When should I start to worry?

This is The Boy's ppd skin test to check for tuberculosis. To me it looks just like a mosquito bite. There is a slight bump but nothing resembling a nodule like in the pictures I googled online.

I took him to the clinic last night. The older nurse who looked at it immediately declared it positive and went to get a doctor. Not our doctor who wasn't in, which annoyed me. Why did they schedule the appointment when she wouldn't be there? She is the only doctor at the clinic with any experience with the BCG vaccination and third world countries.

The doctor examined it for a few minutes and declared it negative. He said the same thing I did - there is a bump but there is no distinct nodule. The redness means nothing at all - it's the size of the bump that matters.

The conclusion was to bring him back this morning to let our doctor examine it.

I could tell from her expression that it wasn't what I wanted to hear. A different nurse got calipers and measured the slight bump to be 10mm and the entire mark to be 20mm. In a child, 10mm or bigger is when you pursue a chest xray.

"I am going to confer with the other doctors and I'll give you a call. We will probably order a chest xray, but he is a healthy, strong little guy. I don't want you to worry."

Really? Can you tell me exactly how to not worry?