A 58-year-old man has the sudden onset late one evening of severe pain in his left great toe. There is no history of trauma. On examination there is edema with erythema and pain on movement of the left 1st metatarsophalangeal joint, but there is no overlying skin ulceration. A joint aspirate is performed and on microscopic examination reveals numerous neutrophils. Over the next 3 weeks, he has two more similar episodes. On physical examination between these attacks, there is minimal loss of joint mobility. Which of the following laboratory test findings is most characteristic for his underlying disease process?