TB gurus: is there a role for using interferon gamma release assays to diagnose active TB?
guidelines & textbooks *don’t* generally recommend this (eg https://www.tandfonline.com/doi/full/10.1080/24745332.2022.2035638#appendixes)
but in practice I see this done a *lot* 🧐
#IDsky #pulmsky
guidelines & textbooks *don’t* generally recommend this (eg https://www.tandfonline.com/doi/full/10.1080/24745332.2022.2035638#appendixes)
but in practice I see this done a *lot* 🧐
#IDsky #pulmsky
Comments
This can be falsely negative in active TB, and positive in the absence of it (eg latent TB)
- does this patient have functional T cells that can release interferon gamma to known antigens?
AND
- have those T cells seen MTB before?
AND
- do they remember it?
a choice to use it as part of your probability titration is not unreasonable, but you must understand what the test tells you
Not sensitive enough for ne IGRA to rule out active TB.
Not specific for active disease as possible in LTBI (and previous disease).
Rarely MIGHT be helpful in swinging balance of probabilities in difficult cases where microbiology/ molecular testing not possible.
This was our paper, but reported by many elsewhere too https://academic.oup.com/ofid/article/11/4/ofad697/7516258
Got TB =IGRA+
Had TB? =IGRA+
Latent TB? =IGRA+
BCG vaccinated against TB? =IGRA+
Looked at a picture of a mycobacterium in medical school? =IGRA+
- this pt has functional T cells that can release IFN-g to known antigens
- the T cells have seen MTB antigens before & they remember it
IGRA -
- this pt has functional T cells that can release IFN-g to known antigens
- the T cells don’t remember seeing MTB antigens before
BUT doesn't distinguish between active disease and LTBI, or past disease.
It is useful for what it was designed for - screening for LTBI.
as an additional argument to take into account within a bundle of arguments, it does slightly alters the probability of TB in one way or another
But not for pulmonary TB
And imo IGRA can be helpful in the common scenario where there’s a TB-compatible syndrome without + micro (for TB or alternate).
(-) doesn’t rule out but reduces slightly
(+) is it really a 3% coincidence?
There’s no such thing as a bad test, just a bad user!
Clearly, it can sometimes contribute usefully to decision making where microbiological confirmation is challenging, e.g. occular TB