Test Details: The formation of methemoglobin from haemoglobin within red cells is an ongoing oxidative process that results from exposure of haemoglobin to a variety of highly reactive molecules (oxygen free radicals), produced during normal cell metabolism. The only difference between haemoglobin and methemoglobin is that one or more of the four iron atoms in the methemoglobin molecule are in the ferric (Fe3+) rather than the ferrous (Fe2+) state and are therefore incapable of binding oxygen.