Calcium Kidney Stones

A 43-year-old man presents for evaluation of recurrent kidney stones. He passed his first stone 9 years earlier and has had two additional symptomatic stones. Analysis of two stones showed that they contained 80% calcium oxalate and 20% calcium phosphate. The urinary volume was 1.54 liters per day, and the urine pH was 5.6. The patient had been treated with 20 to 40 mmol of potassium citrate daily since he passed his first stone. How should he be further evaluated and treated?