TB Test Practice and Application 3

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?

Accepted Answer

IGRA is not affected by prior BCG vaccination

Answer

IGRA can diagnose active TB disease directly

Answer

IGRA requires no blood draw and is less invasive

Answer

IGRA results are available within 30 minutes

Question 2

When reading a TST result, the clinician should measure which physical finding?

Accepted Answer

The diameter of induration (palpable raised area)

Answer

The diameter of erythema (redness)

Answer

The combined diameter of induration and erythema

Answer

The height of the bleb in millimeters

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?

Accepted Answer

Positive; close contacts of active TB use the 5 mm threshold

Answer

Negative; children require a 10 mm threshold

Answer

Positive; all children under 5 use the 5 mm threshold regardless of exposure

Answer

Indeterminate; children under 5 require IGRA confirmation

Question 4

Which of the following is a contraindication to performing a TST?

Accepted Answer

A previously documented positive TST result

Answer

Recent BCG vaccination within the past 5 years

Answer

Current active TB infection being treated

Answer

Allergy to penicillin

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?

Accepted Answer

12 mm — the induration at the 48–72 hour reading

Answer

6 mm — the initial bleb measurement

Answer

The average of both measurements: 9 mm

Answer

The smaller of the two measurements: 6 mm

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?

Accepted Answer

Positive; high-risk immigrants use the 10 mm threshold

Answer

Negative; the threshold is 15 mm for healthy adults

Answer

Positive; any foreign-born person automatically uses the 5 mm threshold

Answer

Indeterminate; occupational risk requires IGRA confirmation

Question 7

Which of the following substances contains the correct dose of tuberculin for a standard Mantoux test in the US?

Accepted Answer

5 tuberculin units (TU) of purified protein derivative (PPD)

Answer

1 TU of whole tuberculin extract

Answer

10 TU of PPD in 0.2 mL

Answer

2 TU of PPD in 0.1 mL