Investigation

MetHb: The gold standard investigation is a MetHb level which may be available as standard on ABG co-oximetry or may have to be specifically requested.

ABG:

Will show a normal PaO2 (or raised if receiving supplemental oxygen) and reduced SaO2*. This allows calculation of the Saturation Gap.

* some older ABG machines calculate SaO2 from SpO2, pH and bicarbonate. These machines give a falsely high reading for SaO2 as they assume Hb is normal. Most modern ABG machines are combined with a co-oximeter which measures light absorption at four (or more) wavelengths and allows direct measurements of COHb (600nm) and MetHb (631nm). Thus they give an accurate SaO2 and MetHb.

FBC: Check Hb levels. MetHb will be less well tolerated in anaemia.

ECG: Rule out differentials or contributory causes. MetHb can cause arrhythmias secondary to hypoxia. Toxin exposure can also cause conduction disturbances.

CXR: Rule out differentials or contributory causes to hypoxia

bHCG: Pregnancy has implications for treatment thresholds and the need for full discussion of risks and benefits to gain informed consent

Learning bite

Get an ABG early for accurate SaO2 and MetHb level.