Scratch tests tend to be prefered because they give quicker results (that day) & can test more things, with blood the amount of blood to test say 20 things is quite a lot from a baby (like doing say an adult blood donor amount) & also take much longer to get results & the cost is greater. From memory in Aus only 4 antigens are covered by medicare & they are "grouped" (dust mite allergy, grasses, high foods (dairy, peanut, shellfish,egg, yeast etc) & animal) so then you have to go further & test greater groups & the cost is quite significant. Where as for skin prick/scratch tests they can far more effectively test for individual food groups.