We did a RAST test at about 9 mos. It was reliable, in that the SPT we did at 14 mos came up with the same results.
It was also accurate, in that when we removed the 3 positive foods from my diet and his, his eczema cleared up a LOT. (not entirely, so we were missing some things, but they tested for only 3 things with the RAST test and about 8? 10? foods with the SPT)
RAST test at 27 mos showed the same 3 food (and same 3 environmental) allergens, for what it's worth.
SO, he had at least 3 IgE food allergens that were reliably and validly tested by RAST under 12 mos and SPT under 18 mos.
BUT, he has never shown a positive RAST or SPT or in-office-challenge to dairy, yet he does react to dairy. I believe the reaction is lessening over time, but it's still there.
I think a RAST test is a fairly easy test to do - it's only one prick - and it CAN give you a starting point. It will also give you a value that you can track over time, so you can see if things are getting worse or better. The problem comes when it's negative - indicating no IgE allergy - but, we all know that the child can still be reacting - intolerant or IgG - to foods, so you have to fight the relatives, DH, doc, if that comes up negative.