Types of Tuberculosis Tests
There are two main categories of tuberculosis tests: tests for TB infection (latent or active) and tests for active TB disease, with the primary infection screening tests being the Tuberculin Skin Test (TST) and Interferon-Gamma Release Assays (IGRAs), while active disease diagnosis relies on microbiological methods including smear microscopy, culture, and molecular tests.
Tests for TB Infection (Screening for Latent TB or Active TB)
Tuberculin Skin Test (TST)
The TST uses purified protein derivative (PPD) tuberculin injected intradermally, producing a delayed-type hypersensitivity reaction that becomes detectable 2-12 weeks after infection 1. The reaction is read at 48-72 hours, measuring induration (not redness) 2.
Key limitations:
- False positives occur with BCG vaccination and nontuberculous mycobacterial infections 1
- Sensitivity approaches 100% in immunocompetent persons with latent TB, but specificity is reduced in BCG-vaccinated populations 1
- Requires two patient visits (placement and reading)
Interferon-Gamma Release Assays (IGRAs)
Two FDA-approved IGRAs are available: QuantiFERON-TB Gold In-Tube (QFT-GIT) and T-SPOT.TB 3. These blood tests measure interferon-gamma release when blood is exposed to M. tuberculosis-specific antigens (ESAT-6, CFP-10, and TB7.7 for QFT) 4.
Advantages over TST:
- Not affected by BCG vaccination 3, 4
- Single patient visit required
- More specific than TST in BCG-vaccinated populations 5
Current recommendations for infection testing 3, 6:
- Adults: Either IGRA or TST is acceptable; consider dual testing where a positive result from either would be considered positive
- Children <5 years: TST is preferred, though IGRA or TST is acceptable
- IGRAs may be used in all circumstances where TST is currently used 3
Tests for Active TB Disease
Microscopy (AFB Smear)
Direct smear examination for acid-fast bacilli using Ziehl-Neelsen, Kinyoun, or auramine-rhodamine fluorescent staining 7. Sensitivity is 80-90% in extensive cavitary disease but requires high bacterial loads 7. Results available within 24 hours 2.
Culture
The gold standard for TB diagnosis with 80% sensitivity and >90% specificity 7. Both liquid and solid culture methods should be performed on all specimens 6. Major limitation: results take weeks 2.
Nucleic Acid Amplification Tests (NAATs)
Rapid molecular diagnostics are now recommended for all suspected TB cases 8, 6:
- Xpert MTB/RIF and Xpert MTB/RIF Ultra: Automated, low-complexity tests detecting M. tuberculosis and rifampin resistance simultaneously 9
- Truenat platforms: Alternative low-complexity automated NAATs 9
- TB-LAMP: Low-complexity manual NAAT option 9
Performance 6:
- Overall sensitivity: 0.85, specificity: 0.97
- In AFB smear-positive patients: positive NAAT provides presumptive evidence of TB
- In AFB smear-negative patients with intermediate-to-high suspicion: positive NAAT supports diagnosis, but negative NAAT cannot exclude TB
Rapid Molecular Drug Susceptibility Testing
Xpert MTB/XDR detects resistance to rifampin, isoniazid, fluoroquinolones, and other drugs 10, 9. Targeted next-generation sequencing is now WHO-endorsed for comprehensive drug susceptibility testing at reference laboratories 9.
Indications for rapid molecular DST 6:
- Previous TB treatment
- Born in or lived ≥1 year in countries with TB incidence ≥20/100,000 or MDR-TB prevalence ≥2%
- Contact of MDR-TB patient
- HIV infection
Specialized Tests
For extrapulmonary TB 8:
- Appropriate specimens from suspected sites for microscopy, rapid molecular tests, culture, and histopathology
- Pleural biopsy for culture (sensitivity 85% percutaneous, 98% thoracoscopic) 7
- Adenosine deaminase in pleural fluid (sensitivity and specificity 95%) 7
For children and sputum-scarce populations 9:
- Stool, gastric aspirate, nasopharyngeal samples
- Novel specimen types including tongue swabs 9
For HIV-positive patients:
- Urine lipoarabinomannan (LAM) assays improve diagnostic yield, particularly in advanced HIV disease 9
Imaging
Chest radiography with computer-aided detection is a WHO-recommended screening and triage tool 9. Findings suggestive of TB warrant microbiological testing 8.
Common Pitfalls
- Never use fluoroquinolones as empiric therapy when TB is suspected—they can cause transient improvement and delay diagnosis 8
- Negative NAAT in smear-negative patients does not exclude TB 6
- IGRAs cannot distinguish recent from remote infection or predict progression to active disease 3
- Culture remains essential even when molecular tests are positive, for complete drug susceptibility testing 6