A 32-year-old woman reports easy hair loss during combing over the past 3 years, recurrent painless oral mucosal ulcers, and episodes of finger pallor, pain, and subsequent cyanosis upon cold exposure in winter for the past 2 years, which later resolve. During history-taking, the doctor notes that the patient develops facial skin erythema after sun exposure in summer, raising suspicion of SLE. Which additional laboratory finding would confirm the diagnosis?