Tb

[xjacob]

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I am thoroughly confused about PPD's for TB.
If a pt with no significant Hx is found to have a positive PPD (whatever cut-off we use), but his CXR is completely normal, what does that mean? (assume that he was not vaccinated w/ CBG or anything like that).
Does it mean that:
a) He was exposed to TB, got infected (subclinically), TB went to the lungs, and a Ghon complex formed there, but it is too small to see on CXR.
b) He was exposed to TB, got infected (subclinically), but this occured in a tissue other than the lung.
c) He was exposed to TB, but the bacteria never had an opportunity to multiply anywhere, so he didn't really get "infected."
d) He just has a reaction to PPD without really having been exposed.

I know that + PPDs are pretty darn common, but with no CXR change, people don't get treated with INH, and they are declared false +. But is a false + an idiosyncratic reaction, or does it mean that there was exposure/infection/whatever but it does not have any consequences?
Also, note that this is not a question I got from somewhere...I just made it up, since I dunno the answer - so maybe it's some other option.

 

[xjacob]

Bump. Thought I'd give this one more try. Any inf. disease enthusiasts out there, who want to help a "i-dunno-anything-about-bugs" dude?

 

[anon-y-mouse]

I am thoroughly confused about PPD's for TB.
If a pt with no significant Hx is found to have a positive PPD (whatever cut-off we use), but his CXR is completely normal, what does that mean? (assume that he was not vaccinated w/ CBG or anything like that).
Does it mean that:
a) He was exposed to TB, got infected (subclinically), TB went to the lungs, and a Ghon complex formed there, but it is too small to see on CXR.
b) He was exposed to TB, got infected (subclinically), but this occured in a tissue other than the lung.
c) He was exposed to TB, but the bacteria never had an opportunity to multiply anywhere, so he didn't really get "infected."
d) He just has a reaction to PPD without really having been exposed.

I know that + PPDs are pretty darn common, but with no CXR change, people don't get treated with INH, and they are declared false +. But is a false + an idiosyncratic reaction, or does it mean that there was exposure/infection/whatever but it does not have any consequences?
Also, note that this is not a question I got from somewhere...I just made it up, since I dunno the answer - so maybe it's some other option.

Having researched this thoroughly recently, thought I'd answer what I know. There is no hard and fast rule about PPD interpretation. Everyone's going to get a spot of redness, but the main criteria is induration, and the diameter of said induration is measured. There are *several* definitions of a positive test, based on history (country, potential exposure, age, immune system status), and "positive" induration criteria change depending on this history.

What does it mean if someone is "positive"?

A) false positive. This can happen if the body's immune system has "forgotten" that it fought TB, and hence the PPD tests "positive". This is resolved by taking the PPD again the following week and applying the same criteria.

B) "actual" positive (adjusted for history, as I mentioned before).

- The person was exposed to another mycobacterium other than the tuberculosis one. Certainly possible.
- The person was exposed, but the infection was subclinical, as you said. This means the disease hasn't manifested itself... the bacteria remain latent, but if the body's immune system becomes compromised, the bacteria may flare up and cause the manifestations in the chest.

What we consider tuberculosis is actually the manifested disease in the lungs... hence the chest x-ray. Many doctors and health professionals in 3rd world countries would probably 'fail' a PPD, but the infection is latent... much like much of the population has HSV1. This is known as an LTBI -- latent TB infection. Nothing to worry about, which is why on your health forms, if you fail the PPD, you have to get a CXR.

The closest situation is your "(C)". A is not a possibility. When TB hits, it hits hard (I've seen so many cases in the third world!! with a mask of course). B isn't really how TB works. D is kind of vague, but false positives are somewhat common.

As for the vaccination, it's BCG not CBG... recent studies have shown that BCG isn't really going to cause a difference on PPD results if the PPD is performed on an adult, i.e. since the immunization occurred decades prior.

 

[xjacob]

Thanks a lot for the thorough reply!
So I am assuming that when I learned that "patients that test positive for TB, are placed on INH monotherapy for 12 months (or more)" they meant patients, who actually have a positive CXR, not just the PPD.
One thing that confuses me is why you say A) is not a possibility. I was taught that "primary TB infection is usually of no serious consequence, exceptions being young children, eldery, and immunocompromised patients" and that most tuberculosis, as we know it, is actually secondary TB.

