A 57 year old man with history of idiopathic panniculitis on long-term steroid treatment presented to the hospital with a 1 month history of dyspnea, malaise, nausea and vomiting. He was hypotensive and tachycardic with an acute kidney injury. Prior to this presentation, he had been hospitalized for 2 weeks at a nearby hospital with similar symptoms and had been found to have multifocal ground glass opacities on CT of the chest. His condition after admission improved with fluid resuscitation, stress-dose steroids, and treatment for presumed sepsis. Infectious and rheumatologic workups were negative. He was noted to have bilateral pulmonary infiltrates ( Fig. 79.1a, b). One week after discharge, however, he presented again with systemic inflammatory response syndrome (SIRS). A bronchoscopy revealed only acute and chronic organizing pneumonia, with no evidence of infection. A bone marrow biopsy was performed, which demonstrated a hypercellular marrow. Pulse dose steroids were initiated for organizing pneumonia and he improved significantly over the next week with continuation of empiric antimicrobials and tapering of steroids. After 1 week of this regimen, however, he again decompensated with hypoxia and hypotension. Worsening infiltrates were noted on his chest radiograph ( Fig. 79.1c). Ferritin was elevated at 7652 ng/mL, and sIL-2R was elevated at 3432 U/ mL. Trigylcerides were normal and fibrinogen was elevated. Given his unremarkable bone marrow biopsy 7 days before and elevated fibrinogen, the possibility of hemophagocytic lymphohistiocytosis (HLH) was dismissed as his hyperferritinemia and elevated sIL-2R were attributed to history of blood transfusion and occult infection. He continued to improve with empiric antimicrobials and steroids, and was discharged from the hospital 5 days after admission to the ICU to complete a course of levofloxacin on 40 mg of prednisone daily.He presented to the hospital for the third time 2 days after discharge with hypotension and hypoxia requiring endotracheal intubation and vasopressors, and antibiotics and steroids were restarted. He was admitted to the ICU and his pulmonary status rapidly improved. After 3 days, however, he again decompensated with hypotension and an elevated lactate. He received intravenous fluids and intravenous steroids. Given the unclear association between changes in his antimicrobial regimen, steroid dosing, and clinical condition, his steroid dose was decreased from 40 mg prednisone daily by 10 mg each day. A repeat bone marrow biopsy was performed, which showed rare hemophagocytic macrophages but no evidence of malignancy. The next day, in the setting of reducing his steroid dose, he again had an episode of hypoxia and hypotension. Ferritin was measured at that time and was 9148 ng/mL, increased from 4074 ng/mL the day before ( Fig. 79.2). sIL2-R was measured and was elevated at 3494 U/mL. A diagnosis of HLH was made, and he was started on chemotherapy with etoposide and high dose dexamethasone. Eight weeks had elapsed since his initial presentati...