The alteration of urine composition due to stone material present in the urinary tract

Based on a recently introduced mathematical model approach we show that uroliths located in the urinary tract (UT) can substantially deplete the urinary concentrations of lithogenic constituents by continuous growth.

To illustrate the influence of the urinary depletion effect on urinalysis, a "typical" calcium oxalate (CaOx) stone former, presently not stone-free, was investigated. Serum and urine samples were analyzed. Several metabolic tests were performed. Crystallization risk according to the BONN-Risk-Index (BRI) and the relative supersaturation in respect to CaOx (RS(CaOx)) were determined. X-ray films taken before and after a period of six months were evaluated in order to estimate stone size and average stone growth rate taken place within that period.

Regarding CaOx urolithiasis the urine samples show a pathologically altered pH, a low 24h-citric acid excretion, and a low 24h-urine volume. Neither calcium nor oxalate excretion data reflect values indicating these substances as risk factors. However, BRI and RS(CaOx) reflect a high crystallization risk. The patient's intestinal oxalate absorption is high. Applying the depletion model, the patient's 24h-urine composition shifts from normooxaluric to hyperoxaluric.

In the patient, a hitherto undetected hyperoxaluria is diagnosed when in-vivo stone growth is taken into account. This is a clear indication that the metabolic status can only be evaluated correctly in "stone-free" patients. Any stone material present in the UT must be considered in urinalysis interpretation as the stone-related urinary depletion effect is of high clinical relevance. A helpful nomogram is presented, allowing the estimation of the extent of urinary depletion caused by CaOx uroliths.