Influence of a variable differential function on the stone-growth-related urinary depletion effect.

Abstract

Recently, we showed that urine flowing past growing stone material on its way through the urinary tract becomes systematically depleted of its lithogenic substances (1). This effect can be large enough to be clinically relevant (2). However, only the altered urine can be assessed by urine collection, which leads to a urinalysis that does not sufficiently reflect the patient’s true metabolic status. The excessively low concentrations and excretion values of the lithogenic urinary constituents may lead to “optimistic” misinterpretation of the patient’s actual health status and overestimation of treatment success. Apart from geometric and physiologic simplifications, the previously used model assumes that the differential volume function of the kidneys amounts to 50%. Consequently, both the stone-free and the stone-forming kidney contribute equally to the total urine volume. However, even in healthy individuals, the differential function varies by ±6%. Parenchymal defects, such as tumors, renal arteriopathy, and posttraumatic failures, or morphologic abnormalities, such as pyeloureteral junction stenosis and vesico-ureteral-renal reflux, can lead to a dramatic decrease in renal function, causing a reduction in the glomerular filtration rate. Patients suffering from these diseases often present with calcium nephrolithiasis and proteinuria, occasionally with progression to nephrocalcinosis, urinary concentrating defects, and renal insufficiency caused by tubular atrophy, intestinal fibrosis, and glomerulosclerosis. The process of kidney stone formation follows fundamental physical principles such as mass conservation. In a recently introduced model (1), we evaluated the influence of in vivo-growing uroliths on urinary composition. The material from which the stones are formed originates completely from the urine streaming through the kidneys; therefore, the urinary concentrations of the lithogenic components, such as calcium and oxalate, in the excreted urine must be depleted by the mass deposited in the stone(s). As long as no method exists to measure those concentrations in vivo, mathematical models describing the depletion process …