Abstract / Summary
Infants with asymptomatic renal stones smaller than 5 mm are often managed conservatively with ultrasonographic follow-up and hydration counselling. In diapered children, repeated outpatient urine assessment is difficult because first-morning samples, 24-hour urine collections, and laboratory osmolality testing are often impractical. Handheld total dissolved solids (TDS) meters provide rapid conductivity-based readings from small urine volumes, but the biological meaning of these readings in infant urine is uncertain and they are not validated equivalents of established urine concentration measures. We hypothesized that infants younger than 24 months with ultrasound-detected renal stones smaller than 5 mm would have higher mean handheld urine total dissolved solids values than stone-negative infants from the same ultrasonography setting. This pilot study investigated this preliminary association and was not designed to validate total dissolved solids as a diagnostic test or hydration biomarker. This prospective pilot comparative study included infants younger than 24 months with asymptomatic, no obstructing, ultrasound-detected renal stones smaller than 5 mm and stone-negative controls recruited consecutively from the same outpatient ultrasonography setting. The 5- mm threshold was selected a priori to define a conservatively followed small-stone cohort. Non-first-morning daytime spot urine samples were collected on three non-consecutive days within a 10-day period and mean handheld total dissolved solids values were calculated. Receiver operating characteristic analysis was used descriptively to assess sample-level separation, and multivariable logistic regression adjusted for age and sex was used to examine association. Seventy-four infants were included: 37 cases and 37 controls. Mean urine TDS was higher in infants with stones than in controls (480.6 ± 171.6 vs. 306.8 ± 67.9 ppm; p < 0.001), corresponding to a large effect size (Cohen’s d ≈ 1.3). ROC analysis showed an area under the curve of 0.80 (95% CI 0.696–0.900). In multivariable analysis adjusted for age and sex, each 100 ppm increase in TDS was associated with higher odds of stone presence (adjusted OR 2.92; 95% CI 1.73–4.93; p < 0.001). In this pilot cohort, handheld urine total dissolved solids values were higher in infants with ultrasound-detected renal stones smaller than 5 mm than in stone-negative controls. These findings indicate an association between stone status and a device-derived conductivity signal, but they do not establish diagnostic validity or confirm that total dissolved solids reflect urine concentration alone. Future studies should first validate handheld total dissolved solids against urine osmolality, refractometric specific gravity, and urinary solute composition before evaluating any clinical monitoring role.