Key investigations for methaemoglobinaemia
Pulse oximetry and arterial blood gas analysis may be only slightly abnormal in methaemoglobinaemia. The apparent oxygen saturation does not usually fall below 85% even with moderate methaemoglobinaemia. Suspect methaemoglobinaemia in patients with generalised cyanosis, but normal partial pressure of oxygen (PaO2) on arterial blood gas analysis.
Dual-wavelength pulse oximetry does not measure methaemoglobin. Do not rely on newer pulse oximeters that claim to detect methaemoglobinaemia and carboxyhaemoglobinaemia by using co-oximetry because they are inaccurate.
Many blood gas analysers can accurately measure methaemoglobin. A simple colour chart method is also available that is accurate to within 10%1.