A 51-year-old woman presents with xerostomia and dry eyes for 1 year, intermittent bilateral parotid gland swelling, and scattered red rashes on both lower limbs for 1 month, accompanied by generalized arthralgia without obvious joint swelling. Physical examination reveals scattered rice-sized red rashes on both lower limbs that do not blanch on pressure. Lab tests: normal UA, ESR 120mm/h, RF positive 1:640, ANA positive 1:320, elevated α-globulin, IgG, IgM, and IgA. Which of the following tests is most significant for diagnosis?