Use of urinary concentrations in mg/dl in relation to absolute values in 24-hour samples for the evaluation of lithogenic factors in stone forming patients.

OBJECTIVE: The aim of this study is to analyze urine concentrations (mg/dl) of different lithogenic factors in a sample of 24 h as a predictor of these changes rather than absolute values depend on the volume of diuresis.

METHODS: A total of 131 patients from the North Almeria Health Management Area (Spain) with urinary calstone disease in whom a metabolic study was indicated were included from June 2014 to May 2015. The concentrations of calcium, oxalate, uric acid, citrate and magnesium were measured in the urine, and the calcium/citrate ratio was calculated. The classifications used were: hypercalciuria (>260mg/24h), hyperuricosuria (>750mg/24h), hyperoxaluria (>40mg/24h), hypocitraturia (<320mg/24h) and hypomagnesuria (<35mg/24h). The statistical analysis was performed using SPSS 17.0.

RESULTS: A cut-off point of 12.55mg/dl, with a sensitivity of 90% and a specificity of 85% and a relative risk (RR) of 51.2 (13.9-188.4), was estimated for urinary calcium. For oxalate the cut-off point was 1.86mg/dl, with a sensitivity of 91% and a specificity of 84% with an estimated RR of 67.2 (8.3-540.6). As regards the uric acid concentration in urine, a cut-off point of 31.2mg/dl was estimated, with a sensitivity of 85% and a specificity of 70% and a RR of 12 (3.8-37.6). For citrate the cut-off point was 18.8mg/dl, with a sensitivity and specificity of 82% and 74%, respectively, with a RR of 13.7 (4.4- 42.6). The cut-off point for magnesium was 2.26mg/dl with a sensitivity of 95% and specificity of 78%, with a RR of 67.6 (11.4-398.3).

CONCLUSION: The determination of urine concentrations, instead of absolute values, depends to a large extent on urine output, appears to be useful when estimating classic metabolic alterations and should be taken into account in the evaluation of patients with urinary stone disease.