Spectrophotometry of hydroxocobalamin and hemoglobin reveals production of an unanticipated methemoglobin variant

Introduction. Cyanide-poisoned patients often require pulse oximetry and co-oximetry to measure oxyhemoglobin, deoxyhemoglobin, carboxyhemoglobin, and methemoglobin. These and other critical laboratory measurements can be confounded by the cyanide antidote hydroxocobalamin. The postulated mechanism of this confounding is direct optical interference. Methods. The spectra of swine hemoglobin with and without hydroxocobalamin were measured from 450–800 nm. The resulting complex spectrum was divided into hemoglobin, hydroxocobalamin, and a remainder spectrum. Results. The remainder spectrum appears to be a methemoglobin variant quantitatively dependant on the amount of hydroxocobalamin added to the hemoglobin solution and the presence of oxygen. The Pearson's correlation coefficient comparing the known swine methemoglobin spectrum with the remainder spectrum reveals a very high degree of correlation (r2 = 0.986). Conclusion. This is the first study to document methemoglobin formation caused by hydroxocobalamin. Further studies are needed in vitro and in vivo to assess this previously unreported methemoglobin variant.