Immunology: interferon-gamma release assay interpretation: practice question

A Microbiology question in the style of the NEET PG paper. Answer it first, then read why the right option is right and why each of the others is wrong.

Try it

Microbiology · Immunology: interferon-gamma release assay interpretation

Sample item

A 35-year-old man with advanced HIV (CD4 count 40 cells/mm3) is screened for latent tuberculosis. The interferon-gamma release assay reports an indeterminate result because the positive mitogen control produced little interferon-gamma. How should this be interpreted?

Original practice item, not a recalled exam question. Nothing here is recorded.

The answer and the reasoning

A. Uninterpretable, because failure of the mitogen control reflects poor T-cell response and does not exclude infection

The assay measures interferon-gamma released by sensitised T cells after stimulation with ESAT-6 and CFP-10, with a mitogen as positive control. A low mitogen response, common in severe lymphopenia, shows the T cells cannot respond, so a negative antigen response cannot be trusted. Such results are reported as indeterminate and should be repeated or supplemented by clinical assessment.

Option by option

  • A. Uninterpretable, because failure of the mitogen control reflects poor T-cell response and does not exclude infection (correct)

    A failed positive control makes the test invalid, so latent infection is neither confirmed nor excluded.

  • B. Negative, because the tuberculosis antigens produced no response and infection is excluded

    A negative result requires an adequate mitogen response, which is absent here.

  • C. Positive for non-tuberculous mycobacterial infection

    These antigens are present in Mycobacterium tuberculosis and a few species such as M. kansasii and M. marinum, and this indeterminate result does not identify any species.

  • D. Positive, because background interferon-gamma is raised from BCG vaccination

    ESAT-6 and CFP-10 are absent from BCG, so BCG does not produce a positive result.

Source note: Original NextPrep sample item — pending expert review. This item is original practice material awaiting independent expert review, so confirm important facts in a standard textbook.

More Microbiology questions

Questions about this Microbiology question

What is the answer to this Microbiology question?

The best answer is A, Uninterpretable, because failure of the mitogen control reflects poor T-cell response and does not exclude infection. The assay measures interferon-gamma released by sensitised T cells after stimulation with ESAT-6 and CFP-10, with a mitogen as positive control.

Why is "Negative, because the tuberculosis antigens produced no response and…" not the answer?

A negative result requires an adequate mitogen response, which is absent here.

Why is "Positive for non-tuberculous mycobacterial infection" not the answer?

These antigens are present in Mycobacterium tuberculosis and a few species such as M. kansasii and M. marinum, and this indeterminate result does not identify any species.

Why is "Positive, because background interferon-gamma is raised from BCG vacc…" not the answer?

ESAT-6 and CFP-10 are absent from BCG, so BCG does not produce a positive result.

What concept does this question test?

It tests Immunology: interferon-gamma release assay interpretation in Microbiology. A low mitogen response, common in severe lymphopenia, shows the T cells cannot respond, so a negative antigen response cannot be trusted. Such results are reported as indeterminate and should be repeated or supplemented by clinical assessment.

How hard is this question?

We tag it as a multi-step question: it asks you to combine more than one idea. The tag is our own editorial judgement, not an official difficulty rating, so read the reason for every option, not only the answer.

How should I study Immunology: interferon-gamma release assay interpretation?

Make a one-line identity card for each organism: shape and stain, defining test, toxin, disease and drug. Then use the flashcards and the other Microbiology questions on this site to test what stuck.

Is this a real NEET PG question?

No. It is an original practice item (editorial status: sample, version 1). NextPrep does not publish recalled or leaked questions, and independent faculty review of this item has not been completed yet, so confirm important facts in a standard textbook.