What are the current recommended diagnostic methods for suspected pulmonary tuberculosis in adults and children, including HIV‑positive patients?

Newer Tuberculosis Diagnosis Methods

Core Diagnostic Algorithm for Suspected Pulmonary TB

All patients with suspected pulmonary tuberculosis should undergo simultaneous testing with AFB smear microscopy, rapid molecular testing (NAAT), and mycobacterial culture on both liquid and solid media from respiratory specimens. 1

Specimen Collection Strategy

• Collect at least 3 respiratory specimens (not necessarily on consecutive days, but can be same-day collection) to maximize diagnostic yield, as the first specimen detects 53.8% of cases, the second adds 11.1%, and the third adds 2-5% additional yield 2, 1

• Obtain at least one early morning sputum sample (12% more sensitive than spot specimens) with a minimum volume of 3 mL, though optimal volume is 5-10 mL 2, 1

• Use concentrated specimens and fluorescence microscopy rather than conventional methods, as concentration increases sensitivity by 18% and fluorescence microscopy is 10% more sensitive 2, 1

• For children unable to produce sputum, collect gastric aspirates on 3 consecutive mornings (40-50% yield overall, up to 90% in infants), or use sputum induction with bronchodilator or nasopharyngeal aspiration (20-30% yield) 3, 1

Rapid Molecular Testing (The "Newer" Method)

Nucleic acid amplification testing (NAAT), specifically the Xpert MTB/RIF assay, represents the most significant advance in TB diagnostics and should be performed on the initial respiratory specimen from all patients with suspected TB. 1, 4

NAAT Performance Characteristics:

• In AFB smear-positive patients: 96-97% sensitivity, 85-95% specificity 4
• In AFB smear-negative patients: 66-79% sensitivity, 80-98% specificity 4
• Provides results within 1 day compared to 2-6 weeks for culture 2, 4
• Simultaneously detects rifampicin resistance (96% sensitivity, 98% specificity for resistance detection), serving as a marker for MDR-TB in approximately 90% of cases 3, 4

NAAT Result Interpretation Algorithm:

• AFB smear-positive + NAAT positive: Presume TB and initiate treatment immediately (positive predictive value >95%) 4
• AFB smear-negative + NAAT positive: Use as presumptive evidence of TB disease; consider testing additional specimens, with two or more positive NAATs allowing presumptive diagnosis 1, 4
• AFB smear-positive + NAAT negative: Evaluate for inhibitors and collect new specimen; this discordance suggests either non-tuberculous mycobacteria or technical issues 4
• AFB smear-negative + NAAT negative: Cannot exclude TB in patients with intermediate-to-high clinical suspicion; proceed with culture and clinical evaluation 1, 4

Culture Requirements (Still Essential)

Both liquid and solid mycobacterial cultures must be performed on every specimen, as culture remains the gold standard despite molecular advances. 1, 4

• Liquid culture: 88-90% sensitivity, faster time to detection (13-15 days), but higher contamination rate (4-9%) 1
• Solid culture: 76% sensitivity, longer time to detection (25.8 days), but serves as safeguard against contamination and detects some isolates missed by liquid culture 1
• All molecular resistance results must be confirmed by phenotypic culture-based drug susceptibility testing 1, 3, 4

Rapid Molecular Drug Susceptibility Testing

Perform rapid molecular DST for rifampicin with or without isoniazid on respiratory specimens from patients who are AFB smear-positive or NAAT-positive AND meet any of these criteria: 1

• Previous TB treatment
• Born in or lived ≥1 year in a country with TB incidence ≥20 per 100,000 or MDR-TB prevalence ≥2%
• Contact of MDR-TB patient
• HIV-infected

Special Considerations for HIV-Positive Patients

• Lower threshold for bronchoscopy due to atypical presentations and lower smear sensitivity 2
• Expedited diagnostic evaluation is essential, and if clinical evidence strongly suggests TB, initiate treatment empirically while awaiting results 1
• NAAT sensitivity may be lower in HIV-positive patients with paucibacillary disease, making culture even more critical 5

Extrapulmonary TB Diagnosis

For all patients with suspected extrapulmonary TB, obtain appropriate specimens from suspected sites for microscopy, rapid molecular tests, culture, species identification, and DST. 1

• NAAT sensitivity for extrapulmonary specimens: 56-62% with approximately 98% specificity 4
• Measure adenosine deaminase levels on pleural, cerebrospinal, ascitic, or joint fluid 1
• Perform cell counts and chemistries on amenable fluid specimens 1
• For miliary TB: CB-NAAT yields positive results in only 50-60% of cases (false-negative rate 40-50%), so negative NAAT cannot exclude disease; obtain multiple specimen types and do not withhold treatment if clinical suspicion is high 4

Bronchoscopy Indications

Perform flexible bronchoscopy when: 1

• Induced sputum is AFB smear-negative or cannot be obtained
• Rapid diagnosis is essential in seriously ill patients
• Suspected miliary TB with no alternative accessible lesions

Bronchoscopic sampling should include: BAL plus brushings for most patients; add transbronchial biopsy when rapid diagnosis is essential 1

Collect postbronchoscopy sputum specimens from all adults undergoing bronchoscopy for suspected TB 1

Critical Pitfalls to Avoid

• Never rely on a single negative sputum specimen to exclude TB—three specimens are required 2
• Never use TST or IGRA to exclude active TB disease—these detect latent infection, not active disease 2
• Never assume negative AFB smears exclude TB—approximately 40% of culture-positive cases are smear-negative 2
• Never delay empiric treatment in seriously ill patients while awaiting culture results if clinical suspicion is high 1, 2
• Never forget to collect specimens for culture and DST before starting treatment, as this guides definitive therapy 2
• Never use fluoroquinolones as empiric broad-spectrum antibiotics in patients with suspected TB, as they are active against M. tuberculosis and may cause transient improvement, delaying diagnosis 1
• A negative NAAT cannot definitively exclude TB, especially with moderate-to-high clinical suspicion 4

Quality Assurance Requirements

• Use only WHO-recommended rapid diagnostic assays (Xpert MTB/RIF, Hologic Amplified MTD) 1
• Ensure testing in quality-assured laboratories with established proficiency and appropriate turnaround times 1, 3
• NAAT results should be available within 48 hours of specimen collection 4
• Positive NAAT results are critical values requiring immediate reporting to clinicians and public health authorities 4