A novel use for measurement of carboxyhemoglobin and methemoglobin

Blood gas analyzers often have an incorporated CO-oximeter that allows accurate determination of carboxyhemoglobin (COHb) and methemoglobin (MetHb) from the arterial blood sample submitted for routine blood gas analysis. For patients with suspected acute pulmonary embolism (PE), arterial blood gas analysis is often ordered, but only to determine pH, pCO₂ and pO₂; scant, if any, attention would normally be paid to the simultaneously generated COHb and MetHb values. 

The notion that COHb and MetHb measurement might be useful for assessment of patients with acute PE is suggested for the first time by the results of a recently published retrospective study of 156 non-smoking patients who had suffered acute PE. 

Investigators retrieved the admission blood gas results, including COHb and MetHb values for each of these patients, along with other laboratory results and clinical details that allowed assessment of individual disease severity scores. The outcome (survival or in-hospital death) for each study patient was also retrieved. 

Analysis of these accumulated data revealed a statistically significant correlation between disease severity scores and both COHb and MetHb. In the case of COHb the correlation was negative and in the case of MetHb the correlation was positive. Univariate logistic regression analysis revealed that both COHb and MetHb were both significantly associated with increased risk of death. 

Multivariate regression analysis revealed that COHb (but not MetHb) was an independent predictor of death in hospital. As the authors of this study concede, their study has a number of limitations (small size, retrospective in nature, etc.) but they provide preliminary data to suggest that for patients with acute PE, admission COHb/MetHb values represent potentially valuable (and readily available) prognostic information that might help refine the assessment of patients with acute PE. 

In discussion of their study, the authors consider possible mechanisms operating to determine that the lower the COHb and the higher the MetHb, the greater is the severity of acute PE.