What are the recommended guidelines for using the Cartridge‑Based Nucleic Acid Amplification Test (CBNAAT) to diagnose tuberculosis and assess rifampicin resistance?

CBNAAT Guidelines for Tuberculosis Diagnosis

CBNAAT (Cartridge-Based Nucleic Acid Amplification Test) should be performed on at least one respiratory specimen from every patient with suspected pulmonary tuberculosis, with results interpreted in correlation with AFB smear status to guide immediate treatment decisions. 1

Core Testing Algorithm

When to Order CBNAAT

Perform CBNAAT on patients with:

• Signs and symptoms of pulmonary TB where diagnosis is not yet established
• Cases where results would alter case management or TB control activities (contact investigations)
• At least one respiratory specimen, preferably the first diagnostic specimen 1

Critical timing requirement: Results must be available within 48 hours of specimen collection 1

Specimen Processing

• Collect respiratory specimens (sputum) routinely
• Process specimens (liquefy, decontaminate, concentrate) for AFB smear, culture, AND CBNAAT simultaneously
• Do not delay specimen collection or testing while awaiting CBNAAT results 1
• Suspend specimen in sufficient buffer volume for all planned tests 1

Result Interpretation Framework

Scenario 1: CBNAAT Positive + AFB Smear Positive

Action: Presume TB and begin anti-TB treatment immediately while awaiting culture

• Positive predictive value >95% 1
• This is the strongest indication for treatment initiation

Scenario 2: CBNAAT Positive + AFB Smear Negative

Action: Use clinical judgment for treatment initiation

• Consider testing additional specimen with CBNAAT for confirmation
• Presume TB if ≥2 specimens are CBNAAT positive 1
• In smear-negative patients with intermediate-to-high clinical suspicion, positive CBNAAT serves as presumptive evidence 2

Scenario 3: CBNAAT Negative + AFB Smear Positive

Action: Test for inhibitors and retest with additional specimen

• If inhibitors detected: CBNAAT provides no diagnostic help; use clinical judgment 1
• If no inhibitors detected: Consider nontuberculous mycobacteria if second specimen is also smear-positive/CBNAAT-negative 1

Scenario 4: CBNAAT Negative + AFB Smear Negative

Action: Use clinical judgment; negative CBNAAT cannot exclude TB

• CBNAAT sensitivity only 50-80% in smear-negative, culture-positive cases 1
• Do not use single negative result to definitively exclude TB, especially with moderate-to-high clinical suspicion 2

Rifampicin Resistance Testing

Perform rapid molecular drug susceptibility testing for rifampicin on specimens from patients who are AFB smear-positive or CBNAAT-positive AND meet any of these criteria 2:

1. Previously treated for tuberculosis
2. Born in or lived ≥1 year in countries with TB incidence ≥20 per 100,000 OR MDR-TB prevalence ≥2%
3. Contacts of MDR-TB patients
4. HIV-infected

CBNAAT accuracy for rifampicin resistance: Sensitivity 100%, Specificity 99.4% 3

Critical Caveats

Culture Remains Essential

• Culture is the gold standard and required for drug susceptibility testing and genotyping 1
• Always reserve sufficient specimen portions for culture regardless of CBNAAT result 1
• For extrapulmonary TB, CBNAAT should be accompanied by culture wherever possible, irrespective of CBNAAT result 4

Limitations to Recognize

Inhibitors: 3-7% of sputum specimens contain inhibitors causing false-negative results 1

Low clinical suspicion: Do not order CBNAAT routinely when TB suspicion is low—positive predictive value <50% in such cases 1

Extrapulmonary TB: Sensitivity varies significantly by specimen type 3:

• CSF: 88.2% sensitivity, 96.0% specificity (high-certainty evidence)
• Pleural fluid: 74.0% sensitivity, 88.1% specificity (low-certainty evidence)
• Lymph node aspirate: 71.3% sensitivity, 97.4% specificity

Reporting Requirements

Treat initial positive CBNAAT as a critical test value:

• Immediately report to clinician and public health authorities
• Laboratorians must be available for consultation on interpretation 1

Practical Implementation

Do not use CBNAAT alone: The test accelerates diagnosis from 1-2 weeks to 1-2 days, enabling earlier treatment initiation and interruption of transmission 1. However, clinical judgment incorporating radiological, histopathological findings, and treatment response remains essential, particularly for smear-negative cases and extrapulmonary TB 5.

Consultation threshold: Seek TB expert consultation if inexperienced in NAA test interpretation or TB diagnosis/treatment 1