Tuberculin Skin Test (Mantoux Test) for TB

Tuberculin Skin Test

• A positive result is indicated by a delayed hypersensitivity reaction, evident 48-72 hours after intradermal injection of purified protein derivative (PPD), resulting in a raised, indurated lesion >6 mm diameter in non-vaccinated adults.
• In bacille Calmette-Guérin (BCG)-vaccinated adults, a positive result is indicated by a raised, indurated lesion >15 mm.
• False-negative results can occur in immunosuppressed individuals, such as those with HIV infection, sarcoidosis, or taking certain medications, and at the extremes of age or in active disease.
• False-positives can occur due to cross-reactivity with non-tuberculous mycobacteria and BCG Vaccine.

Interferon-Gamma Release Assays (IGRAs)

• IGRAs detect T-cell secretion of IFN-γ following exposure to M.tuberculosis-specific antigens (ESAT-6, CFP-10).
• The test does not differentiate between active and latent infection.
• IGRAs have high specificity compared to the TST and similar or better sensitivity, requiring only a single visit.

Microbiological Diagnosis

• Serial sputum samples should be collected on at least three occasions, ideally immediately upon waking.
• Bronchoscopy and lavage can be used to obtain respiratory secretions.
• Auramine-rhodamine staining is more sensitive (though less specific) than Ziehl-Neelsen staining, and is widely used.
• Auramine-rhodamine staining requires fluorescence microscopy and highlights bacilli as yellow-orange on a green background.
• Mycobacterial culture can be done using liquid/broth culture (1-3 weeks) or solid media (Lowenstein-Jensen slopes or Middlebrook agar) (3-8 weeks).

Tuberculin Skin Test

• A positive result is indicated by a delayed hypersensitivity reaction, evident 48-72 hours after intradermal injection of purified protein derivative (PPD), resulting in a raised, indurated lesion >6 mm diameter in non-vaccinated adults.
• In bacille Calmette-Guérin (BCG)-vaccinated adults, a positive result is indicated by a raised, indurated lesion >15 mm.
• False-negative results can occur in immunosuppressed individuals, such as those with HIV infection, sarcoidosis, or taking certain medications, and at the extremes of age or in active disease.
• False-positives can occur due to cross-reactivity with non-tuberculous mycobacteria and BCG Vaccine.

Interferon-Gamma Release Assays (IGRAs)

• IGRAs detect T-cell secretion of IFN-γ following exposure to M.tuberculosis-specific antigens (ESAT-6, CFP-10).
• The test does not differentiate between active and latent infection.
• IGRAs have high specificity compared to the TST and similar or better sensitivity, requiring only a single visit.

Microbiological Diagnosis

• Serial sputum samples should be collected on at least three occasions, ideally immediately upon waking.
• Bronchoscopy and lavage can be used to obtain respiratory secretions.
• Auramine-rhodamine staining is more sensitive (though less specific) than Ziehl-Neelsen staining, and is widely used.
• Auramine-rhodamine staining requires fluorescence microscopy and highlights bacilli as yellow-orange on a green background.
• Mycobacterial culture can be done using liquid/broth culture (1-3 weeks) or solid media (Lowenstein-Jensen slopes or Middlebrook agar) (3-8 weeks).