LAUSR

We read with great interest the recent study published by Hulse et al. [1] in your journal and wish to share our experience about detecting methemoglobin by CO-oximeter. Although Masimo company has determined a measurement accuracy range of 1–15± 1% for the detection of SpMet by Radical-7R non-invasive pulse CO-oximeters, preliminary results even questioned methemoglobin detected by them.[2,3] Level of accuracy has not been validated in the patients who need methylene blue as an antidote (i.e., those with SpMet >20%).[4] Thus, the authors’ interpretation regarding probable inefficacy of the CO-oximeters in the patients with moderate to severe methemoglobinemia seems correct and in accordance with the manufacturers’ catalogue. It has been proposed that patient movements or decreased perfusion of the tissue decreases the efficacy of CO detection by CO-oximeters. Although this has not been studied for MetHb detection, such parameters may reduce the detection of oxygen saturation and pulse rate and affect MetHb detection, as well.[4] We measured MetHb by the same device in aluminum phosphide-poisoned patients. During the first 24 hours post admission, Sp-Met had a minimum sensitivity of 81.1% and specificity of 83.3% in determination of the ill-health criteria in our study (SBP 80mmHg or pH 7.2 or HCO3 15). Irrespective of post-ingestion time, all cases with more than 13% Sp-Met died.[5] At the same time, we also measured SpCO in our patients and discovered that although there was no carbon monoxide by spectrophotometric methods, a high SpCO was detected by pulse Co-oximetry – probably due to the formation of a dyshemoglobin which interfered with CO light absorption – and accompanied a poor prognosis.[5] We are currently using SpCO as a sensitive method for early detection of life-threatening phosphine exposure in our center. There was no false positive SpMet in Hulse et al.’s study.[1] It means that although the CO-oximeter frequently underestimated the MetHb percentage, it never overestimated the measurements and antidote administration based on CO-oxiymetry was correct. Let us say oximeters are noninvasive instruments with lots of limitations and inaccuracy, but let us provide better care particularly when spectrophotometric methods are not readily available for the detection of methemoglobinemia. Applying this instrument may reveal methemoglobinemia earlier and enables us to administer methylene blue on time. It should not be used as a confirmatory test and must be confirmed by spectrophotometric or even color charts.