We report a case of methemoglobinemia with significant hemoglobin desaturation in a young female with AIDS who was being treated for Pneumocystis jiroveci pneumonia. A review of the etiology, pathophysiology, and treatment of methemoglobinemia is presented. 1. Background Methemoglobinemia is the presence of a significant amount of oxidized iron (Fe3+, met-Hgb) within hemoglobin (Hgb) in the blood, rendering it unable to bind oxygen. It is caused by a number of medications and toxins and can quickly degrade oxygen transport sufficiently enough to cause or aggravate severe tissue hypoxemia. Its hallmark features are hemoglobin desaturation out of proportion to blood partial pressure of oxygen and “chocolate brown” blood. Definitive diagnosis can be made quickly and easily with co-oximetry, but must be suspected, as co-oximetry is not routinely performed in patients. It is important to recognize, as correction of the pathologic hemoglobin redox state with methylene blue is simple, rapid, effective, and lifesaving. We present a case of methemoglobinemia secondary to primaquine, which was successfully treated with methylene blue and discontinuation of the drug. 2. Case Presentation A 32-year-old female with a history of intravenous drug abuse and AIDS (last CD4 count = 26/mm3) was admitted to a local hospital for cough, fever, and respiratory distress. Initial blood cultures grew gram-positive cocci in clusters, and endocarditis was suspected. Vancomycin was started, but a transthoracic echocardiogram was nondiagnostic, and the patient was transferred to our institution for further evaluation. The patient was unmarried and had been HIV positive for three years. She had no history of opportunistic infections and had been in good health for the past year taking no medications. She smoked one pack of cigarettes/day, used heroin intravenously 3–5 times/week, and denied alcohol use. She reported an intolerance to sulfa drugs. On arrival, we found a slender female in moderate to severe respiratory distress. Temperature was 99.2°F, respiratory rate of 35/min, blood pressure 110/62？mmHg, and heart rate 140/minute. Oxygen saturation measured by pulse oximetry was 88% on a 100% nonrebreather mask. She could speak only in short sentences. Cardiac examination revealed tachycardia without murmur. Lung examination revealed coarse crackles throughout. Abdomen was benign and the extremities were without cyanosis or edema. Skin showed evidence of chronic intravenous needlesticks without cellulitis. The remainder of the physical examination was unremarkable. Arterial blood
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