What diagnostic tests can be used to diagnose Tuberculosis (TB)?

From the Guidelines

Tuberculosis (TB) can be diagnosed through several tests, including the tuberculin skin test (TST), interferon-gamma release assays (IGRAs), chest X-rays, sputum microscopy, sputum culture, and nucleic acid amplification tests (NAATs) like GeneXpert MTB/RIF, with the choice depending on the suspected type of TB and patient factors, as recommended by the most recent guidelines 1.

Diagnostic Tests for TB

The following tests can be used to diagnose TB:

• Tuberculin skin test (TST) or Mantoux test: involves injecting a small amount of TB protein under the skin and checking for a reaction after 48-72 hours
• Interferon-gamma release assays (IGRAs) like QuantiFERON-TB Gold and T-SPOT.TB: blood tests that measure immune response to TB proteins
• Chest X-rays: can show lung abnormalities suggestive of TB
• Sputum microscopy: examines sputum samples for TB bacteria using acid-fast bacilli (AFB) staining
• Sputum culture: the gold standard, grows TB bacteria from samples but takes 2-8 weeks for results
• Nucleic acid amplification tests (NAATs) like GeneXpert MTB/RIF: provide rapid results within hours and can detect drug resistance

Additional Diagnostic Considerations

For extrapulmonary TB, tissue biopsies or fluid samples from affected areas may be needed, and drug susceptibility testing determines which medications will effectively treat the infection, as noted in the guidelines 1.

Test Selection and Interpretation

The selection of diagnostic tests and interpretation of results should be based on clinical presentation, TB risk factors, and test availability, with consideration of the most recent guidelines and expert recommendations 1.

From the Research

Diagnostic Tests for Tuberculosis

• The following tests can be used to diagnose tuberculosis:
  • GeneXpert MTB/RIF assay: a rapid, automated test that can detect both TB and rifampicin resistance within two hours 2, 3, 4, 5, 6
  • Ziehl-Neelsen (ZN) smear microscopy: a conventional method for detecting acid-fast bacilli (AFB) in sputum specimens 2, 6
  • Lowenstein-Jensen (LJ) culture: a conventional method for culturing Mycobacterium tuberculosis (MTB) from clinical specimens 6
  • MTB culture: a method for culturing MTB from sputum specimens 2

Performance of Diagnostic Tests

• GeneXpert MTB/RIF assay:
  • Sensitivity: 88-98% for pulmonary TB, 68-81% for extrapulmonary TB 3, 4, 5, 6
  • Specificity: 93-100% for pulmonary TB, 78-100% for extrapulmonary TB 3, 4, 5, 6
• Ziehl-Neelsen (ZN) smear microscopy:
  • Sensitivity: 72.1% for pulmonary TB, 63.2% for extrapulmonary TB 6
  • Specificity: 81.3% for pulmonary TB, 70.5% for extrapulmonary TB 6
• Lowenstein-Jensen (LJ) culture:
  • Used as a gold standard for diagnosing TB 6

Advantages and Limitations of Diagnostic Tests

• GeneXpert MTB/RIF assay:
  • Advantages: rapid, automated, highly sensitive, and specific 2, 3, 4, 5, 6
  • Limitations: more expensive than conventional sputum microscopy, may not detect all cases of TB 5
• Ziehl-Neelsen (ZN) smear microscopy:
  • Advantages: conventional method, widely available 6
  • Limitations: less sensitive and specific than GeneXpert MTB/RIF assay 6
• Lowenstein-Jensen (LJ) culture:
  • Advantages: used as a gold standard for diagnosing TB 6
  • Limitations: time-consuming, requires specialized equipment and expertise 6