Girls, can you tell me how to take blood for allergen detection? Or do…

Girls, can you tell me how to take blood for allergen detection? Or do they want something? The baby has red cheeks around a month old sometimes to the crusty, I smear with moisturizing cream and give Fenistil sometimes. Sometimes I don't give her fish, she refused milk porridge, I don't understand what could be an allergy, nothing new was introduced.