Diagnosis requires high index of suspicion; lack of improvement in oxygen saturation with high-flow oxygen plus discrepancy in oxygen saturation >5% between measurements by ABG vs pulse oximetry ("saturation gap") should raise suspicion. Diagnosis is then confirmed by measuring level of<unk> moglobin in blood
Co-oximetry
Measures relative levels of oxyhemoglobin, carboxyhemoglobin,<unk> moglobin, reduced hemoglobin
Usually performed on arterial blood sample, but fingertip CO-oximeter devices are available
Normal PaO2
SpO2 will initially be falsely elevated
Once<unk>  >30% → SpO2 will fall to 80-85%
Mid-80s SpO2 due to light absorption of both oxyHb and deoxyHb
"Chocolate brown" color of blood
Measures relative levels of oxyhemoglobin, carboxyhemoglobin,<unk> moglobin, reduced hemoglobin
Usually performed on arterial blood sample, but fingertip CO-oximeter devices are available
Once<unk>  >30% → SpO2 will fall to 80-85%
Mid-80s SpO2 due to light absorption of both oxyHb and deoxyHb
Methemoglobinemia
Consider rare entity of sulfhemoglobinemia
Sulfhemoglobinemia
Sulfonamides, sulfasalazine, phenazopyridine
Dyshemoglobinemia
Occupational exposure
Hypoxemia
Conventional co-oximerty misidentifies sulfhemoglobin as methemoglobin so diagnosis is suspected when patient does not respond to methylene blue