Abstract
Background:
Abdominal tuberculosis (TB) includes peritoneal and gastrointestinal TB. Diagnosis of Extra pulmonary tuberculosis including abdomainl TB always remained challenging thus leading to missed and delayed diagnosis. Point of care test such as CBNAAT may help in early diagnosis.

Aim:
To evaluate the performance of CBNAAT (GeneXpert Ultra) in peritoneal and abdominal lymph node tuberculosis.

Methods:
The prospective study was conducted in the Department of Microbiology after the ethical committee’s approval. All specimens with a requisition for CBNAAT ultra from clinically suspected admitted patients of abdominal TB, were included. The specimens were subjected to microscopy (ZN stain), culture and CBNAAT (Xpert MTB/RIF assay Ultra) and processed as per standard protocols. The data was analysed statistically.

Results:
Among 331suspected gastrointestinal TB patients, 65.3% were males and the maximum number of patients (33.80%) belonged to the age of more than 60 years. The most common presenting symptom was fever (98.8%) followed by abdominal discomfort (84.5%). Comorbidities such as hypertension (56.5%) and diabetes mellitus (55.6%) were observed. ADA (Adenosine Deaminase>39 U/L) was raised in 20.5 % of patients. Out of the total samples tested 86%were ascitic fluid, 10.9% were lymph nodes and 3% were pus samples. Among these CBNAAT was positive in 14 samples, LJ (Lowenstein Jensen) culture in 6 samples and none was AFB smear positive. Out of 331 patients, 34met the criteria of Composite Reference Standard (CRS). On comparison with culture, CBNAAT showed sensitivity, specificity, PPV and NPV of 100%, 97.54%, 42.86% and 100% respectively. While considering CRS as the gold standard CBNAAT showed a sensitivity of 38.24%, specificity of 99.66%, PPV of 92.86% and NPV of 93.38%.

Conclusion:
CBNAAT ultra-detected cases that have been missed by other diagnostic methods and increased the diagnostic yield. Still, low sensitivity was observed with CRS criteria. Hence, a combined clinical, radiological, laboratory and treatment response is the best approach to diagnose the disease

Key words: Xpert MTB/RIF Ultra assay, Abdominal tuberculosis, Sensitivity, Specificity, Composite reference standard