A 58-year-old female presents with progressively worsening xerostomia and fatigue over the past year. Physical exam reveals multiple dental caries, residual black tooth roots, and generalized lymphadenopathy. Laboratory tests show ESR 89mm/h, serum γ-globulin 28.5%, ANA 1:80 positive, and anti-SSA antibody positive. Which of the following tests is LEAST useful for diagnosing this condition?