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!