Lymphocyte-Predominant Ascites as a Clue to Peritoneal Tuberculosis: Two Case Reports and a Literature Review

Background: Peritoneal tuberculosis (PTB) is difficult to diagnose because its clinical and imaging features are nonspecific and may mimic peritoneal malignancy or other intra-abdominal diseases. In addition, microbiological and molecular tests of ascitic fluid may be negative in paucibacillary disease. Case Presentation: We report two patients with PTB whose initial diagnostic evaluation was inconclusive. Both presented with abdominal distension and imaging findings suspicious for alternative diagnoses. In Case 1, acid-fast bacilli (AFB) staining and Mycobacterium tuberculosis ( M. tuberculosis ) nucleic acid amplification test (NAAT) of ascitic fluid were negative; targeted next-generation sequencing (tNGS) was not performed. Ascitic fluid cytology showed a nucleated cell count of 7900 × 10 6 /L, with 76% lymphocytes, mesothelial cells less than 1%, and no malignant cells. The patient improved after empiric treatment, and the diagnosis was later confirmed by positive ascitic fluid culture for M. tuberculosis . In Case 2, AFB staining and tNGS performed on pleural and ascitic fluid samples did not detect M. tuberculosis or other pathogens. Ascitic fluid cytology showed a nucleated cell count of 6,726 × 10 6 /L, with 82% lymphocytes, mesothelial cells less than 1%, and no atypical cells. PTB was ultimately confirmed by laparoscopic biopsy with histopathology and positive tissue AFB staining. Conclusion: In the appropriate clinical setting, lymphocyte-predominant ascitic fluid with a low mesothelial cell proportion may provide a useful supportive clue to PTB when initial tests are negative or nondiagnostic. However, these cytological findings are nonspecific and should be interpreted together with clinical, microbiological, and histopathological findings.