TB Test Practice and Application 2 — Questions and Answers
Question 1: A TST is placed on a patient who received BCG vaccination as a child. The induration measures 12 mm. What is the correct interpretation?
- Disregard BCG history; 12 mm is positive for a standard-risk person (Correct answer)
- The result is negative because BCG causes false positives
- The result must be confirmed with a chest X-ray before reading
- BCG vaccination automatically invalidates the TST result
Correct answer: Disregard BCG history; 12 mm is positive for a standard-risk person
In the US, BCG history is generally disregarded when interpreting TST because BCG is not routinely used and its effect wanes; 10 mm or more is positive for most standard-risk adults.
Question 2: Which anatomical site is the standard location for placing a TST (Mantoux test)?
- Volar surface of the forearm (Correct answer)
- Deltoid muscle
- Dorsal surface of the forearm
- Antecubital fossa
Correct answer: Volar surface of the forearm
The Mantoux TST is injected intradermally on the volar (inner) surface of the forearm, 2–4 inches below the elbow.
Question 3: A healthcare worker's TST shows 9 mm induration. She has no known risk factors. How should this result be classified?
- Negative, because the threshold for healthcare workers is 10 mm (Correct answer)
- Positive, because any induration in a healthcare worker is significant
- Positive, because the threshold is 5 mm for all tested individuals
- Indeterminate; repeat in 1–3 weeks
Correct answer: Negative, because the threshold for healthcare workers is 10 mm
For healthcare workers without additional risk factors, 10 mm or more is the positive threshold; 9 mm is therefore negative.
Question 4: What is the recommended needle gauge for administering a Mantoux tuberculin skin test?
- 26–27 gauge (Correct answer)
- 18–20 gauge
- 22–23 gauge
- 30 gauge
Correct answer: 26–27 gauge
A short, 26–27 gauge needle with a short bevel is recommended to ensure proper intradermal injection.
Question 5: After correctly placing a TST, the nurse observes no bleb (wheal) at the injection site. What action is most appropriate?
- Repeat the test immediately at a site at least 2 inches from the first (Correct answer)
- Document the result as 0 mm and schedule a 48–72 hour reading
- Apply pressure to encourage bleb formation
- Report the result as invalid and do not repeat for 4 weeks
Correct answer: Repeat the test immediately at a site at least 2 inches from the first
If no bleb forms, the injection was likely subcutaneous rather than intradermal, and the test must be repeated at a different site.
Question 6: A patient with HIV has a TST induration of 4 mm. How should this result be interpreted?
- Negative; the threshold for HIV-positive individuals is 5 mm
- Positive; HIV-positive individuals use a 5 mm threshold (Correct answer)
- Negative; HIV-positive individuals use a 10 mm threshold
- Positive; any reaction in an HIV-positive patient is significant
Correct answer: Positive; HIV-positive individuals use a 5 mm threshold
For HIV-positive individuals, a TST induration of 5 mm or more is classified as positive, so 4 mm is negative—but just barely.
Question 7: Which scenario best illustrates the 'booster phenomenon' in TST testing?
- A second TST 1–3 weeks after a negative first TST shows increased induration (Correct answer)
- A patient develops a large blister at the TST site indicating allergy
- Repeated TST tests permanently sensitize a patient to tuberculin
- A TST result reverts from positive to negative over several years
Correct answer: A second TST 1–3 weeks after a negative first TST shows increased induration
The booster phenomenon occurs when a remote TB infection causes a negative initial TST but a larger reaction on a follow-up test 1–3 weeks later due to immune memory stimulation.
TB Test Practice and Application 2
A TST is placed on a patient who received BCG vaccination as a child.
The induration measures 12 mm.
What is the correct interpretation?