Received on April 8, 2004
Accepted on September 10, 2004

Case Conference

Laboratory Assessment of Oxygenation in Methemoglobinemia

¹ Department of Pathology and Immunology, Washington University School of Medicine, 660 South Euclid Ave., St. Louis, MO 63110

^* To whom correspondence should be addressed. E-mail: mscott{at}pathology.wustl.edu.

Background: This case conference reviews laboratory methods for assessing oxygenation status: arterial blood gases, pulse oximetry, and CO-oximetry. Caveats of these measurements are discussed in the context of two methemoglobinemia cases.

Cases: Case 1 is a woman who presented with increased shortness of breath, productive cough, chest pain, nausea, fever, and cyanosis. CO-oximetry indicated a carboxyhemoglobin (COHb) fraction of 24.9%. She was unresponsive to O₂ therapy, and no source of carbon monoxide could be noted. Case 2 is a man who presented with syncope, chest tightness, and signs of cyanosis. His arterial blood was dark brown, and CO-oximetry showed a methemoglobin (MetHb) fraction of 23%.

Issues: Oxygen saturation (SO₂) can be measured by three approaches that are often used interchangeably, although the measured systems are quite different. Pulse oximetry is a noninvasive, spectrophotometric method to determine arterial oxygen saturation (S_aO₂). CO-oximetry is a more complex and reliable method that measures the concentration of hemoglobin derivatives in the blood from which various quantities such as hemoglobin derivative fractions, total hemoglobin, and saturation are calculated. Blood gas instruments calculate the estimated O₂ saturation from empirical equations using pH and PO₂ values. In most patients, the results from these methods will be virtually identical, but in cases of increased dyshemoglobin fractions, including methemoglobinemia, it is crucial that the distinctions and limitations of these methods be understood.

Conclusions: SO₂ calculated from pH and PO₂ should be interpreted with caution as the algorithms used assume normal O₂ affinity, normal 2,3-diphosphoglycerate concentrations, and no dyshemoglobins or hemoglobinopathies. CO-oximeter reports should include the dyshemoglobin fractions in addition to the oxyhemoglobin fraction. In cases of increased MetHb fraction, pulse oximeter values trend toward 85%, underestimating the actual oxygen saturation. Hemoglobin M variants may yield normal MetHb and increased COHb or sulfhemoglobin fractions measured by CO-oximetry.