A20 year old female has an erythematous rash over her face on both cheeks and across the bridge of her nose.This rash is made worse by sunlight exposure when she is outdoors. Along with the rash, she has had muscle and joint pain for several month. However, radiographs of the joints does not show any abnormalities, and she has normal joint mobility without deformity. Which of the following laboratory test findings is most characteristic of the disease?