NAAT

Definition And Core Principles

Nucleic Acid Amplification Test (NAAT) is a molecular diagnostic method that detects and amplifies specific nucleic acid sequences of the Mycobacterium tuberculosis complex (MTBC) directly from clinical specimens.
It is the preferred initial diagnostic tool in pediatric tuberculosis due to the paucibacillary nature of the disease in children.
The mechanism involves DNA or RNA extraction from the sample, followed by target amplification utilizing specific primers and probes.
Common gene targets include the rpoB gene (utilized for MTB detection and rifampicin resistance), IS6110, and 16S rRNA.
Detection is achieved via fluorescence, hybridization, or colorimetric methods in automated or semi-automated platforms.

Platform	Characteristics And Utility
Cartridge-Based NAAT (CBNAAT / GeneXpert MTB/RIF)	Utilizes semi-nested real-time PCR. Detects MTB and rifampicin resistance via five overlapping rpoB probes. World Health Organization recommended as an initial test. Provides results within 2 hours.
Truenat MTB/RIF	Portable, battery-operated, chip-based real-time PCR. Highly suitable for peripheral laboratories and point-of-care use in children.
Line Probe Assay (LPA)	Includes GenoType MTBDRplus and MTBDRsl for first- and second-line drug resistance detection. Can be performed from positive cultures or direct specimens. Generates results in 24 hours.
Loop-Mediated Isothermal Amplification (LAMP / TB-LAMP)	A simple, equipment-light method utilized primarily for resource-limited settings.

All children presenting with presumptive pulmonary or extrapulmonary TB (e.g., fever, cough lasting greater than 2 weeks, weight loss, contact history).
Smear-negative or smear-scanty cases, which represent the majority of pediatric presentations.
High-risk groups including HIV co-infection, severe acute malnutrition, immunocompromised states, and suspected drug-resistant TB.
Extrapulmonary TB evaluation utilizing specific samples like cervical lymphadenopathy (FNAC), tuberculous meningitis (CSF), pleural or pericardial effusion, abdominal TB, and osteoarticular TB.
Critically ill children requiring rapid diagnosis prior to the availability of culture results, which typically take 6-8 weeks.

Pulmonary TB: Two to three early-morning gastric aspirates (preferred in children less than 8 years old, requiring 5-10 ml volume), induced sputum using 3% hypertonic saline nebulization with chest physiotherapy, or bronchoalveolar lavage.
Extrapulmonary TB: Fine-needle aspiration cytology (FNAC) for lymph nodes yields high cellularity. Cerebrospinal fluid (minimum 2-3 ml for tuberculous meningitis), sterile body fluids, and tissue biopsies are also utilized.
Direct testing is possible without prior decontamination in most automated platforms.
Multiple samples increase diagnostic yield; gastric aspirates must be collected after overnight fasting with proper nasogastric tube placement.

NAAT Result	Clinical Interpretation And Action
MTB Detected	Initiate daily Anti-Tubercular Therapy (ATT). If rifampicin resistance is identified, initiate shorter or longer MDR-TB regimen per current protocols.
MTB Not Detected	Does not definitively exclude TB. Integrate with clinical features, chest radiograph, Mantoux/IGRA, histopathology, and overall treatment response.
Rifampicin Resistant	Avoid rifampicin in the treatment regimen. Confirm resistance with phenotypic Drug Susceptibility Testing (DST) on culture.
Invalid Or Error	Repeat the test using a fresh sample. Check for technical issues or presence of inhibitors in the sample.

Feature	Advantages	Limitations
Diagnostic Yield	High sensitivity of 60-90% in pulmonary TB (surpassing smear microscopy at 30-50%) and 40-70% in extrapulmonary TB. High specificity greater than 95%.	Lower sensitivity in smear-negative extrapulmonary TB and culture-negative cases, presenting a false negative rate of 20-40%.
Drug Resistance	Enables simultaneous detection of rifampicin resistance, facilitating early initiation of MDR-TB regimens.	Does not provide full phenotypic DST; traditional culture remains the gold standard for second-line drugs.
Viability Assessment	Rapid turnaround time (hours versus weeks for culture). Works efficiently on small-volume, paucibacillary pediatric samples.	Cannot distinguish live from dead bacilli; a positive result may persist post-treatment. Not utilized for routine treatment monitoring.

Infants and young children have a higher proportion of extrapulmonary and disseminated TB; diagnostic yield improves significantly with multiple gastric aspirates and lymph node sampling.
Gastric lavage technique is critical, requiring proper tube placement, early morning collection, and neutralization if processing is delayed.
In neonates, use smallest volume platforms (like Truenat) combined with chest radiograph and CSF analysis.
In HIV-exposed infants, NAAT is mandatory to differentiate TB from other opportunistic infections causing failure to thrive and pneumonia.