See also under XIVa, b, e. Cyanosis is slate-greay rather that purple. SpO2 is low or spurious/erratic. Cyanosis is resistant to oxygen therapy. SpO2 low or spurious. PaO2 measured by the Clarke electrode is normal. The hallmarks of methemoglobinemia. Diagnosis is confirmed by chocolate-brown color of blood; which does not turn bright red when bubbled with O2 and/or by spectrophotometric measurement of blood methemoglobin (normally <2%). Four-wavelength pulse CO-oxymetry can also measure methemoglobin and COHb. Methemoglobinemia can be life-threatening or fatal if >40%. Treatment is with oxygen and methylene blue. Severe cases may require HBO or exchange transfusion. Oxidant-induced hemolytic anemia can be found in association.