Rasburicase-induced methemoglobinemia: The eyes do not see what the mind does not know.

Rasburicase is indicated for the prevention and treatment of tumor lysis syndrome which can be a potentially life-threatening emergency. The drug has oxidizing potential and as an adverse effect, it can convert the ferrous form of iron in erythrocytes to its ferric form resulting in the formation of methemoglobin which makes the heme component incapable of carrying oxygen. Patients with glucose-6-phosphate dehydrogenase enzyme deficiency are at high risk of methemoglobinemia from oxidizing agents. Symptoms of methemoglobinemia range from none to life-threatening hypoxemia, cyanosis and respiratory compromise. Treatment is indicated at levels above 20% and at lower levels if the patient is significantly anemic. We present a case of a 60-year-old male with diffuse large B cell lymphoma at high risk of tumor lysis syndrome. Rasburicase was administered to prevent renal failure and further rise in uric acid. Twenty-four hours later, a bedside pulse oximetry showed an oxygen saturation ranging from 60 to 65% with minimal cyanosis. Co-oximetry revealed a methemoglobin level of 9.8%. Methylene blue was administered and the methemoglobin level decreased to 2.6%. However, the patient developed hemolysis several hours later, likely secondary to rasburicase and methylene blue, requiring transfusion support. We discuss this potentially fatal and initially asymptomatic adverse effect of rasburicase along with diagnostic and treatment considerations, and review the cases described in the current literature.