What are the diagnostic tests for tuberculosis (TB)?

Diagnostic Testing for Tuberculosis

For suspected active pulmonary TB, obtain at least three sputum specimens for AFB smear microscopy, mycobacterial culture (preferably both liquid and solid media), and perform nucleic acid amplification testing (NAAT) on the initial specimen, with chest radiography to support clinical assessment. 1

Essential Diagnostic Tests for Active TB Disease

Sputum Collection and AFB Smear Microscopy

• Collect three sputum specimens on separate days, with first morning specimens preferred as they yield 12% greater sensitivity than single spot specimens 1, 2
• Request at least 3 mL sputum volume, optimally 5-10 mL to address the pervasive issue of poor sample quality 1
• Use concentrated specimens and fluorescence microscopy rather than conventional methods, as concentration increases sensitivity by 18% and fluorescence microscopy is 10% more sensitive 1
• AFB smear microscopy achieves approximately 70% sensitivity with three specimens (first specimen 53.8%, second adds 11.1%, third adds only 2-5%) 1, 2

Critical caveat: A negative AFB smear does not exclude pulmonary TB due to false-negative rates, and a positive smear does not confirm TB due to potential nontuberculous mycobacteria (specificity ≥90% but PPV varies 70-90%) 1

Mycobacterial Culture - The Gold Standard

• Perform both liquid and solid culture methods on all specimens, as liquid cultures have higher sensitivity (88-90%) compared to solid cultures (76%) 1, 2
• Liquid media provide results in average 10-14 days, while solid media require 3-4 weeks 1
• Culture is essential even when molecular tests are performed because it enables drug susceptibility testing and confirms diagnosis 1, 2
• The standard of care requires ≥90% of adult patients with clinical TB diagnosis should have culture confirmation 1

Nucleic Acid Amplification Testing (NAAT)

• Perform NAAT on the initial respiratory specimen from patients with suspected pulmonary TB, with results available within 1-2 days 1
• In AFB smear-positive patients, a positive NAAT provides rapid confirmation 1
• In AFB smear-negative patients with intermediate-to-high suspicion, a positive NAAT serves as presumptive evidence of TB disease, but a negative NAAT cannot exclude pulmonary TB 1
• Appropriate NAATs include the Hologic Amplified MTD test and Cepheid Xpert MTB/RIF test 1

Rapid Molecular Drug Susceptibility Testing

Perform rapid molecular testing for rifampin resistance (with or without isoniazid) using respiratory specimens in patients who are AFB smear-positive or NAAT-positive AND meet any of these criteria: 1

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

Results are available within 1-2 days 1

Clinical and Radiographic Assessment

When to Suspect TB

Consider TB diagnosis in patients with: 1

• Persistent cough lasting ≥3 weeks (hallmark symptom)
• Hemoptysis (bloody sputum)
• Constitutional symptoms: fever, night sweats, weight loss, anorexia

Chest Radiography

• Obtain chest X-ray in all patients with suspected TB to identify radiological findings consistent with TB 2, 3
• Advanced imaging (CT, MRI, ultrasound, PET-CT) may be useful for extrapulmonary TB or when chest X-ray is normal or inconclusive 2, 3, 4

Testing for Latent TB Infection (Not Active Disease)

Important distinction: TST (tuberculin skin test) and IGRA (interferon-gamma release assays) detect TB infection, not active disease, and should NOT be used to exclude active TB diagnosis in symptomatic patients 1

PPD/Tuberculin Skin Test (TST)

• Administer 0.1 mL of PPD (5 TU) intracutaneously on the volar or dorsal forearm surface, creating a 6-10 mm wheal 1
• Read results at 48-72 hours by trained personnel (not patient self-reading), measuring only induration (not erythema) in millimeters 1
• Interpretation depends on patient risk factors, with positive thresholds of ≥5mm, ≥10mm, or ≥15mm based on clinical context 1, 3

IGRA Testing

• IGRAs are recommended in combination with TST for latent TB infection screening 3
• For adults, either IGRA or TST is acceptable; for children <5 years, TST is preferred 1

Special Populations

Children

• Obtain early morning gastric aspirates (or washings with 10-20 mL sterile water) when children <10 years cannot produce expectorated sputum, with expected yield of 50% 1, 2
• Extra efforts for culture confirmation are warranted when there is no culture-positive index case or MDR-TB is suspected 1
• Microbiological confirmation may not be necessary for children with uncomplicated pulmonary TB identified through recent contact investigation if the source case has drug-susceptible TB 1

Culture-Negative TB

• When cultures remain negative but clinical/radiographic findings strongly suggest TB, initiate treatment with INH, RIF, PZA, and EMB even with negative smears 1
• Reassess at 2 months for clinical or radiographic improvement; if present with no alternative diagnosis, continue treatment 1
• Alternative diagnoses must be carefully considered, potentially requiring bronchoscopy with bronchoalveolar lavage and biopsy 1

Diagnostic Algorithm Summary

1. Collect ≥3 sputum specimens (first morning preferred, 5-10 mL volume) 1
2. Perform AFB smear microscopy (concentrated, fluorescence method) on all specimens 1
3. Perform NAAT on initial specimen for rapid results 1
4. Culture all specimens using both liquid and solid media 1, 2
5. Obtain chest radiography to support clinical assessment 2, 3
6. Perform rapid molecular drug susceptibility testing in high-risk patients 1
7. Identify isolates and perform first-line drug susceptibility testing on all initial isolations 1, 3

This comprehensive approach enables rapid laboratory confirmation, earlier treatment initiation, improved patient outcomes, and reduced transmission 2