Methemoglobin measurements are underestimated by the Radical 7 co-oximeter: experience from a series of moderate to severe propanil poisonings

Abstract

Introduction: In Asia methemoglobinemia (MetHb) is commonly caused through self-poisoning with the pesticide propranil. MetHb can cause hypoxia, coma and death, but usually responds to methylene blue. It is therefore vital to have accurate methods to measure blood MetHb to guide appropriate treatments. The gold standard to measure MetHb utilizes a spectrophotometer, but recent bedside tests have been developed e.g., pulse co-oximeter probe and blood color chart.

Methods: Nine propanil poisoned patients had data collected from hospitals in Sri Lanka during 2008. Several MetHb readings were taken from each patient from admission up to 50 hours using spectrophotometry (Unico UV-Vis model no. 2800), pulse co-oximetry (Radical-7, Masimo, CA), and color chart.

Results: The co-oximeter underestimated the MetHb percentage when compared with spectrophotometry and the color chart, especially when the average MetHb was greater than 20%. The color chart demonstrated acceptable accuracy compared with formal spectrophotometry with the majority of values showing no more than 10% difference.

Conclusion: This small cohort highlights the potential for extreme inaccuracy of the Radical-7 co-oximeter, especially with a MetHb greater than 20%. Pulse co-oximeters should be required to be validated for the complete range of MetHb prior to regulatory approval.

Keywords:

• Methemoglobinemia
• pulse oximetry
• SpMet
• non-invasive co-oximeter spectrophotometry
• diagnostic accuracy

Acknowledgements

Elspeth Hulse analysed the data and wrote the first and subsequent drafts of this work. Fathima Shihana was involved with the study design, gathering of data and latter drafts of the commentary. Nicholas A. Buckley was involved in the study design, data analysis and all drafts of this commentary.

Disclosure statement

None of the authors have any declaration of interest.