A practical method for the accurate determination of methemoglobin in blood containing carboxyhemoglobin.

Kinetics of the oxidation of carboxyhemoglobin (HbCO) by potassium ferricyanide was studied photometrically in a weakly acid solution. An increase in the absorbance at 630 nm reached a maximum within 10 min when over a 100-fold excess of ferricyanide to hemoglobin iron was used. A slight decrease in the absorbance was observed after completion of the reaction when over a 500-fold excess of the reagent was used. In the presence of 0.4% Sterox SE, the absorbance began to decrease without complete oxidation. From these findings, a simple, rapid and accurate method for the determination of methemoglobin (Met-Hb) in blood was devised. The method was compared with two other methods, using 11 blood samples containing various amounts of HbCO, and proved to be suitable for blood containing elevated HbCO as well as for ordinary blood.