This makes sense in that many stones have a COM core but have COD on the outside. This may indicate that urinary calcium levels may be dynamic, with layers of COD forming during periods of calcium supersaturation and resorbing during periods where urine is undersaturated with calcium. In addition, stones that are initially COD may “convert” to COM with time as this dynamic process plays out, explaining why “older” stones tend to be COM.
There are also implications in terms of management. If we know a patient has made a predominately COD stone, efforts to reduce urinary calcium by dietary modification or medical therapy may be indicated. This may be especially relevant in third world countries where 24-hour urine tests may not be readily available and the patient has had recurrent COD stone disease. COM stone formers, on the other hand, may be better managed in the absence of a 24-hour urine test with a modified oxalate and low animal protein diet along with increased citrus intake in an effort to try to modify urine pH, increase urinary citrate levels, and reduce urinary oxalate levels. This thought-provoking research needs to be continued and verified by larger longitudinal studies of stone-formers and correlated with the investigation of stone micro-architecture.
Written by: Mantu Gupta, MD, FRCS, Site Chair, Urology, The Mount Sinai Hospital, New York, New York, United States
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