TB question


I've been a nurse for a few years now, and I thought I knew the procedure to read TB tests until a more experienced nurse that I greatly respect told me I've been doing it wrong (she said a doctor showed her how to do it).

Basically what I thought, was that you read the induration which will appear at the top of the skin which will often look similar to the initial wheal that was placed, which you can feel with a feather-touch, which often looks like this, clearly raised on top of the skin.

Positive TB Skin Test Picture

and not to measure the redness/erythema or swelling beneath the skin.

I had a patient who's TB test was red and itchy, and she admitted she had been scratching at it. There was a large area of redness and significant localized swelling below the surrounding skin. This more experienced nurse told me that you are supposed to measure if there is swelling beneath the skin, and that is what could determine if she was considered positive. I questioned that and showed her a tb poster which shows a induration at the top of the skin(like the picture linked above), and she repeated that I was confusing it and this lady's swollen area was considered her induration.

Isn't the swelling just a inflammatory/histamine reaction to the itching/scratching though, and not considered an actual positive? I mean, we don't read the erythema...

The nurse told me that we can't always rely on sight but have to use our fingers. I told her I knew this, but thought that that was because you may have trouble seeing it with some people, especially those with darker skin, not because it could be under the skin.

I watched a CDC video on how to read tb tests,

but I still have the same question. Look at 1:20-2:00 and 5:46-6:10, they say not to measure swelling.

Could someone please clarify? Do you really measure swelling below the skin, or only the induration on top? For those who have seen a lot of positive TB tests, do they almost always look like the picture above or do they look differently at times?

Also, sometimes I have felt a half-piece-of-rice sized lump under the skin if I press slightly harder at the area, I figured that was just minor hematoma or swelling, have I been misreading that too? I thought we were only to use a feather-touch so that we only feel what's on the top portion of skin?

I'm embarrassed because I have told a new nurse in the past to only read what's at the top of the skin but before I go to her and tell her I've given her bad information, please help me determine if I really am wrong! Thanks for your help!

highlandlass1592, BSN, RN

1 Article; 647 Posts

Specializes in Critical Care. Has 13 years experience.

I was taught like you were but as I read your post, began to doubt myself. So I went to the CDC website and found the following information for you:

Couple of links I found for you:



I think the second link is right on point as to what you were looking for. Induration appears to be the key, no pun intended. That wall chart may be a good thing to print up to keep on hand. As the CDC sets the guidelines for us to follow, I felt going straight to the source might be best. Hope this helps.


344 Posts

If the TB test site, is not raised, but has a reddened itching area....I would read that test as negative for TB.

However, the patient is having a hypersensitivity to the TB serum/ allergic reaction. And that should be reported to MD. Also it IS possible to have an anaphylactic reaction to TB serum, so according to the CDC website, the patient should remain in the clinic setting for 15 minutes following injection, to monitor for this type of reaction. This was something i did not learn in school, but after my anaphylactic reaction to TB serum occurred!! Very Scary.


63 Posts

Has 2 years experience.

What about a sputum culture?


2 Posts

Thank you for the links. The patient's tb site itself was not raised, however the tissue under and around it was. Think of like a swollen ankle, that's how it looked from the side but from above it looked only red. Patient had no sputum to collect. Leelee, do you mean you yourself had an anaphylactic reaction to the tb test?

So in the future then it seems I should keep reading the tb tests as I have been?

Thanks for the replies.

Specializes in chemical dependency detox/psych.

A positive TB test has a firm lump (induration)that you can palpate (and then measure). Don't read the redness or generalized swelling from the patient having a reaction to the serum. I read TB tests every day, and I have them present their forearm to me, and I take my pointer and index finger to palpate the area in a downward stroke. To be honest, even with all the TB tests that I've read, I've yet to get a positive, but I've had my fellow nurses describe the feeling and look for me (and one that's been there for 15 years said she's only found 1), and it's more a lump than anything that anyone would describe as swelling or redness.

nerdtonurse?, BSN, RN

3 Articles; 2,043 Posts

Specializes in ICU, Telemetry.

I always turn red, but never get the dreaded lump; the employee health nurse said it's an immune system reaction to some component of the injection. She told me as long as I didn't feel "stacked quarters" I was ok.