Pulse CO-Oximetry Unreliable for Carbon Monoxide Diagnoses

Thomas R. Collins

October 18, 2022

BERLIN — A new analysis shows that pulse CO-oximetry is not reliable enough to be used to diagnose carbon monoxide poisoning, researchers said at the European Emergency Medicine Congress (EUSEM) 2022. Pulse CO-oximetry is a non-invasive way to detect carboxyhemoglobin, which forms in red blood cells when carbon monoxide is inhaled.

A review of the literature found that the approach, while simple and easy to do, does not identify carbon monoxide poisoning almost a quarter of the time.

"This method is not accurate enough and should not be used in clinical practice," said Mathilde Papin, MD, a researcher in the emergency department at Nantes University in Nantes, France.

Carbon monoxide poisoning is one of the world's most common causes of poisoning deaths. When people are exposed to the colorless, odorless carbon monoxide, it enters their bloodstream and attaches to hemoglobin, a transporter of oxygen, thereby starving the body of oxygen. It can be treated with oxygen, but the problem is that its symptoms can be similar to infections, such as the flu, and can be difficult to diagnose.

A blood test is available, measuring the amount of hemoglobin that's attached to carbon dioxide. But, the researchers said, a test is needed that can be done quickly right in the ambulance or emergency room.

"A blood test is reliable, but not practical," Papin said.

One quick and simple method is pulse CO-oximetry, in which a device is placed on the finger. A lower level of oxygen saturation could be a sign of oxygen displacement due to carbon monoxide poisoning. However, the approach has delivered mixed results.

Researchers led a systematic review and meta-analysis and searched for all trials that compared pulse oximetry with blood tests. They found 19 and were able to combine the results of 11 of them, which amounted to data on more than 2000 people, some with carbon monoxide poisoning and some healthy.

Pulse CO-oximetry had a sensitivity of 77% and a specificity of 83%, with an overall accuracy of 86%. Papin said this is not good enough, pointing to the relatively low sensitivity.

"At 23%, the false negative rate with pulse oximetry is too high for reliably triaging patients with suspected carbon monoxide poisoning," she said. 

Papin noted that the findings were not all that surprising, given the experience at her own center.

"The idea of this meta-analysis came from the clinical impression that this device was not as accurate as it should be in everyday use in our ED," she said. Pulse oximetry is also routinely used in pre-hospital care, she said, adding to the importance of finding an alternative approach. Papin and her team are now planning to evaluate a method of faster screening of carbon monoxide levels in the capillaries.

James Chenoweth, MD, assistant professor of emergency medicine at the University of California, Davis, who has researched carbon monoxide treatment, said the findings show how the clinical value of pulse oximetry is limited in this context.

"In the right clinical setting" — such as smoking inhalation — "a low oxygen saturation on pulse CO-oximetry may suggest the need for definitive testing for CO poisoning, but it can't be used to definitively rule in or out the presence of carboxyhemoglobin," he said. "I wouldn't say that it should not be used, but clinicians should be aware of the potential pitfall of being falsely reassured if the pulse CO-oximetry is normal."

This study did not receive any specific funding. Papin and Chenoweth have disclosed no relevant financial relationships.

European Emergency Medicine Congress (EUSEM) 2022: Abstract OA085. Presented October 18, 2022.

Tom Collins is a freelance writer in South Florida who has written about medical topics from nasty infections to ethical dilemmas, runaway tumors to tornado-chasing doctors. He travels the globe gathering conference health news and lives in West Palm Beach.

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