INTRODUCTION: Peritoneal tuberculosis (TB) represents about 6% of extrapulmonary Mycobacterium tuberculosis infections in the United States. Clinically, peritoneal TB presents with ascites, abdominal pain, fever and weight loss. Predominant lymphocytosis (sensitivity, 68%) and adenosine deaminase (ADA) >30 U/L (94%) in ascites fluid suggest TB but do not differentiate from cancer or sarcoidosis. Definitive diagnosis requires peritoneal fluid or biopsy that is positive for M. tuberculosis via culture or a nucleic acid amplification test (NAAT). Treatment consists of standard anti-TB therapy. CASE DESCRIPTION/METHODS: A 27-year-old, recently incarcerated, healthy man, born in a country with endemic TB, presented to the hospital with abdominal pain and new ascites. An abdominal computed tomography (CT) scan showed colonic wall thickening, moderate-volume ascites, diffuse peritoneal enhancement, omental infiltration and normal liver (Figure 1). Diagnostic paracentesis was significant for a serum-ascites albumin gradient (SAAG) < 1.1 mg/dL with high protein, elevated white blood cell count with lymphocytosis, negative cytology and ADA of 12 U/L (Table 1). He then became febrile and underwent a second paracentesis with peritoneal biopsy, given concern for a primary malignancy, which revealed >500 neutrophils and non-necrotizing granulomatous inflammation. He was given antibiotics for spontaneous bacterial peritonitis and discharged. The patient returned to the hospital with failure to thrive and recurrent fevers. A chest CT scan showed a small right pleural effusion (Figure 2). An abdominal CT scan showed a slight increase in ascites with peritoneal thickening. A third paracentesis with peritoneal fluid analysis showed similar findings to previous (Table 1). A purified protein derivative (PPD) skin test and interferon gamma release assay (IGRA) were positive, after which the patient started anti-TB therapy. Peritoneal fluid culture and NAAT later confirmed M. tuberculosis. DISCUSSION: The differential diagnosis for ascites fluid with high protein and lymphocytes includes malignancy, sarcoidosis and TB. Peritoneal TB in a young, immunocompetent man with no prior medical history is rare. The lack of clinical or radiologic signs of pulmonary TB, or definitive evidence of sarcoidosis and malignancy, presents a diagnostic dilemma. A high index of suspicion for TB needs to be maintained in patients with incarceration or travel history to endemic countries; early testing with IGRA or PPD can help prevent delay in treatment.Figure 1.: A computed tomography (CT) scan of the abdomen and pelvis with intravenous contrast showing apparent abnormal colonic wall thickening mostly affecting the transverse colon. Moderate volume ascites and diffuse peritoneal enhancement and omental infiltration. Findings may reflect a peritonitis.Figure 2.: A computed tomography (CT) scan of the chest with intravenous contrast showing a small right pleural effusion and bibasilar atelectasis, right greater than left. No enlarged mediastinal lymph nodes. Enlarged right cardiophrenic lymph node, possibly reactive.Table 1.: Peritoneal fluid findings from paracentesis and biopsy