Total IgE: A measurement of the total amount of IgE circulating in a patient's blood can be helpful but is elevated in only one-half of allergic patients. RAST testing is a more specific blood test for allergy. This is a radio-immune assay where a patient's serum is incubated with different allergens and antigen/antibody complexes are then measured. This is not as sensitive as skin testing and certainly more costly, and cannot therefore be used as a screening tool. A smear of nasal mucus, checked for eosinophils (allergy mediating cells), can be helpful if the percentage of eosinophils is over 5%. An adenoid x-ray to assess how large the airway is at the back of the nose is also helpful. Physical examination can sometimes distinguish allergy from viral upper respiratory infections, but this is often quite difficult.