TB Test Assessment and Evaluation 2 — Questions and Answers
Question 1: A patient with a 14mm induration on TST has no risk factors and was born in a low-prevalence country. How should this result be interpreted?
- Negative — threshold for low-risk individuals is 15mm (Correct answer)
- Positive — any induration ≥10mm is positive
- Positive — any induration ≥5mm is positive
- Inconclusive — requires IGRA confirmation
Correct answer: Negative — threshold for low-risk individuals is 15mm
For persons with no known risk factors in low-prevalence settings, the positive threshold is ≥15mm induration.
Question 2: Which IGRA result pattern indicates a borderline or indeterminate finding that requires retesting?
- IFN-γ nil <0.35 IU/mL with TB antigen response ≥0.35 IU/mL
- IFN-γ nil ≥8 IU/mL causing invalid background (Correct answer)
- TB antigen minus nil ≥0.35 IU/mL with mitogen response ≥0.5 IU/mL
- TB antigen minus nil <0.35 IU/mL with mitogen response ≥0.5 IU/mL
Correct answer: IFN-γ nil ≥8 IU/mL causing invalid background
An indeterminate IGRA result occurs when the nil tube IFN-γ is excessively elevated (≥8 IU/mL), invalidating the test.
Question 3: A chest X-ray shows upper lobe fibrotic changes in a patient with a positive TST. What is the most appropriate next step?
- Repeat TST in 8 weeks
- Order sputum smear and culture to rule out active TB (Correct answer)
- Initiate LTBI treatment immediately without further workup
- Perform IGRA to confirm TST positivity
Correct answer: Order sputum smear and culture to rule out active TB
Fibrotic upper lobe changes on CXR in a TST-positive patient raise concern for active or prior TB, requiring microbiologic evaluation.
Question 4: During a TB contact investigation, which classification describes a person exposed to an infectious TB case who has a negative TST but should be retested in 8–10 weeks?
- Window period contact (Correct answer)
- Confirmed LTBI contact
- Non-reactor contact
- Recently converted contact
Correct answer: Window period contact
Window period contacts had recent exposure and may not yet have mounted a detectable immune response, so retesting after the window period is required.
Question 5: Which of the following conditions can cause a false-negative TST result despite true LTBI?
- Recent BCG vaccination within the past year
- Advanced HIV infection with CD4 count <200 cells/µL (Correct answer)
- Prior positive TST documented 5 years ago
- Testing performed more than 72 hours after placement
Correct answer: Advanced HIV infection with CD4 count <200 cells/µL
Severe immunosuppression such as advanced HIV reduces T-cell response, causing anergy and false-negative TST results.
Question 6: A two-step TST is performed. The first test is negative at 72 hours, and a second test two weeks later shows 12mm induration. How should this second result be interpreted?
- New positive — the patient recently converted and has LTBI
- Boosted reaction — the patient likely has remote LTBI, not new infection (Correct answer)
- False positive due to repeat antigen exposure
- Indeterminate — requires IGRA confirmation
Correct answer: Boosted reaction — the patient likely has remote LTBI, not new infection
A positive second TST after a negative first in two-step testing is considered a boosted reaction reflecting remote prior infection, not recent conversion.
Question 7: When evaluating a healthcare worker for annual TB surveillance, which scenario would prompt an immediate evaluation regardless of prior test results?
- Annual TST showing induration 2mm larger than last year
- New onset of cough lasting 3 weeks plus night sweats and weight loss (Correct answer)
- Positive IGRA result in a worker who had BCG vaccination
- Negative IGRA result following a known TB exposure 4 weeks ago
Correct answer: New onset of cough lasting 3 weeks plus night sweats and weight loss
Symptoms of prolonged cough, night sweats, and weight loss are classic active TB symptoms requiring immediate clinical evaluation regardless of prior testing.
TB Test Assessment and Evaluation 2
A patient with a 14mm induration on TST has no risk factors and was born in a low-prevalence country.
How should this result be interpreted?