A 53-year-old male has been admitted to a hospital for an attack of renal colic which has repeatedly occurred throughout the year. Objectively: in the region of auricles and the right elbow some nodules can be seen that are covered with thin shiny skin. Ps- 88/min, AP- 170/100 mm Hg. There is bilateral costovertebral angle tenderness (positive Pasternatsky's symptom). The patient has been scheduled for examination. What laboratory value would be most helpful for making a diagnosis?