Consider high urine oxalate in the evaluation of kidney stones

A 46-year-old man had an anunual physical examination and reported to his new physician that he had had kidney stones in the past. A 24-hour urine for calcium and urate was normal and the patient was referred for further evaluation of the possible cause for his stone. His personal medical history was otherwise negative and there was no family history of stones. He was 6'3” tall, weighed 226 lbs., BP 115/70, pulse 68 bpm and regular, and the remainder of the physical examination was normal.

A 24-hour urine collection was sent to the lab not only for calcium and urate, but also sodium, creatinine, oxalate and citrate.

All values were normal except the urine oxalate, which at 70 mg/24 hours was nearly twice the reported upper limit of normal (45). Urine calcium was normal at 174 mg (upper limit of normal is 4mg/kg body weight).

I did a "Google" search for the term "low oxalate diet" and found a very patient-friendly monograph produced by the University of Pittsburgh Medical Center: www.patienteducation.upmc.com/Pdf/LowOxalateDiet.pdf. I shared it with the patient and watched his facial expression as he went from item to item and found many high oxalate foods that he enjoyed on a regular basis.

Small increments in urine oxalate make a greater contribution to stone formation than larger increments in urine calcium.