Eosinophils are a type of white blood cell. People who have allergies, such as allergic rhinitis, sometimes have more eosinophils in their blood and other body fluids than do people who don't have allergies. Nasal cytology, or a nasal smear test, checks for these cells. Doctors can use the test to confirm a diagnosis of allergic rhinitis.

The test is very simple. You blow your nose on a piece of plastic wrap. A lab technician examines the nasal discharge under a microscope. If there are many eosinophils, you may have allergies. Finding few eosinophils (negative result), however, does not mean that allergies are not causing your symptoms.