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A patient with a complex urological history presented with abdominal pain and respiratory distress after catheter dysfunction. She underwent emergency surgery for a ruptured urinary pouch. Sepsis was later diagnosed based on clinical deterioration, including tachycardia, fever, an elevated respiratory rate and raised inflammatory markers, but blood cultures remained negative. A metagenomic microbial cell-free DNA (cfDNA) assay (iSEP-SEQ), performed early from plasma as part of a research protocol, identified Klebsiella and Pseudomonas at the genus level. Results were obtained retrospectively and were not available in real time; therefore, they did not alter immediate management. These findings were confirmed by cultures from drainage fluid and urine. Broad-spectrum antibiotic treatment led to clinical improvement. This case highlights the limitations of conventional microbiological methods in culture-negative sepsis and illustrates the role of cfDNA-based metagenomic testing as an adjunctive and complementary diagnostic tool for early, accurate pathogen detection. Early use of such tools may support timely and targeted management in complex infectious disease presentations.