Negative blood test after positive skin test

My son is 3 1/2 and has had 2 possible reactions to peanut butter. One when playing with peanut butter playdoh and he touched his eye and it swelled. The other eating a cheese peanut butter cracker and his lip swelled and blistered. He has had PB only a handful of other times, with no reaction noted.

We have been to the allergist and had the skin prick test. He was positive for peanuts (a 4+ on a scale of 0 to 4) and a 2 for tree nuts. We just got the blood tests results - I believe it was the RAST test - and it was negative for peanuts and tree nuts. The allergist said it was possible to have a positive skin prick and then a negative blood test. My understanding is that in order to have an allergic reaction you have to have the antibodies to the food, so how could his IgE number be 0 when he has had a reaction?

The allergist said to continue to avoid peanuts and she would repeat the blood test in 6 months. If that was negative then they would do a food challenge to be sure. Has anyone had an experience with a negative blood test after positive skin test??? Any advice would be much appreciated.