text: Methemoglobin is expressed as a concentration or a percentage. Percentage of methemoglobin is calculated by dividing the concentration of methemoglobin by the concentration of total hemoglobin. Percentage of methemoglobin is likely a better indicator of illness severity than overall concentration, as underlying medical conditions play an important role. For example, a methemoglobin concentration of 1.5 g/dL may represent a percentage of 10% in an otherwise healthy patient with a baseline hemoglobin of 15 mg/dL, whereas the presence of the same concentration of 1.5 g/dL of methemoglobin in an anemic patient with a baseline hemoglobin of 8 g/dL would represent a percentage of 18.75%. The former patient will be left with a functional hemoglobin concentration of 13.5 g/dL and potentially remain asymptomatic while the latter patient with a functional hemoglobin concentration 6.5 g/dL may be severely symptomatic with a methemoglobin of less than 20%.
  * 1–2% Normal
  * Less than 10% metHb - No symptoms
  * 10–20% metHb - Skin discoloration only (most notably on mucous membranes)
  * 20–30% metHb - Anxiety, headache, dyspnea on exertion
  * 30–50% metHb - Fatigue, confusion, dizziness, tachypnea, palpitations
  * 50–70% metHb - Coma, seizures, arrhythmias, acidosis
  * Greater than 70% metHb - High risk of death
  This may be further compounded by the "functional hemoglobin's" decreased ability to release oxygen in the presence of methemoglobin. Anemia, congestive heart failure, chronic obstructive pulmonary disease, and essentially any pathology that impairs the ability to deliver oxygen may worsen the symptoms of methemoglobinemia.