Fever of unknown origin (FUO) due to a solitary cavitary lung lesion: The deadly ferritin-laced doughnut

Fever of unknown origin (FUO) is the clinical designation for patients who have fevers >101F that have persisted for >3 weeks that remain undiagnosed, after an intensive ambulatory/in-hospital workup. Fevers of unknown origin may be due to wide variety of infectious, neoplastic, or rheumatic/inflammatory disorders. The most common causes of FUOs in elderly patients are infectious and neoplastic diseases. With FUOs, the clinical presentation and routine laboratory tests are usually sufficient to narrow differential diagnostic possibilities. We present a case of an elderly Italian woman who presented with an FUO and a solitary, thick-walled cavitary lesion on chest x-ray (CXR). The infectious disease differential diagnosis of her FUO included lung abscess, M. tuberculosis (TB), systemic mycoses, and echinococcal-cyst (or hydatid-cyst) disease. The malignancy and neoplastic differential diagnosis included bronchogenic carcinoma, lymphoma, and metastatic carcinoma. Her nonspecific laboratory tests indicated a highly elevated erythrocyte sedimentation rate (ESR) >100 mm/hour, chronic thrombocytosis, relative lymphopenia, and highly elevated serum ferritin levels. Excluding highly elevated serum ferritin levels, the differential diagnosis of her FUO with a solitary, thick-walled cavitary lesion was lung abscess vs tuberculosis. However, her highly elevated serum ferritin levels proved to be the critical diagnostic clue in predicting the diagnosis of squamous-cell carcinoma. We conclude that serum ferritin levels are an important part of the laboratory workup. As with other nonspecific laboratory tests, the diagnostic significance of highly elevated ferritin levels depends associated clinical features in the clinical presentation.