The diagnosis of methemoglobinemia is based on clinical symptoms and laboratory testing. Peripheral and central cyanosis is almost always present when there is a minimum methemoglobin level of 15% in the blood.
2,4-6,10 These clinical signs typically result from the methemoglobin molecule causing a functional anemia, as well as from the methemoglobin molecule's inability to bind to oxygen. As the concentration of methemoglobin in the blood increases to 45%, symptoms of dizziness, fatigue, headache, tachycardia, and weakness are expected.
4-6,10 Acidosis, cardiac arrhythmia, dyspnea, seizures, and eventually coma become evident as the methemoglobin level approaches 70%.
4-6,10
Arterial blood gas analysis paired with oxygen saturation analysis by pulse oximetry are now considered the definitive measures for making a correct diagnosis of methemoglobinemia. Blood gas analyses in patients with methemoglobinemia reveal normal to elevated levels of PaO
2 with low oxyhemoglobin saturation.
2,4-6