13 mo old reacted to PB but has negative skin test and immunocap. what now??

My DD is 13 mo old and had eaten small bits of PB about 3-4 times prior to a reaction. On this particular day she had just had her 12 mo shots (dtap, hib and prevnar) and had just finished a 10 day course of amoxicillan due to an ear infection. At any rate she had PB around 11 am and 3pm and went to bed without incident at 8:30 pm. She woke the next am with hives that continued to come and go for 3 days. We were concerned about PB but also about whether the hives could have come from the recent shots or antibiotics. She was skin tested after being off benadryl for a week and tested - for peanuts. Her Immunocap was also - for peanuts, walnuts, almonds, and cashews. After her skin test her allergist asked us to give her a bit of PB to see what happened. She did get one tiny hive on her arm after a few minutes. ????? He said that this had to be some sort of coincidence as her tests were - nad they are very reliable. He wants to do another oral challenge. I am so confused. I read conflicting info. on the web about the reliabiltiy of these tests, if I should just stop all peanut products until age 3 or if it is truly best to know for sure now and that maybe I should continue PB to prevent an allergy. Please help, I am very confused and our appt. is tomorrow pm. Thanks

[National Jewish Health]

It is less likely that the hives that developed the next morning after peanut ingestion at 11 AM and 3 PM were due to peanut.  Almost all allergic reactions which are IgE-mediated will occur within 2 hours of ingestion.  Also, hives from foods do not usually last 3 days unless ongoing ingestion occurs.  The hives could have been due to the viral infection that led to the ear infection.

The only true test to see if someone is allergic to a food is to do a food challenge, regardless of positive or negative skin or blood tests.  People can certainly react with negative skin and immunocap levels, although it is not as common.  You may consider having your allergist perform a skin test with peanut butter to see if this is positive.  It is possible that after 3 or 4 exposures to peanut your child became sensitized to peanut.  The only way to determine this is to do the challenge.  However I am unable recommend or not recommend that you do the challenge as I have not seen your child.  If you want your child to eat peanuts now, then the food challenge is the only way to determine if she is truly allergic.  We do not usually do challenges to peanut until children are much older, 3 or 4 years old, and the recommendation from the American Academy of Pediatrics is to wait until 3 years old to introduce peanut.  However it is possible that your child could become allergic to peanut in the future, and there is no guarantee that giving her peanut now would prevent this.

If you do decide to do the challenge, then the safest way to do it is a graded challenge, giving small increasing doses separated by 20 or 30 minutes.  Also, about 8 grams of peanut protein should be given over the course of the challenge, as this represents a normal serving of peanut protein.