Methylene Blue (see Methylene Blue): methylene blue can impart a bluish discoloration to the skin after use (note: this does not imply treatment failure when methylene blue is used to treat methemoglobinemia)
Sulfhemoglobin, in concentrations greater than 0.5 g/dL also causes “cyanosis” with a normal PaO2 and may be erroneously measured as methemoglobin
Sulfhemoglobin can be distinguished from methemoglobin by virtue of its peak absorption at 620 nm which, unlike methemoglobin, is not abolished by the addition of cyanide.
Cyanosis is clinically apparent when methemoglobin levels exceed 8-12% (at a normal hemoglobin concentration): equivalent to >1.5 g/dL
Note: in contrast, a deoxyhemoglobin level of 5 g/dL is required to produce clinical cyanosis
In patients with severe anemia, a higher percentage of methemoglobin is required for cyanosis to be clinically apparent: these patients are more likely to exhibit signs of hypoxemia and have less degrees of cyanosis, as compared to non-anemic patients