AFB-M.Tb detection (M.Tb/NTM Detection) by CBNAAT Ascitic Fluid

Bacterial/ Viral

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No Fasting Required

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Cartridge-based PCR test detecting Mycobacterium tuberculosis vs non-tuberculous mycobacteria in ascitic fluid.

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AFB-M.Tb detection (M.Tb/NTM Detection) by CBNAAT Ascitic Fluid - Comprehensive Guide

Why is it done?
- Test Purpose: Detects Mycobacterium tuberculosis (M.Tb) and non-tuberculous mycobacteria (NTM) in ascitic fluid using CBNAAT (Cartridge Based Nucleic Acid Amplification Test) technology
- Primary Indications: Suspected peritoneal tuberculosis (TB peritonitis)
- Evaluation of ascites of unknown etiology with clinical suspicion of tuberculosis
- Investigation of exudative ascites with lymphocytic predominance
- Diagnosis in immunocompromised patients (HIV/AIDS) with ascites and fever
- Confirmation of TB peritonitis in patients with compatible clinical and radiological findings
- Timing of Test: Performed when ascitic fluid is obtained during diagnostic paracentesis; usually done urgently in acute presentations with clinical suspicion of TB
Normal Range
- Normal Result: NEGATIVE - No M.Tb or NTM detected in ascitic fluid
- Abnormal Result: POSITIVE - M.Tb and/or NTM detected; may include Rifampicin resistance status
- Units of Measurement: Qualitative - reported as DETECTED or NOT DETECTED
- Interpretation Categories:
- NEGATIVE (Not Detected) - Absence of mycobacterial nucleic acid; consistent with absence of TB peritonitis
- M.Tb DETECTED - Confirms presence of Mycobacterium tuberculosis; diagnostic of TB peritonitis
- NTM DETECTED - Identifies non-tuberculous mycobacteria; requires different treatment approach
- Rifampicin RESISTANT - If present, indicates drug-resistant TB requiring modified therapy
- Rifampicin SUSCEPTIBLE - If present, indicates TB susceptible to standard first-line therapy
Interpretation
- NEGATIVE Result:
- Does not exclude TB peritonitis (sensitivity ~55-80%); may occur in paucibacillary disease or inadequate specimen collection
- Clinical correlation with ascitic fluid biochemistry, cell count, and imaging remains essential
- M.Tb DETECTED Result:
- Highly specific (>95%) for tuberculosis diagnosis; confirms TB peritonitis
- Rapid diagnostic result (typically <2 hours) enabling prompt initiation of anti-TB therapy
- Rifampicin susceptibility testing guides first-line or second-line drug selection
- NTM DETECTED Result:
- Indicates infection with atypical mycobacteria; requires species identification and susceptibility testing
- Common NTM in peritonitis include MAC (Mycobacterium avium complex) especially in immunocompromised hosts
- Treatment differs significantly from TB; requires prolonged multi-drug regimens
- Factors Affecting Results:
- Volume and quality of ascitic fluid specimen - minimum 3-5 mL recommended
- Delay in processing - should be processed promptly; refrigeration acceptable if delays unavoidable
- Bacterial load (bacillary burden) - higher sensitivity with higher bacterial load
- Prior anti-TB therapy - may reduce bacterial detection
- Immune status - better sensitivity in immunocompromised patients with higher bacillary loads
- Clinical Significance:
- CBNAAT is WHO-endorsed as the first-line diagnostic test for TB peritonitis
- Superior to AFB smear microscopy (which has low sensitivity in ascitic fluid); faster than culture
- Enables drug-resistant TB detection, guiding appropriate therapeutic selection
- Critical in resource-limited settings and high TB-burden regions for rapid diagnosis
Associated Organs
- Primary Organ System: Peritoneal cavity and abdominal organs (visceral and parietal peritoneum, bowel, omentum, liver)
- Secondary Involved Systems:
- Lymphatic system - often with concurrent lymphadenitis and peritoneal lymph node involvement
- Hepatobiliary system - TB abscess, cirrhosis, or granulomatous hepatitis
- Respiratory system - pulmonary TB as primary site with hematogenous spread
- Renal and genitourinary system - potential dissemination or concurrent renal TB
- Diseases Diagnosed/Monitored:
- Tuberculous (TB) peritonitis - most common extrapulmonary TB presentation in endemic regions
- Multidrug-resistant TB (MDR-TB) - with Rifampicin resistance detection
- Extensively drug-resistant TB (XDR-TB) - if Rifampicin-resistant; requires confirmatory susceptibility testing
- MAC infection (Mycobacterium avium complex) - especially in HIV/AIDS patients
- Other NTM peritonitis - including M. marinum, M. kansasii, M. abscessus
- Potential Complications of Abnormal Results:
- Severe malnutrition and metabolic derangements from chronic peritoneal infection
- Peritoneal fibrosis and adhesions - causing intestinal obstruction, reduced peritoneal function
- Ascites reaccumulation and recurrent peritonitis if treatment suboptimal or drug-resistant
