A 38-year-old woman presents with repeated episodes of sore throat. She is on no medications, does not use ethanol, and has no history of renal disease. Physical examination is normal. Hb is 9.0 g/dL, MCV is 85 fL (normal), white blood cell count is 2000/mL, and platelet count is 30,000/mL. Which of the following is the best approach to diagnosis?