Abstract Background Karius testing is a plasma cell-free DNA metagenomic sequencing (cfDNA/mNGS) assay that detects and quantifies microbial DNA directly from a single blood sample. It is noninvasive and does not require prior knowledge of the suspected pathogen, making it a valuable tool when conventional diagnostics are inconclusive. However, for pulmonary or disseminated tuberculosis (TB), plasma cfDNA/mNGS demonstrates lower sensitivity compared to respiratory samples due to low mycobacterial DNA burden and interference from background nontuberculous mycobacterial DNA. Studies have shown that mNGS on respiratory samples such as bronchoalveolar lavage fluid or tissue achieves pooled sensitivity of 60-83% and specificity of 98-99%, comparable to Xpert mycobacterium tuberculosis (MTB)/rifampicin (RIF) assay, but plasma-based cfDNA/mNGS remains less sensitive. Case Presentation A 29-year-old woman with rheumatoid arthritis presented with fever, chills, and abdominal pain. Laboratory evaluation revealed leukopenia, hyponatremia, and hepatocellular injury. CT imaging demonstrated significant mediastinal, bilateral hilar, and abdominal lymphadenopathy, prompting Karius testing to assess for infectious etiologies. The plasma cfDNA/mNGS assay was negative for microbial DNA. Given ongoing concern for infection, a supraclavicular excisional lymph node biopsy was performed, revealing non-caseating granulomas and positive PCR for MTB, confirming disseminated TB. The patient was initiated on RIPE therapy but developed acute hypoxic respiratory failure requiring intubation on hospital day 13. She remained on RIPE therapy with cefepime for possible superimposed bacterial infection. Following stabilization, she was successfully extubated on day 16 and discharged on day 21 in stable condition, with plans for continued outpatient TB management. Discussion Disseminated TB presents diagnostic challenges due to nonspecific symptoms and limited sensitivity of available tests. This case highlights the imperfect sensitivity of plasma cfDNA/mNGS for detecting MTB, particularly in extrapulmonary or disseminated disease. Traditional diagnostics, including culture and nucleic acid amplification tests, may also yield false negatives, underscoring the need for a multimodal approach. Current ATS/IDSA/CDC guidelines recommend integrating clinical, radiologic, and laboratory findings to establish diagnosis. Conclusion A negative plasma cfDNA/mNGS result does not exclude TB, especially in extrapulmonary or disseminated presentations. Clinicians should maintain a high index of suspicion and pursue tissue diagnosis when clinical concern persists despite negative molecular testing. This abstract is funded by: None
Foster et al. (Fri,) studied this question.