- Secondary bacterial peritonitis superimposed on TB peritonitis
- Systemic sepsis and septic shock if untreated
- Drug-related adverse effects from prolonged second-line TB therapy in MDR cases
- Immune reconstitution inflammatory syndrome (IRIS) in HIV patients initiating antiretroviral therapy
Follow-up Tests
- Recommended Follow-up Tests if M.Tb DETECTED:
- Ascitic fluid culture for M.Tb - gold standard; enables drug susceptibility testing and species confirmation
- Sputum CBNAAT/culture - to exclude or confirm concurrent pulmonary TB
- Comprehensive drug susceptibility testing (DST) - especially if Rifampicin-resistant; tests fluoroquinolones, aminoglycosides, injectables
- Ascitic fluid adenosine deaminase (ADA) and LDH - supportive diagnostic markers
- Abdominal imaging (CT scan) - assess peritoneal thickening, loculated ascites, lymphadenopathy
- Chest X-ray - evaluate for pulmonary TB or miliary disease
- HIV testing - in endemic areas and if risk factors present
- Recommended Follow-up Tests if M.Tb NEGATIVE but High Clinical Suspicion:
- Repeat CBNAAT from fresh ascitic fluid - improves sensitivity if initial specimen marginal
- AFB smear microscopy with Ziehl-Neelsen staining - less sensitive but may confirm suspicion
- Peritoneal biopsy with histology and culture - if clinical suspicion remains high; tissue shows caseating granulomas in TB
- ADA level in ascitic fluid - high ADA (>10 IU/L) supportive of TB diagnosis
- Ascitic fluid total protein, LDH, glucose - assess for exudative pattern consistent with TB
- Recommended Follow-up Tests if NTM DETECTED:
- Mycobacterial culture and species identification - confirm species (MAC, M. marinum, M. abscessus, etc.)
- Specific drug susceptibility testing for NTM - varies by species; guides macrolide, fluoroquinolone therapy
- HIV testing and CD4 count - NTM peritonitis common in AIDS (CD4 <50 cells/μL)
- Sputum and blood cultures - exclude disseminated NTM disease or concurrent infections
- Monitoring During Treatment:
- Clinical monitoring - monthly assessment of symptoms, weight, abdominal symptoms during first 2-3 months
- Repeat imaging (CT/ultrasound) - at 3-6 months to assess ascites resolution and treatment response
- Repeat ascitic tap if ascites reaccumulates - to exclude recurrent/persistent TB or secondary infection
- Liver function tests and renal function - monitor for hepatotoxicity and nephrotoxicity from anti-TB drugs
- Treatment duration: 6 months standard regimen for TB peritonitis; longer for MDR-TB (18-20 months)
Fasting Required?
- Fasting Status: NO - Fasting is NOT required for this test
- Rationale: Ascitic fluid is obtained by direct paracentesis; does not rely on blood specimens; unaffected by oral intake
- Patient Preparation:
- Empty bladder prior to paracentesis - reduces risk of bladder perforation; patient should void before procedure
- Light clothing or gown - for easy abdominal access
- Obtain informed consent - explain paracentesis procedure, risks, benefits
- Review coagulation studies - INR, PT, platelet count; postpone if severely abnormal (INR >1.5, platelets <20,000)
- Discontinue anticoagulants if possible - withhold antiplatelet agents and anticoagulants 24-48 hours prior if clinically feasible
- Medications NOT Required to Discontinue:
- Antibiotics - continue; does not interfere with test
- Diuretics - may be continued per clinical judgment
- Anti-TB drugs - continue if already initiated; does not affect detection
- Specimen Collection Instructions:
- Minimum specimen volume - at least 3-5 mL ascitic fluid for optimal test sensitivity
- Sterile collection - obtained via aseptic paracentesis technique using sterile needle and syringe
- Specimen container - sterile tube; EDTA (purple top) tube for culture preservation if significant delay
- Sample handling - refrigerate at 2-8°C if processing delayed; process within 24 hours for best results
- Label clearly - include patient demographics, collection time, clinical indication
- Paracentesis-Related Instructions:
- Position - patient supine or semi-recumbent; may be standing or sitting depending on ascites distribution
- Ultrasound guidance - recommended to identify optimal fluid collection site; increases safety and success
- Needle size - typically 18-20 gauge for ascitic fluid collection
- Local anesthesia - 1% lidocaine for skin infiltration reduces procedure discomfort
- Post-procedure observation - monitor vital signs; patient should rest 30-60 minutes; discharge after clinical assessment
- Contraindications/Precautions to Paracentesis:
- Uncorrectable coagulopathy - significantly elevated INR or low platelets (consult hematology)
- Skin infection at puncture site - defer procedure until infection resolved
- Bowel perforation risk - careful ultrasound assessment; consider alternative if high risk
- Minimal ascites - ensure adequate fluid volume by ultrasound; may need therapeutic tap deferral

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