TB- PPD Skin Test Question


just a little confused....as per my book "induration at the injection site after the test means that the person has been exposed to TB and developed antibodies"

what makes sense to me is that IF a person's test is POSITIVE on ppd- neg x-ray and neg AFB test that they have been infected with the disease but have antibodies

however a person can be exposed to the disease with out having a positive ppd etc....

is this correct? is there an easy way to figure this out??

my teacher said that:

1) you can be exposed to the disease with out being infected( which in my mind means a neg. ppd reading, neg x-ray and neg AFB)

2) you can be exposed to the disease and become infected and have antibodies against TB which will show as granulomas (sorry for the spelling) (which in my mind means a positive ppd reading, negative results on any other test) thus it is a LTBI latent TB infection


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ghillbert, MSN, NP

3,796 Posts

Specializes in CTICU. Has 27 years experience.

Not my area of expertise, but from my layperson understanding there are four possibilities:

1. Not exposed: negative PPD, no further tests required.

2. Exposed: positive PPD, negative AFB/CXR (developed antibodies as seen by PPD, but did not develop TB).

3. Latent infection: positive PPD, positive AFB/CXR, no active symptoms (latent infection not currently active).

4. Active infection: positive tests, active symptoms.

I am very willing to be corrected though!


148 Posts

thanks for your input-

my teacher said that you can be exposed but not have a postive ppd, is this correct? as I see it there are 5 steps

0- not infected- neg everything, never been exposed etc

1- not infected- neg PPD, neg x-ray, neg AFB but has history of exposure- a nurse in a hospital would be a good example....

2- latent TB infection- positive PPD- neg x-ray and neg AFB test- body has developed antibodies

3- active Tb- postive PPD, positive x-ray and postive AFB

4- not active TB- history of TB in past- this could also be someone on drug therapy

5- tb suspect pending test results

but.....for instance if a healthy person had a 7 mm induration- this would not be considered positive because they are not in the high risk catergory.....BUT my question is would this be considered exposed to and not infected with?? or could this also be LTBI??

on the other hand if a healthy person had a 16 mm induration I would consider this positive and infected with TB or could this mean LTBI??

to my understanding my book says that induration of the PPD test would state that the person has been exposed to the disease, but I thought that an induration meant that you were infected with the disease- whether you developed antibodies or not

Daytonite, BSN, RN

4 Articles; 14,603 Posts

Specializes in med/surg, telemetry, IV therapy, mgmt. Has 40 years experience.

you can read about this on the national institute health website. it has to do with the way the bacteria infects the body. tb is slow growing which gives a healthy person's body time to wall off the infecting bacteria and why you get people who have had the bacteria actually enter their body get a positive skin test but experience no active disease. their immune system walled off and isolated the bacteria before the disease could manifest itself and they could even become ill. people who get active tb infections in today's world have compromised immune systems or are continually bombarded with (exposed to) the bacteria.

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ghillbert, MSN, NP

3,796 Posts

Specializes in CTICU. Has 27 years experience.

Just having a positive PPD (eg your 16mm example) does NOT mean that you have been infected with the disease, just exposed enough to develop antibodies. You'd need +ve AFB/CXR to be "infected".


28 Posts

I have a question..........I just started at a new hospital here in NC......I will be working in the ER.........They give TB skin test every 3 months......I have never heard of this before!! I have worked in many ERs, some do twice a year and some do once a year............Is it safe to do this every 3 months? I ask them and of course they said yes,......and that they have a high TB population[raleigh] I never heard of that either. I worked in Florida.......Palm Beach county, which has a high rate and we only did it once a year!! I feel this is really over reacting my poor arms..........are going to look like a junkie!

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