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 the first and the last stones showed that they contained 80% calcium oxalate and 20% calcium phosphate. Analysis of a 24-hour urine collection while the patient was not receiving medications revealed a calcium level of 408 mg (10.2 mmol), an oxalate level of 33 mg (367 mu mol), and a volume of 1.54 liters; 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?