Rog,
Now that I am wandering aimlessly around the www, I thought I continue with some of the medical discussion posts that I did previously. Hopefully they will be useful educationally for others who read your site.
No doubt Queensland Health is on the critical list .. but a few thoughts on your post with regards to diagnosing active Tb.
I can see how the comment on not using TST and IGRA to diagnose active Tb seems in contrast to what we know about Quantiferon (QFT) that in studies it can assist in the diagnosis.
The Korean paper you reported recently showed that QFTG was 84% sensitive and 82.9% specific in active Tb….clearly useful if you look at the numbers.
However what Konstantinos is doing is making a statement in a general publication, and I suspect one of the concerns would be that if you suggested the use of the tests it could lead to over reliance.
He does say “initial,” and there would be a place just don’t “hang your hat” on the result.
It is worth remembering that QFT has a significant advantage over the skin test (TST) in active Tb with its ability to show that the patients “immune system is sick”.
In active Tb, the patient can be very sick, and their immune system is suppressed so they cannot mount a response to the skin test. Therefore the skin test in a sick patient with Tb may come back negative because the patient is sick (with Tb !). This is called anergy.
With QFT, one of the tubes used has a group of proteins (antigens - mitogens) that everyone responds to. So you get 2 readings…. can they respond to anything and can they respond to the Tb antigen.
So a sick patient, may not respond to either tube (the mitogen or the Tb antigen) as their immune response is so weak. In this case the test is reported as indeterminate. An unfortunate choice of terms in my opinion as it is telling you something - there is something wrong, most likely with the immune system ! A very different result to what the skin test showed in this situation where you might have just reported it as negative and as Konstantinos is indicating, been falsely reassured.
The other QFT options are:
1) mitogen response positive, Tb response negative = QFT negative (and that’s about 80% specific for saying the patient doesn’t have active Tb)
2) mitogen response positive, Tb response positive = QFT positive (and that’s positive about 80% of the time when the patient does have active Tb).
There is a lot more to say following on from the Konstantinos paper. It raises some good points about what happens after infection, and that raises the very important issue of the use of QFT in contact tracing, a paper that is worth highlighting again.
A final practical point that perhaps the Korean paper didn’t make is the issue of diagnosing active pulmonary Tb (and note that I said pulmonary or lung Tb as some Tb is outside the lungs).
The diagnosis of pulmonary Tb requires identification of the Tb organism, that’s why you can’t rely just on QFT.
When a patient is seen, there is often a sequence (that may vary depending on the signs and tests in each case).
The history may suggest the diagnosis:
- patient likely to get Tb (but not always) e.g from high risk area
- productive cough and fever
- recent exposure to someone with Tb
Tests may be supportive
- the chest Xray often looks like Tb rather than an ordinary pneumonia although as in the Korean study, sometimes you are not sure
- the sputum may show Tb under the microscope either using the old special stains or the new very accurate techniques… but the problem is that up to 50% of patients do not produce sputum. They have used various tricks over the years to get the sputum including chest physiotherapy to get them to cough it up or even sucking it out of the stomach in the morning after they wake up ! Sometimes they look down inside the lungs with a scope…
It is when the diagnosis isn’t immediately apparent, especially with the saliva tests, that you might start to think about using QFT.
The problem that Konstantinos was alluding to is that you can be QFT positive and have a chest infection but that doesn’t mean it is active Tb – until you get the Tb organism in the sputum or you grow it. If you can’t grow the Tb organism from the lungs, it may just be latent Tb and an ordinary chest infection.
Caution is needed. Tb is a notifiable disease and if the diagnosis is made, you have to test contacts, and stop it spreading elsewhere.
Look forward to your future posts and further discussion;
skorpian
