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Table 2 Symptoms observed at increasing levels of methemoglobin

From: Acquired methemoglobinemia from phenazopyridine use

Methemoglobin % Symptoms
< 3% (Normal) None
3–15% Possibly asymptomatic, low saturations on pulse oximeter, gray skin
15–20% “Chocolate-brown” blood, cyanosis
20–50% Dizziness, syncope, headache, dyspnea, exercise intolerance, weakness
50–70% Central nervous system depression, dysrhythmias, acidosis, seizures, coma
> 70% Profound hypoxia, death