TB skin test

[quickfeet]

If a healthcare worker has a positive TB skin test (but has a series of negative tests in the past, since they get one every year), why isn't it standard to confirm the result with a quantiferon gold/Igra test? Why is it necessary to jump right to chest X-ray even if they are asymptomatic?

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[dempty]

I don't know for a fact, but probably because they're in a higher risk group (higher pretest probability), so QF testing might be seen as extraneous testing. I would see the past tests as irrelevant because exposure could have occurred between the most recent negative and first positive test.

 

[mvenus929]

Because in the US, if you've been exposed and convert your PPD, you are going to be treated regardless. It's just a matter of whether you're treated for latent TB (negative CXR), or active TB (positive x-ray, symptoms).

 

[quickfeet]

Because in the US, if you've been exposed and convert your PPD, you are going to be treated regardless. It's just a matter of whether you're treated for latent TB (negative CXR), or active TB (positive x-ray, symptoms).

Question 1 - What if you don't know if you've been exposed, are asymptomatic, and have clear CXR?

Question 2 - What if you don't know if you've been exposed, are asymptomatic, have clear CXR, and negative Quantiferon Gold?

 

[mvenus929]

Question 1 - What if you don't know if you've been exposed, are asymptomatic, and have clear CXR?

Well, that's why we do PPDs.

I've spoken with people in Europe who do the Quantiferon Gold as the first line test. I don't know the overlap of sensitivity/specificity and cost, but I imagine that it's not financially worthwhile for most institutions to revert to the blood test if you screen positive with a PPD. Especially because medical professionals are reasonably reliable enough to take a medicine every day for 9 months.

 

[Lexy101]

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[mehc012]

Question 1 - What if you don't know if you've been exposed, are asymptomatic, and have clear CXR?

Question 2 - What if you don't know if you've been exposed, are asymptomatic, have clear CXR, and negative Quantiferon Gold?

But...you do know you've been exposed. Or at least, you are willing to accept the margin of error of the PPD that indicates you've been exposed.

 

[mehc012]

Also remember that institutional regulations don't always need to make absolute scientific sense. Some places have it written in their policies that you need a PPD, not a Quantiferon, and so they can only use the results of a PPD. I briefly worked at one hospital which used PPD and another which used quantiferon. I had to get both tests every year, because neither institution would accept the test that wasn't their officially required one. I tried showing my quantiferon results to the second hospital and they were basically like "great, so we know you don't have TB...now please get 2 PPDs that will tell you nothing new, so that you can start working in patient care areas."

 

[Foot Fetish]

Also remember that institutional regulations don't always need to make absolute scientific sense. Some places have it written in their policies that you need a PPD, not a Quantiferon, and so they can only use the results of a PPD. I briefly worked at one hospital which used PPD and another which used quantiferon. I had to get both tests every year, because neither institution would accept the test that wasn't their officially required one. I tried showing my quantiferon results to the second hospital and they were basically like "great, so we know you don't have TB...now please get 2 PPDs that will tell you nothing new, so that you can start working in patient care areas."

I just hope I don't end up at an institution that tries to make me get a chest x-ray every year. I received the BCG vaccine as a kid, so I test falsely positive on PPD tests. I've had to get blood drawn for Quantiferon every year for the past 5 years or so, which is kind of annoying but probably more convenient than having to make a second visit for a PPD check.

 

[Peach Newport]

After a positive PPD, if you’ve already gotten a negative quantiferon, most institutions require a chest X-ray - they -actually don’t care about the quantiferon.

The reason is that they don’t care about latent TB. Lots of people have latent TB, whatever. they care about ACTIVE, currently-dissolving-your-parenchyma-TB, which also happens to be spectacularly contagious. Chest x-ray is really good for detecting that. Quantiferon is not a very sensitive test, especially for active TB- it’s strength lies in detecting latent TB, and in patients who have received the BCG vaccine.

 

[mehc012]

After a positive PPD, if you’ve already gotten a negative quantiferon, most institutions require a chest X-ray - they -actually don’t care about the quantiferon.

The reason is that they don’t care about latent TB. Lots of people have latent TB, whatever. they care about ACTIVE, currently-dissolving-your-parenchyma-TB, which also happens to be spectacularly contagious. Chest x-ray is really good for detecting that. Quantiferon is not a very sensitive test, especially for active TB- it’s strength lies in detecting latent TB, and in patients who have received the BCG vaccine.

Latent TB should have a positive quantiferon too, no? The advantage of quantiferon theoretically being slightly higher sensitivity (debatable) and nonreaction with BCG vaccine history (and requires less followup, depending on who you're trying to test...all at a higher cost).
I thought that neither PPD nor quantiferon told you active vs latent - that's what the CXR is for - but if you have a positive PPD due to BCG some hospitals accept a negative quantiferon rather than yearly CXR.

