Tuberculosis infection testing (IGRA, PPD)

5 ASCP MLS practice questions on Tuberculosis infection testing (IGRA, PPD), part of Immunology. The first 5 are below; the first 5 with answers and explanations (tap Show answer after you have chosen), the rest in Learn with a free account.

1. Question 1 · Easy

   Why is an IGRA preferred over the tuberculin skin test for a person who received BCG vaccine?

   1. AIts antigens are absent from BCG strains
   2. BIt shows boosting after repeat testing
   3. CIt is unaffected by immunosuppression
   4. DIt detects active disease, not infection

   Show answerHide answer

   Answer: A · Its antigens are absent from BCG strains

   ESAT-6 and CFP-10 are encoded in a genome region deleted from all BCG strains and absent from most nontuberculous mycobacteria. An IGRA is therefore not made positive by BCG. PPD contains many antigens shared with BCG, so the skin test can be falsely positive after vaccination. M. kansasii, M. marinum and M. szulgai do carry these antigens and can cause positive IGRAs.

   • B: Boosting is a feature of repeated skin tests, not of IGRAs.
   • C: Immunosuppression lowers the sensitivity of both tests.
   • D: Neither test separates latent infection from active disease.

2. Question 2 · Medium

   In a person with cough and weight loss, what does a positive IGRA show?

   1. ATB infection, but not whether it is latent or active
   2. BLatent TB infection, with active disease excluded
   3. CPast BCG vaccination rather than infection
   4. DActive pulmonary tuberculosis needing isolation

   Show answerHide answer

   Answer: A · TB infection, but not whether it is latent or active

   Neither IGRAs nor the skin test can tell latent infection from active disease. Active tuberculosis is diagnosed with symptoms, chest imaging and sputum AFB smear, culture and NAAT. A negative IGRA does not rule out active tuberculosis either. With these symptoms, sputum testing is needed whatever the IGRA result.

   • B: A positive IGRA cannot exclude active disease.
   • C: IGRA antigens are not found in BCG strains.
   • D: A positive IGRA does not prove active disease.

3. Question 3 · Easy

   Interferon-gamma release assays (IGRAs) measure interferon-gamma released by:

   1. AMacrophages infected with live mycobacteria
   2. BSensitized T cells exposed to ESAT-6 and CFP-10
   3. CB cells exposed to purified protein derivative
   4. DNeutrophils exposed to mycobacterial cell wall lipids

   Show answerHide answer

   Answer: B · Sensitized T cells exposed to ESAT-6 and CFP-10

   Whole blood or isolated cells are incubated with peptides of ESAT-6 and CFP-10, antigens of M. tuberculosis. Memory T cells from an infected person respond by releasing interferon-gamma. This is measured by ELISA (QuantiFERON) or counted as spots by ELISpot (T-SPOT.TB). Nil and mitogen controls check background and the ability of T cells to respond.

   • A: No live organisms are used; the antigens are peptides.
   • C: B cells do not produce interferon-gamma, and PPD is the skin test antigen.
   • D: Neutrophils are not the source of interferon-gamma in this test.

4. Question 4 · Hard

   Each tuberculin skin test was read at 48 to 72 hours. Using CDC cutoffs for induration, which result is positive?

   1. AHospital nurse with no other risk factor, 8 mm
   2. BAdult with no risk factors, 13 mm
   3. CHIV-positive adult, 7 mm
   4. DRecent immigrant from a high-burden country, 9 mm

   Show answerHide answer

   Answer: C · HIV-positive adult, 7 mm

   A cutoff of 5 mm or more applies to people with HIV, recent contacts of a case, the immunosuppressed and those with fibrotic chest X-ray changes. A cutoff of 10 mm applies to recent immigrants from high-burden countries, health care workers, people who inject drugs and residents of congregate settings. People with no risk factors need 15 mm or more. Only induration is measured, not redness.

   • A: Health care workers need 10 mm or more.
   • B: People with no risk factors need 15 mm or more.
   • D: Recent immigrants from high-burden countries need 10 mm or more.

5. Question 5 · Medium

   QuantiFERON-TB Gold Plus (IU/mL): Nil 0.10; TB1 minus Nil 0.05; TB2 minus Nil 0.08; Mitogen minus Nil 0.20. Cutoffs: TB response 0.35 or more, mitogen response 0.50 or more. How is this reported?

   1. ABorderline
   2. BPositive
   3. CNegative
   4. DIndeterminate

   Show answerHide answer

   Answer: D · Indeterminate

   The TB antigen responses are below 0.35 IU/mL, but this can be called negative only if the mitogen control shows that the T cells can respond. A mitogen minus Nil below 0.50 IU/mL means the cells did not respond, so the result is indeterminate. Common causes are lymphopenia, immunosuppression and delayed or poor sample handling. The test can be repeated or a skin test used.

   • A: Borderline is not a QuantiFERON result category.
   • B: Both TB antigen responses are below 0.35 IU/mL.
   • C: A negative result needs an adequate mitogen response.