TB Test Core Knowledge and Fundamentals 3 — Questions and Answers
Question 1: A healthcare worker's TST shows 8 mm induration. She has no risk factors and has received BCG vaccine. How should this result be interpreted?
- Negative — the cutoff for healthcare workers is ≥10 mm (Correct answer)
- Positive — any induration ≥5 mm is positive for healthcare workers
- Positive — BCG makes the cutoff 5 mm
- Inconclusive — a blood test must confirm
Correct answer: Negative — the cutoff for healthcare workers is ≥10 mm
For healthcare workers without other risk factors, ≥10 mm induration is considered positive; 8 mm falls below that threshold.
Question 2: Which IGRA test measures interferon-gamma release in response to ESAT-6 and CFP-10 antigens?
- QuantiFERON-TB Gold Plus (QFT-Plus) (Correct answer)
- Mantoux TST
- Tuberculin purified protein derivative test
- Chest X-ray with CT enhancement
Correct answer: QuantiFERON-TB Gold Plus (QFT-Plus)
QFT-Plus and T-SPOT.TB are IGRA blood tests that detect IFN-γ released by sensitized T cells in response to M. tuberculosis–specific antigens.
Question 3: A person with HIV has a TST result of 4 mm. This is classified as:
- Negative (Correct answer)
- Positive
- Borderline — requires repeat in 1–3 weeks
- Inconclusive — IGRA required
Correct answer: Negative
For people living with HIV, ≥5 mm induration is positive; 4 mm does not meet that threshold and is classified negative.
Question 4: What does a 'booster effect' mean in two-step TST testing?
- A larger positive reaction on the second test due to immune memory stimulated by the first test (Correct answer)
- A false negative caused by repeated injections
- An allergic reaction to tuberculin
- A sign of active TB disease
Correct answer: A larger positive reaction on the second test due to immune memory stimulated by the first test
The booster effect occurs when an initial TST stimulates immune memory so a second test weeks later gives a larger (boosted) reaction, not a new infection.
Question 5: Which of the following is a contraindication to the Mantoux TST?
- Documented previous severe reaction (vesiculation, necrosis) to TST (Correct answer)
- Current mild upper respiratory infection
- BCG vaccination more than 1 year ago
- Age over 65
Correct answer: Documented previous severe reaction (vesiculation, necrosis) to TST
A prior severe reaction (vesiculation, ulceration, or necrosis) to TST is a contraindication to repeat testing.
Question 6: Which organism characteristic makes Mycobacterium tuberculosis resistant to drying and able to survive in room air?
- High lipid content in the cell wall (Correct answer)
- Production of endospores
- Gram-positive thick peptidoglycan layer
- Capsule made of polysaccharides
Correct answer: High lipid content in the cell wall
The mycolic acid–rich lipid cell wall of M. tuberculosis resists desiccation, harsh chemicals, and many disinfectants, enabling airborne survival.
Question 7: IGRA testing is preferred over TST in which of the following situations?
- Patients who have received BCG vaccine (Correct answer)
- Patients currently taking anti-TB therapy
- Infants under 2 years of age
- Patients with active TB disease
Correct answer: Patients who have received BCG vaccine
IGRA antigens (ESAT-6, CFP-10) are not encoded in BCG, so IGRA results are not confounded by prior BCG vaccination, unlike TST.
TB Test Core Knowledge and Fundamentals 3
A healthcare worker's TST shows 8 mm induration.
She has no risk factors and has received BCG vaccine.
How should this result be interpreted?