Background 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. This pilot study investigated whether handheld urine TDS values are associated with ultrasound-detected renal stones smaller than 5 mm in infants younger than 24 months. Methods This prospective pilot comparative study included infants younger than 24 months with and without ultrasound-detected renal stones smaller than 5 mm. Non-first-morning daytime spot urine samples were collected on three non-consecutive days within a 10-day period, and mean TDS values were calculated. Receiver operating characteristic (ROC) analysis was used descriptively to assess sample-level separation, and multivariable logistic regression adjusted for age and sex was used to examine association. Results 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 < 0.01), 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 < 0.0001). Conclusions In this pilot cohort, handheld urine TDS 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 TDS reflects urine concentration alone. Direct analytical validation and longitudinal studies are needed before any clinical role can be considered.