My med school, for example, will accept either a negative PPD or a negative quantiferon, but they will not pay for the quantiferon unless you have documentation of a prior positive PPD. So if you have a positive PPD, you have the option to get quantiferon each year, instead. If that's positive, you need a CXR yearly.

 

[Peach Newport]

Latent TB should have a positive quantiferon too, no? The advantage of quantiferon theoretically being slightly higher sensitivity (debatable) and nonreaction with BCG vaccine history (and requires less followup, depending on who you're trying to test...all at a higher cost).
I thought that neither PPD nor quantiferon told you active vs latent - that's what the CXR is for - but if you have a positive PPD due to BCG some hospitals accept a negative quantiferon rather than yearly CXR.

My med school, for example, will accept either a negative PPD or a negative quantiferon, but they will not pay for the quantiferon unless you have documentation of a prior positive PPD. So if you have a positive PPD, you have the option to get quantiferon each year, instead. If that's positive, you need a CXR yearly.

You’re right, pretty much only an x-ray will distinguish between active and latent.

Quantiferon has a not-fantastic sensitivity, but it is fairly specific.

As for why quantiferon is not as good at detecting active TB, I’m not really sure.

However, it was once explained to me like this:

Quantiferon works by showing your T-cells a TB antigen, and then measuring the amount of IFN-g released. IFN-g is a major cytokine involved in forming granulomas - the T-cells produce IFN to “lock up” the TB and keep it latent. No IFN-g means that the TB can break out of their prison and go active (like in HIV, when your Tcells die).

So a quantiferon test relies on having functional T-cells making IFN-g. If you have active TB, this MAY (but obviously not necessarily) be due to some T-cell/IFN-g/immune problem. In a case like this, TB can be active, and quantiferon shows a negative result.

I’m not sure if this is true but it makes sense to me. Someone better at immuno can correct me.

 

[mehc012]

You’re right, pretty much only an x-ray will distinguish between active and latent.

Quantiferon has a not-fantastic sensitivity, but it is fairly specific.

As for why quantiferon is not as good at detecting active TB, I’m not really sure.

However, it was once explained to me like this:

Quantiferon works by showing your T-cells a TB antigen, and then measuring the amount of IFN-g released. IFN-g is a major cytokine involved in forming granulomas - the T-cells produce IFN to “lock up” the TB and keep it latent. No IFN-g means that the TB can break out of their prison and go active (like in HIV, when your Tcells die).

So a quantiferon test relies on having functional T-cells making IFN-g. If you have active TB, this MAY (but obviously not necessarily) be due to some T-cell/IFN-g/immune problem. In a case like this, TB can be active, and quantiferon shows a negative result.

I’m not sure if this is true but it makes sense to me. Someone better at immuno can correct me.

Yeah, the relative results of IGRA vs TST in various brands of immunocompromise (though I tend to associate granuloma formation most strongly with TNFα, which is why those particular drugs are so strongly associated with dissemination/reactivation) is freaking fascinating. The nitpicky issue I have with the above is that the TST also relies on T cell activity, since PPDs are a type IV delayed cell-mediated hypersensitivity reaction, so I don't believe it breaks down quite that cleanly on either side.

Honestly, I can't even get consistent figures on either side with regards to sensitivity and specificity. The more papers I pull on the subject, the more it seems like both tests' utility is strongly affected by prior exposure vs latent infection vs active vs BCG vs immunosuppression vs HIV status vs nutritional status, etc. The review articles generally lament the lack of an actual gold standard for diagnosing latent infection, which might be part of the problem. I mean, how do you differentiate someone with latent infection from someone who was successfully treated (for either LTBI or active disease?)

So, yeah...I could sit here and try to sift the immuno further, and I did try that recently when it came up in class, but I think all I'll accomplish with that is giving myself enough words to backfill whatever conclusion I'm biased towards, because I'm really not seeing a clear consensus, tbh.

Thanks for the interesting discussion, though! If you find any good sources, throw them my way?

 

[68PGunner]

Can we get a consensus explanation from some clinician about this topic? There was never a set explanation about this topic from my PhDs.

 

[Foot Fetish]

After a positive PPD, if you’ve already gotten a negative quantiferon, most institutions require a chest X-ray - they -actually don’t care about the quantiferon.

