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The pulse oximeter serves as an indicator of arterial oxygen saturation. We present two cases in which methylene blue injection during urological surgery appeared to cause arterial oxygen desaturation by pulse oximetry and methemoglobinemia by arterial blood gas co-oximetry. Methylene blue interferes with light absorption and gives a false estimate of the percentage of oxyhemoglobin and arterial oxygen saturation. The co-oximeter interprets methylene blue as methemoglobin and gives a false indication of methemoglobinemia. The surgical team must be familiar with conditions and agents that interfere with their ability to safely monitor surgical patients.

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