16 Questions
What is the expected diameter of a raised, indurated lesion in a non-vaccinated adult with a positive tuberculin skin test result?
Greater than 6 mm
What is a common cause of a false-negative tuberculin skin test result?
HIV infection
What is the advantage of interferon-gamma release assays over tuberculin skin tests?
Higher specificity
What is the purpose of collecting serial sputum samples in microbiological diagnosis?
To confirm the diagnosis of tuberculosis
What is the advantage of Auramine–rhodamine staining over Ziehl–Neelsen staining?
It is more sensitive
What is the significance of ESAT-6 and CFP-10 in interferon-gamma release assays?
They are tuberculosis-specific antigens
What is the time frame in which a delayed hypersensitivity reaction is evident after an intradermal injection of purified protein derivative (PPD)?
48-72 hours
In which of the following individuals is a raised, indurated lesion >15 mm in diameter indicative of a positive tuberculin skin test result?
BCG-vaccinated adults
What is a common cause of false-negative tuberculin skin test results in certain populations?
All of the above
What is the advantage of interferon-gamma release assays compared to microbiological diagnosis?
It requires a single visit
What is the purpose of bronchoscopy and lavage in microbiological diagnosis?
To obtain respiratory secretions
What is the characteristic of Auramine–rhodamine staining?
Less specific but more sensitive
What is the role of M. tuberculosis-specific antigens in interferon-gamma release assays?
To stimulate T-cell secretion of IFN-γ
Why are serial sputum samples collected in microbiological diagnosis?
To increase the sensitivity of diagnosis
What is the significance of fluorecence microscopy in Auramine–rhodamine staining?
Highlights bacilli as yellow-orange on a green background
What is the significance of a positive result in a tuberculin skin test?
Presence of M. tuberculosis infection
Study Notes
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).
This quiz covers the Tuberculin Skin Test, also known as the Mantoux test, used to diagnose tuberculosis (TB). Learn how to interpret the results, including the measurement of the raised, indurated lesion in non-vaccinated and vaccinated adults.
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