A broad-range polymerase chain reaction on a formalin-fixed, paraffin-embedded tissue is a powerful diagnostic tool, but requires cautious interpretation: a case report.

A woman in her mid-50s with Lynch syndrome and a history of malignant neoplasms was found to have a superior vena cava (SVC) thrombus and a necrotic mediastinal mass on surveillance imaging. The initial biopsy yielded necrotic debris, and the procedure was complicated by a mediastinal hematoma. Due to concern for possible infection, the patient was empirically treated with antibiotics while planning further workup. A broad-range 16S polymerase chain reaction (BRPCR) was performed on a formalin-fixed paraffin-embedded (FFPE) tissue, which was negative for fungi and mycobacteria but revealed DNA from normal oral flora. Subsequently, she developed SVC syndrome, raising concern about a mediastinal abscess. A repeated biopsy confirmed the suspected carcinoma with necrosis. This case underscores the importance of proper interpretation of BRPCR results in FFPE (non-sterile) tissue and emphasizes the need to use these results in an appropriate clinical context.