Can a bloodtest pinpoint an allergy ?

my 2 1/2yr has been having a reaction to something for the last 5 days and he was covered in hives pretty much everywere they came in patches he is on steriods for the next 3 days and needs a bloodtest and this is going to bee stressful for him i know i just want to make sure they can pinpoint the allergan befor i put him through this anyone else had there kids tested this way ?