severity of symptoms, COHb levels, and the presence of neurological damage.
• A study published in 1989 2 found that central nervous system dysfunction, including loss of consciousness, and/or cardiovascular dysfunction, was the criteria for hyperbaric oxygen therapy in 59% of patients, regardless of their COHb level.
• Another study published in 2005 3 found that the effectiveness of hyperbaric oxygen therapy in preventing neurological sequelae is still uncertain, and further research is needed to define its role in the treatment of CO poisoning.