What are the diagnostic tests for active tuberculosis (TB)?

From the Guidelines

The most effective diagnostic tests for active tuberculosis include sputum smear microscopy, culture, nucleic acid amplification tests (NAATs) like GeneXpert MTB/RIF, and chest X-rays, with mycobacterial culture being the gold standard for definitive diagnosis 1. The initial approach to diagnosing active tuberculosis typically involves collecting sputum samples for acid-fast bacilli (AFB) smear microscopy, which can provide rapid results but has limited sensitivity.

• Mycobacterial culture remains the gold standard, offering definitive diagnosis but requiring 2-8 weeks for results.
• GeneXpert MTB/RIF provides results within hours with high sensitivity and can detect rifampin resistance.
• Chest X-rays help identify lung abnormalities consistent with TB but aren't specific enough for definitive diagnosis. Additional tests include:
• Liquid culture systems (MGIT)
• Line probe assays for drug resistance
• Interferon-gamma release assays (IGRAs) to detect TB infection For extrapulmonary TB, appropriate specimens from affected sites should be collected. In children or HIV-positive patients where sputum collection may be difficult, gastric aspirates or induced sputum may be necessary 1. Diagnosis often requires combining multiple tests, as no single test is perfect for all situations, particularly in paucibacillary disease or extrapulmonary cases. The choice of diagnostic tests may also depend on the clinical presentation and risk factors of the patient, as well as the availability of resources and expertise 1.

From the Research

Diagnostic Tests for Active Tuberculosis (TB)

The diagnostic tests for active tuberculosis (TB) include:

• Xpert MTB/RIF assay: a rapid and simple technique with high sensitivity and specificity for diagnosing TB and detecting drug resistance in extrapulmonary and smear-negative TB cases 2
• GeneXpert MTB/RIF: a molecular biological diagnostic technique for MTB, recommended by the World Health Organization (WHO) as a standard diagnostic tool for MTB 3
• Transbronchial lung cryobiopsy with GeneXpert MTB/RIF assay: a diagnostic test with high sensitivity and specificity for TB diagnosis, especially in patients with peripheral localized lesions and HIV patients 4
• Sputum smear microscopy: a traditional diagnostic method, but with lower sensitivity compared to Xpert MTB/RIF assay and GeneXpert MTB/RIF 3, 5
• Culture results: considered as the gold standard for diagnosis of TB, and drug susceptibility testing for detection of RIF resistance 2

Characteristics of Diagnostic Tests

The characteristics of these diagnostic tests are:

• Xpert MTB/RIF assay: sensitivity of 86% and specificity of 88.4% for detection of TB 2
• GeneXpert MTB/RIF: sensitivity of 57.4% and 63.9% for bronchial brushings and biopsies, respectively, and a specificity of 100% 3
• Transbronchial lung cryobiopsy with GeneXpert MTB/RIF assay: sensitivity of 81.3% and specificity of 100% 4
• Sputum smear microscopy: sensitivity of 13.1% and 32.8% for sputum smear and bronchial brush smear, respectively 3

Clinical Application

These diagnostic tests can be used in clinical practice to:

• Rapidly diagnose TB and detect drug resistance 2, 3
• Evaluate the efficacy of diagnostic tests for TB diagnosis 3, 4
• Reduce unnecessary isolation for inpatients with presumed tuberculosis 5
• Improve patient outcomes and hospital resource utilization 5