Definition
FMetHb is the ratio between the concentration of MetHb and tHb:
The systematic symbol for arterial blood is FMetHb(a). The analyzer symbol may be MetHb or FMetHb.
What does FMetHb tell you
Methemoglobin is formed when the ferrous ion (Fe2+) in the heme groups is oxidized to the ferric state (Fe3+). Methemoglobin is unable to combine with oxygen, resulting in a decreased oxygen-carrying capacity of the blood. Formation of metheme groups increases the affinity for oxygen of the remaining binding sites.
Reference ranges
FMetHb(a) reference range (adult): 0.0-1.5 % (0.000-0.015)
disclaimer
Clinical interpretation
Methemoglobin levels above 10-15 % can result in pseudocyanosis. Methemoglobinemia may cause headache and dyspnea at levels above 30 % and may be fatal.
Considerations
Most cases of methemoglobinemia are caused by drugs or chemicals containing nitro- and aminogroups, e.g., topical administration of benzocain. Newborns can get methemoglobinemia if given nitrate-containing well water. Excessive methemoglobinemia may be treated with intravenous infusion of methylene blue or by red-cell transfusion.
Be aware of the risk of preanalytical errors (inhomogeneous sample) on FMetHb values.
For more information, go to Preanalytical considerations.
