Met-hemoglobin
Met-hemoglobin is the oxidized form of hemoglobin with iron in the Fe3+ state, formed naturally as blood autoxidizes and darkens over time.
Met-hemoglobin, also known as met-hb, is the oxidized form of hemoglobin where the iron atom is in the ferric (Fe3+) state rather than the ferrous (Fe2+) state. This transition occurs naturally through autoxidation of oxyhemoglobin and is strongly influenced by environmental factors like temperature and humidity. It is a key intermediate in the chemical sequence that causes fresh blood to darken from red to brown, mahogany, and eventually black over time.
What this means in real life
A person exposed to certain pesticides or local anesthetics like benzocaine may develop elevated levels, causing their lips and fingertips to appear bluish-gray even though their oxygen levels are actually normal.
What it isn’t
It is not simply low oxygen in the blood. Rather, the hemoglobin molecule itself is chemically altered and cannot pick up oxygen, even when oxygen is available—a fundamentally different problem than typical hypoxia.
Commonly misused online
Often conflated with general 'low oxygen' or hypoxia on health forums, when the actual issue is that hemoglobin is chemically broken and won't bind oxygen regardless of how much is present.