TB Test Assessment and Evaluation 3 — Questions and Answers
Question 1: A QuantiFERON-TB Gold Plus (QFT-Plus) test reports TB1 and TB2 tube results. What is the significance of the TB2 tube?
- It detects CD4+ T-cell response only, improving sensitivity in immunocompromised patients
- It detects both CD4+ and CD8+ T-cell responses, potentially improving detection in some populations (Correct answer)
- It replaces the nil tube to reduce background interference
- It measures IgG antibody levels against ESAT-6 and CFP-10
Correct answer: It detects both CD4+ and CD8+ T-cell responses, potentially improving detection in some populations
The TB2 tube in QFT-Plus contains peptides that stimulate both CD4+ and CD8+ T-cells, potentially improving sensitivity in immunocompromised individuals.
Question 2: A nurse administers 0.1 mL of PPD tuberculin intradermally and observes a bleb of 6mm immediately after injection. What does this bleb indicate?
- The test is invalid and must be immediately reinjected at a different site
- Correct intradermal injection technique with proper bleb formation (Correct answer)
- Subcutaneous injection occurred and the site should be marked for reading
- The patient is likely sensitized and will have a large induration
Correct answer: Correct intradermal injection technique with proper bleb formation
A pale, raised bleb of 6–10mm immediately after injection confirms correct intradermal placement of the tuberculin.
Question 3: Which of the following populations has the LOWEST positive predictive value for TST, making IGRA generally preferred?
- Foreign-born persons from high-TB-burden countries
- HIV-positive individuals with CD4 count >500 cells/µL
- BCG-vaccinated individuals from countries with routine infant BCG programs (Correct answer)
- Household contacts of a confirmed active TB case
Correct answer: BCG-vaccinated individuals from countries with routine infant BCG programs
BCG vaccination can cause false-positive TST results, lowering the positive predictive value; IGRA antigens (ESAT-6, CFP-10) are absent from BCG strains.
Question 4: A patient's TST placed on Monday is read on Friday (96 hours later). The result shows 12mm induration. How should this be handled?
- Accept the result as positive since induration is present
- Reject the result as invalid; repeat TST immediately at a new site (Correct answer)
- Document as negative since reading beyond 72 hours is unreliable
- Request IGRA to confirm before making any clinical decision
Correct answer: Reject the result as invalid; repeat TST immediately at a new site
TST must be read 48–72 hours after placement; readings after 72 hours are unreliable and the test should be repeated.
Question 5: In a correctional facility outbreak investigation, a newly incarcerated individual tests positive on TST. Which factor is most important in determining whether this represents LTBI acquired inside or before incarceration?
- The size of the induration in millimeters
- Documentation of a negative TST at intake within the past 2 years (Correct answer)
- The inmate's country of origin
- The presence or absence of BCG vaccination scar
Correct answer: Documentation of a negative TST at intake within the past 2 years
A documented negative TST at intake followed by a new positive establishes a conversion that likely occurred during incarceration.
Question 6: When performing a contact investigation for a smear-positive pulmonary TB case, which contacts should be prioritized for immediate TST or IGRA testing?
- Casual contacts with <1 hour cumulative exposure in outdoor settings
- Close contacts sharing the same household or enclosed workspace for extended periods (Correct answer)
- All individuals who have ever met the index case socially
- Only contacts who report symptoms of cough or fever
Correct answer: Close contacts sharing the same household or enclosed workspace for extended periods
Household and close contacts with prolonged shared air exposure are at highest risk of transmission and are the priority for immediate evaluation.
Question 7: A patient previously treated for active TB disease presents with a positive TST. What is the appropriate interpretation?
- Reinfection with a new TB strain requiring full workup
- Expected positive result; TST remains positive after prior TB or LTBI treatment (Correct answer)
- False positive due to residual immune memory from BCG
- LTBI requiring immediate treatment initiation
Correct answer: Expected positive result; TST remains positive after prior TB or LTBI treatment
TST and IGRA can remain positive after successful TB treatment because immunological memory persists; this does not indicate active disease or treatment failure.
TB Test Assessment and Evaluation 3
A QuantiFERON-TB Gold Plus (QFT-Plus) test reports TB1 and TB2 tube results.
What is the significance of the TB2 tube?