Comment on the Overestimation of Methemoglobin Concentrations in Neonatal Samples with the Chiron 800 System CO-Oximeter Module

We wish to comment on the Technical Brief of Lynch et al. (1), who reported overestimation of methemoglobin concentrations in neonatal samples containing fetal hemoglobin and analyzed with the Chiron 800 system CO-oximeter. Overestimation of methemoglobin in the presence of fetal hemoglobin may have serious implications, with the possibility of premature termination of nitric oxide treatment. In our hospital, >1000 methemoglobin analyses are performed annually on request of our neonatal intensive care unit. These measurements have been performed with a CIBA Corning 270 CO-oximeter and, at present, are also performed with a Chiron 865 blood gas analyzer equipped with a CO- oximetry module (Chiron Diagnostics NV). We decided to investigate this alarming report and used both analyzers to measure methemoglobin concentrations in three sets of samples. The first set consisted of 10 samples from our neonatology department. In these samples, the concentration of fetal hemoglobin was 65% or higher. The second and third sets of samples consisted of adult blood and umbilical cord blood, respectively, collected in heparin-containing tubes and divided into two portions. In one portion of each sample, 100% methemoglobin was obtained by adding potassium nitrite (2). The other portion was not treated (<2% methemoglobin). All portions were washed with saline several times and subsequently used to produce samples with nominal percentages of methemoglobin of 2–25%. Samples were measured within 2 min on both blood gas analyzers. The results are given in Table 1.