Influence of urinary stones on the composition of a 24-hour urine sample

Background: It can be assumed that stones in the urinary tract continuously increase in size by incorporating material from urine. Consequently, urine will exhibit depleted concentrations of lithogenic constituents when urinary stones are present in the patient's urinary tract. Methods: To calculate the influence of the depletion effect, we considered two different models of stone growth. In the first model, the increase in stone size depends only on the urinary concentration of a lithogenic substance; the second model also considers the surface area of the growing stone. The case of only one kidney being affected by stone formation is considered separately. We discuss example calculations involving the formation of calcium oxalate. Results: The calculated depletion effects are of a non-negligible order of magnitude. Assuming both a measured oxalate concentration of, e.g., 0.37 mmol/L and a reasonable in vivo stone growing rate of 10 mm(3)/day, a relative underestimation of the real "in situ" oxalate concentration between similar to21% (model 1) and similar to42% (model 2) occurs. The depletion effect increases markedly with increasing stone growth rate. Conclusions: Metabolic status can be evaluated correctly only in patients who have been declared "stone-free", e.g., after stone removal. Because the expected stone-related depletion effect in most cases is of high clinical relevance, we recommend estimating the effect of the order of magnitude of the depletion on actual urinary composition. (C) 2003 American Association for Clinical Chemistry.