QuantiFERON-TB Gold Plus (IU/mL): Nil 0.45; TB1 0.70; TB2 0.78; Mitogen 7.20. Rules: positive if TB1 or TB2 minus Nil is 0.35 or more and at least 25% of Nil; indeterminate if Nil is above 8.0, or if both TB responses are negative and Mitogen minus Nil is below 0.50. The result is:

Negative: neither TB response reaches 0.35

Indeterminate: TB1 and TB2 disagree

Positive: both TB tubes exceed 0.35

Indeterminate: the Nil value is too high

QuantiFERON-TB Gold Plus has two TB antigen tubes. Compared with TB1, the TB2 tube adds short peptides intended to stimulate:

CD8 T cells as well as CD4 T cells

B cells to secrete specific IgG

Neutrophils to release interferon-gamma

The induration of a positive tuberculin skin test is an example of which type of hypersensitivity?

Type IV, delayed T cell-mediated

Type II, antibody-mediated cytotoxic

Type III, immune complex-mediated

When a Mantoux tuberculin skin test is read, the result recorded is the:

Transverse diameter of induration in mm

Area of redness and swelling in mm²

Height of the raised area in mm

New healthcare workers who will have yearly tuberculin skin tests are given two-step testing at baseline. What is its purpose?

To tell a boosted reaction from a later conversion

To tell BCG reactions from TB infection

To lower the chance of a severe skin reaction

To confirm a positive first test before treatment

Tuberculosis infection testing

5 DOH Abu Dhabi (HAAD) lab licensing practice questions on Tuberculosis infection testing, part of Immunology and Serology. 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.

Question 1 · Hard
QuantiFERON-TB Gold Plus (IU/mL): Nil 0.45; TB1 0.70; TB2 0.78; Mitogen 7.20. Rules: positive if TB1 or TB2 minus Nil is 0.35 or more and at least 25% of Nil; indeterminate if Nil is above 8.0, or if both TB responses are negative and Mitogen minus Nil is below 0.50. The result is:
1. AIndeterminate: TB1 and TB2 disagree
2. BNegative: neither TB response reaches 0.35
3. CPositive: both TB tubes exceed 0.35
4. DIndeterminate: the Nil value is too high
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Answer: B · Negative: neither TB response reaches 0.35
Each TB response is corrected for background by subtracting the Nil: TB1 0.70 - 0.45 = 0.25 and TB2 0.78 - 0.45 = 0.33 IU/mL, both below 0.35. The Mitogen response, 7.20 - 0.45 = 6.75 IU/mL, shows the cells can make interferon-gamma, and the Nil is far below 8.0, so the test is valid and negative. Reading the uncorrected TB values would wrongly give a positive result. A negative IGRA does not exclude active tuberculosis in a patient with symptoms.
- A: Both TB tubes are negative after correction, and tube disagreement is not an indeterminate rule.
- C: The Nil must be subtracted first; corrected, both TB responses are below 0.35.
- D: The Nil makes a test indeterminate only above 8.0 IU/mL.
Question 2 · Medium
QuantiFERON-TB Gold Plus has two TB antigen tubes. Compared with TB1, the TB2 tube adds short peptides intended to stimulate:
1. ACD8 T cells as well as CD4 T cells
2. BNatural killer cells directly
3. CB cells to secrete specific IgG
4. DNeutrophils to release interferon-gamma
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Answer: A · CD8 T cells as well as CD4 T cells
TB1 contains long peptides from ESAT-6 and CFP-10 that are presented mainly to CD4 T cells. TB2 contains these plus shorter peptides that can also be presented to CD8 T cells, whose responses are linked to recent infection and active disease. Interferon-gamma from either tube, above the cut-off after the Nil value is subtracted, makes the test positive. The assay still cannot separate latent from active tuberculosis.
- B: The peptides are designed for T-cell recognition, not NK-cell activation.
- C: IGRAs measure interferon-gamma from T cells, not antibody from B cells.
- D: Neutrophils are not the source of the measured interferon-gamma.
Question 3 · Easy
The induration of a positive tuberculin skin test is an example of which type of hypersensitivity?
1. AType II, antibody-mediated cytotoxic
2. BType IV, delayed T cell-mediated
3. CType III, immune complex-mediated
4. DType I, IgE-mediated
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Answer: B · Type IV, delayed T cell-mediated
Tuberculin purified protein derivative injected into the skin is recognized by memory T cells in people previously sensitized to mycobacteria. The T cells release cytokines, especially interferon-gamma, that recruit macrophages and lymphocytes, giving firm induration that peaks at 48 to 72 hours. No antibody is involved, which is why it is a delayed (type IV) reaction. Immunosuppression, such as advanced HIV, can blunt the response and cause false-negative tests.
- A: Type II reactions are antibodies against cell-surface antigens.
- C: Type III reactions are caused by immune complexes and complement, as in the Arthus reaction.
- D: Type I reactions are IgE-mediated and appear within minutes.
Question 4 · Easy
When a Mantoux tuberculin skin test is read, the result recorded is the:
1. ADiameter of the redness in mm
2. BTransverse diameter of induration in mm
3. CArea of redness and swelling in mm²
4. DHeight of the raised area in mm
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Answer: B · Transverse diameter of induration in mm
The reader feels for the edges of the firm induration and measures its diameter across the long axis of the forearm, in millimetres. Redness (erythema) is not measured, because it can occur without induration. The test is read 48 to 72 hours after intradermal injection of tuberculin PPD. No induration is recorded as 0 mm.
- A: Erythema is not measured and can be present without induration.
- C: Results are recorded as a diameter, not an area.
- D: The height of the reaction is not part of the reading.
Question 5 · Medium
New healthcare workers who will have yearly tuberculin skin tests are given two-step testing at baseline. What is its purpose?
1. ATo tell BCG reactions from TB infection
2. BTo tell a boosted reaction from a later conversion
3. CTo lower the chance of a severe skin reaction
4. DTo confirm a positive first test before treatment
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Answer: B · To tell a boosted reaction from a later conversion
In people infected long ago, or sensitized by BCG or nontuberculous mycobacteria, the first tuberculin test can be negative yet recall the immune response, so a second test is positive. If a second baseline test 1 to 3 weeks later is positive, this boosted result is taken as the baseline and evaluated, rather than being mistaken for a new infection at the next yearly test. A negative second test sets a negative baseline, so a later positive suggests a true conversion. Two-step testing does not separate BCG from TB infection.
- A: Boosting occurs after BCG as well as after TB infection, so two steps cannot separate them.
- C: Repeating the test does not reduce reaction severity.
- D: A positive first test is not repeated; the person is evaluated.