Btw, I am doubly interested in this, b/c I got my PPD+, but CXR normal (so not just for the boards). I found it odd, since I come from a low-risk background (I got this before I started going to the hospital, my 3rd world travel Hx is limited, and I lived most of my life in a European country, where TB is even more uncommon than in the US). However, coming from Europe, where we ALL get a vaccine as newborns (I am assuming it's BCG), I attributed this positive PPD to my calmette memory cells (altho I realize that CDC recommends that a vaccination Hx be ignored when interpretting PPDs). So according to what you say, I have likely really "seen the bug" (antigen exposure) but I am probably not carrying it around (no infection - I hope!).
Anyway, THANKS A LOT again!

 

[anon-y-mouse]

Thanks a lot for the thorough reply!
So I am assuming that when I learned that "patients that test positive for TB, are placed on INH monotherapy for 12 months (or more)" they meant patients, who actually have a positive CXR, not just the PPD.
One thing that confuses me is why you say A) is not a possibility. I was taught that "primary TB infection is usually of no serious consequence, exceptions being young children, eldery, and immunocompromised patients" and that most tuberculosis, as we know it, is actually secondary TB.

Btw, I am doubly interested in this, b/c I got my PPD+, but CXR normal (so not just for the boards). I found it odd, since I come from a low-risk background (I got this before I started going to the hospital, my 3rd world travel Hx is limited, and I lived most of my life in a European country, where TB is even more uncommon than in the US). However, coming from Europe, where we ALL get a vaccine as newborns (I am assuming it's BCG), I attributed this positive PPD to my calmette memory cells (altho I realize that CDC recommends that a vaccination Hx be ignored when interpretting PPDs). So according to what you say, I have likely really "seen the bug" (antigen exposure) but I am probably not carrying it around (no infection - I hope!).
Anyway, THANKS A LOT again!

Why don't you have a second PPD done? It's a pretty cheap test... again, the memory factor could have played a part. There are many many reasons for false positives. I had a positive PPD (huge puffy balloon-type thing) many many years ago when I first volunteered at a hospital, but I had one done a few weeks ago, and it was most clearly negative. Redness but absolutely no induration.

And as for your first point, I think the INH monotherapy is indicated mainly for people who have definitely spent time with TB patients and test positive, or people with the positive chest X-ray, immunocompromised people who have been exposed (but haven't developed any chest signs).

 

[Pegsie]

And as for your first point, I think the INH monotherapy is indicated mainly for people who have definitely spent time with TB patients and test positive, or people with the positive chest X-ray, immunocompromised people who have been exposed (but haven't developed any chest signs).

Peope who test positive for PPD are considered to have a "latent" infection. I think a positive skin test defines a latent infection (whatever that is). INH monotherapy for 6-9 months is for people who test positive, but have a clear CXR. This prophylactic therapy cuts your lifetime chance of reactivation from 5% down to 1%.

Personal story...although they don't really take the BCG into account, when I asked my school nurse, she said my induration was so huge that even if my BCG had worked, my reaction was too big for it to be due to the vaccine alone. So, if you know how big your induration is, you might be able to tell if it was your BCG or not. According to the school nurse. 😉

edit: I just checked on the CDC website, they only recommend INH monotherapy for those who are immunocompromised w/ positive PPD, who are more likely to get active disease. When health services talked to me, they made it seem like it was recommended that everybody go through monotherapy while they're still healthy & young. Maybe it's b/c therapy is so cheap and it's so much easier to take while you're young?

 

[xjacob]

I think I am getting more confused again...but that's ok, all the posts were really helpful.
Either way, I got my PPD at college, just before going into med school (i.e. as part of the required health form). My induration was 11mm. The health center went with the 10mm, rather than 15mm cut-off (ahh, foreign-born people are always lumped into one cathergory as "risky", even when I try to tell them that in my part of Europe, TB is considered a "tropical or exotic disease"). After getting a negative CXR, nobody ever talked to me again, let alone offer INH. According to my study material, INH is always given to kids & immunosupressed, and almost never to adults over 55 (everyone else being in the "gray zone," where Rx depends on clinical decision); this best fits with anon's description.
Even now, whenever my med school administers PPDs to my class, I am always exempt (no waiting in a line = great!), but they don't have me do CXR either and just tell me to go home. Maybe they don't care, since I'm not on the wards, just yet.
Finally, as far as BCG goes, the TB-experts at my school (who taught the class), were extremely vague about it. They said that some believe the effect on PPD positivity is huge, while other say the opposite. Bottom line: not enough published data arguing either way, so always ignore BCG history.