The reason is that they don’t care about latent TB. Lots of people have latent TB, whatever. they care about ACTIVE, currently-dissolving-your-parenchyma-TB, which also happens to be spectacularly contagious. Chest x-ray is really good for detecting that. Quantiferon is not a very sensitive test, especially for active TB- it’s strength lies in detecting latent TB, and in patients who have received the BCG vaccine.

But what about people who are known to have false positive results due to prior BCG vaccination? Do they just get one CXR when they first come on board and then quantiferon every year thereafter in lieu of the standard PPD? Like @mehc012 pointed out, institutional policies are not always logical, so my concern is that I will end up at a hospital that forces me to get a CXR every year, thus exposing me to unnecessary radiation. Have you ever heard of that happening?

 

[mehc012]

But what about people who are known to have false positive results due to prior BCG vaccination? Do they just get one CXR when they first come on board and then quantiferon every year thereafter in lieu of the standard PPD? Like @mehc012 pointed out, institutional policies are not always logical, so my concern is that I will end up at a hospital that forces me to get a CXR every year, thus exposing me to unnecessary radiation. Have you ever heard of that happening?

Yeah, plenty of people where I used to work had annual CXRs from prior BCG vaccination. This was the hospital that didn't accept quantiferon at all.

 

[Raryn]

But what about people who are known to have false positive results due to prior BCG vaccination? Do they just get one CXR when they first come on board and then quantiferon every year thereafter in lieu of the standard PPD? Like @mehc012 pointed out, institutional policies are not always logical, so my concern is that I will end up at a hospital that forces me to get a CXR every year, thus exposing me to unnecessary radiation. Have you ever heard of that happening?

I can answer this as someone who had a false positive PPD at age 2 due to a prior BCG vaccine - when I started medical school, I told them that. Then I got a CXR. Every year thereafter, I just filled out a symptom survey. Check no to night sweats, coughing up blood, etc. Nothing else needed. Much easier than my peers who had to do a PPD every year.

When I started residency, same story. CXR x1, then symptom survey every year.

In fellowship, they just got a quantiferon when I started. It's been over a year and I haven't even had to do a symptom survey. (Not every institution has to screen all staff for TB every year. Only if they treat >X TB cases in a year. Guess my current one doesn't qualify for that?)

 

[MS12345]

I can answer this as someone who had a false positive PPD at age 2 due to a prior BCG vaccine - when I started medical school, I told them that. Then I got a CXR. Every year thereafter, I just filled out a symptom survey. Check no to night sweats, coughing up blood, etc. Nothing else needed. Much easier than my peers who had to do a PPD every year.

When I started residency, same story. CXR x1, then symptom survey every year.

In fellowship, they just got a quantiferon when I started. It's been over a year and I haven't even had to do a symptom survey. (Not every institution has to screen all staff for TB every year. Only if they treat >X TB cases in a year. Guess my current one doesn't qualify for that?)

Sorry for commenting on this is an old thread, but just inquire about TB testing.
I tested + on PPD, but had an all clear chest X-ray. I was born out of the US and recieved BCG vaccine 21 years ago. My doctor said BCG immunity wanes with age so a false positive isn't the reason for my + skin test. She is suggesting that I go on antibiotics for months, but I'm not sure if I really want to do that. Would chest x-rays annually be fine? I'm starting med school and worried about exposure in the hospital. What are the real life chances that a latent infection actually turns active for a healthy adult? People make it seem like these meds are the only way to prevent it from happening

 

[MedicineZ0Z]

Well, that's why we do PPDs.

I've spoken with people in Europe who do the Quantiferon Gold as the first line test. I don't know the overlap of sensitivity/specificity and cost, but I imagine that it's not financially worthwhile for most institutions to revert to the blood test if you screen positive with a PPD. Especially because medical professionals are reasonably reliable enough to take a medicine every day for 9 months.

PPD = false positive

No one should get a liver toxic drug without the Quantiferon.

 

[pawprint]

PPD = false positive

No one should get a liver toxic drug without the Quantiferon.

Quoted for emphasis.

To MS 12345, I would get a quantiferon gold before starting treatment. I test positive on PPD's but my Quantiferon is stone cold negative. Quantiferon >>>>>> PPD. Quantiferon is a test that takes your blood, puts it in a container with TB antigens and sees if your white cells react. If your quantiferon is negative, that means your white cells have never been exposed to TB before. You can have skin reactivity to the PPD, someone is poking a needle in your skin...its a terrible test. Its also dependent on a med tech reading it, and the criteria for being positive is subjective based on your environment. Especially with past BCG vaccination....there's many reasons you could be reacting to the PPD. It is too subjective to be the sole reason to start someone on treatment. It may be cost effective for initial screening, but many ID docs are currently changing practice guidelines to go PPD -> Quantiferon -> treatment.