A 58 y/o female presented to her clinician with perioral and nailbed cyanosis. She denied any cardiopulmonary symptoms. She had been taking phenazopyridine daily for dysuria over the past two weeks. Pulse oximetry showed a saturation of 84% on room air. Basic laboratory analysis was unrevealing, and blood gas was normal, but methemoglobin (MetHb) levels were markedly elevated on CO-oximetry. Phenazopyridine was stopped, methylene blue therapy was initiated, and the patient was admitted for observation. Despite this intervention her reported levels of MetHb remained high. The patient remained asymptomatic and a blood specimen sent to a larger healthcare system showed MetHb within normal limits. It was at this time the suspicion of sulfhemoglobinemia, a rare and typically asymptomatic condition, was considered and the patient was discharged and scheduled for follow-up. Sulfhemoglobinemia was confirmed at a separate reference laboratory. Her blue discoloration slowly resolved over the following weeks. Not all CO-oximeters are able to distinguish MetHb from SulfHb and some will report back SulfHb as false-positive MetHb. It is important to distinguish these hemoglobin variants because prognosis and treatment are vastly different. This case emphasizes that one needs to consider sulfhemoglobinemia in a cyanotic, asymptomatic patient that is unresponsive to methylene blue treatment even in the face of elevated methemoglobin levels on CO-oximetry.
Fanta JW, Fanta SE. Pseudo-methemoglobinemia Induced by Phenazopyridine, A Diagnostic Challenge. Aesculapius. 2020 Mar 01; 1(1):Article 1. Available from: https://red.library.usd.edu/aesculapius/vol1/iss1/1. Free full text article.