TB Test Practice and Application 3 — Questions and Answers
Question 1: A clinician orders an IGRA (QuantiFERON-TB Gold Plus) for a patient who had a BCG vaccine abroad. What is the primary advantage of IGRA over TST in this case?
- IGRA is not affected by prior BCG vaccination (Correct answer)
- IGRA can diagnose active TB disease directly
- IGRA requires no blood draw and is less invasive
- IGRA results are available within 30 minutes
Correct answer: IGRA is not affected by prior BCG vaccination
IGRAs measure interferon-gamma release in response to TB-specific antigens (ESAT-6 and CFP-10) not present in BCG, so prior BCG vaccination does not cause false-positive IGRA results.
Question 2: When reading a TST result, the clinician should measure which physical finding?
- The diameter of induration (palpable raised area) (Correct answer)
- The diameter of erythema (redness)
- The combined diameter of induration and erythema
- The height of the bleb in millimeters
Correct answer: The diameter of induration (palpable raised area)
Only the diameter of induration—the firm, palpable swelling—is measured, not the surrounding redness (erythema).
Question 3: A 4-year-old child is a close contact of a confirmed active TB case. Her TST measures 6 mm. What is the correct interpretation?
- Positive; close contacts of active TB use the 5 mm threshold (Correct answer)
- Negative; children require a 10 mm threshold
- Positive; all children under 5 use the 5 mm threshold regardless of exposure
- Indeterminate; children under 5 require IGRA confirmation
Correct answer: Positive; close contacts of active TB use the 5 mm threshold
For close contacts of active TB cases, a TST of 5 mm or more is considered positive regardless of age.
Question 4: Which of the following is a contraindication to performing a TST?
- A previously documented positive TST result (Correct answer)
- Recent BCG vaccination within the past 5 years
- Current active TB infection being treated
- Allergy to penicillin
Correct answer: A previously documented positive TST result
A previously documented positive TST is a contraindication to repeat TST, as severe local reactions (necrosis, ulceration) can occur.
Question 5: A patient's TST bleb measures 6 mm immediately after injection. At the 72-hour reading, induration measures 12 mm. Which measurement is recorded as the official result?
- 12 mm — the induration at the 48–72 hour reading (Correct answer)
- 6 mm — the initial bleb measurement
- The average of both measurements: 9 mm
- The smaller of the two measurements: 6 mm
Correct answer: 12 mm — the induration at the 48–72 hour reading
The official TST result is the induration measured at the 48–72 hour reading, not the immediate post-injection bleb.
Question 6: A recent immigrant from a high-TB-burden country works in a meat processing plant. His TST measures 10 mm. What is this result classified as?
- Positive; high-risk immigrants use the 10 mm threshold (Correct answer)
- Negative; the threshold is 15 mm for healthy adults
- Positive; any foreign-born person automatically uses the 5 mm threshold
- Indeterminate; occupational risk requires IGRA confirmation
Correct answer: Positive; high-risk immigrants use the 10 mm threshold
Recent immigrants from high-prevalence countries and those in certain occupations fall under the 10 mm threshold category for a positive TST result.
Question 7: Which of the following substances contains the correct dose of tuberculin for a standard Mantoux test in the US?
- 5 tuberculin units (TU) of purified protein derivative (PPD) (Correct answer)
- 1 TU of whole tuberculin extract
- 10 TU of PPD in 0.2 mL
- 2 TU of PPD in 0.1 mL
Correct answer: 5 tuberculin units (TU) of purified protein derivative (PPD)
The standard US Mantoux test uses 5 TU of PPD-S (purified protein derivative) in 0.1 mL delivered intradermally.
TB Test Practice and Application 3
A clinician orders an IGRA (QuantiFERON-TB Gold Plus) for a patient who had a BCG vaccine abroad.
What is the primary advantage of IGRA over TST in this case?