Nosocomial Transmission of Cryptococcosis

We appreciate Dr. Grimsley's comments regarding pulse oximetry and methemoglobin.Pulse oximeters estimate oxygen saturation by comparing the absorbance of light at two wavelengths.Infrared light is absorbed by oxyhemoglobin, whereas red light is absorbed by deoxyhemoglobin.The absorbance characteristics of methemoglobin are similar to those of oxyhemoglobin, falsely elevating pulse-oximeter readings. 1,2o-oximeters measure light absorbance at four or more discrete wavelengths, providing accurate measurement of oxygen saturation, methemoglobin, and carboxyhemoglobin.In our patient, the oxygen saturation as measured by co-oximetry was 88.9 percent, the methemoglobin level was 8.8 percent, and the carboxyhemoglobin level was 1.2 percent.A methemoglobin level of 8.8 percent would be expected to yield a pulse-oximeter reading of 92 percent, 3 not 85 percent.A number of factors can impair the function of a pulse oximeter, including motion by the patient, the presence of or an excess of ambient light, increased skin pigmentation, abnormal body temperature, hyperbilirubinemia, and severe anemia. 2,4,5It is possible that one or more of these variables may have contributed to the discrepancy between the predicted oxygen-saturation value and the one recorded in our patient.We agree with Dr. Grimsley that although pulse oximetry provides a useful, noninvasive estimate of oxygen saturation, co-oximetry should be performed in all patients in whom methemoglobinemia is suspected.