Nephrolithiasis risk factors for obese patients on 24-hour urine collection metabolic evaluation

Twenty-four-hour urine collections are obtained as part of the metabolic workup for nephrolithiasis to identify modifiable abnormalities for stone prevention. We sought to discern trends in the prevalence of abnormalities based on body mass index (BMI) (kg/m²).

All unique Litholink™ 24-Hour Urine outcomes for nephrolithiasis prior to medical or dietary therapy obtained at our institution between 2004 and 2020 were retrospectively reviewed. Patients with anthropometric data were classified according to body mass index (BMI) as underweight (<18.5), normal (18.5-25), overweight^25-30 and obese (>30). Litholink™ 24-Hour Urine gender specific reference ranges were used to define abnormalities.

A total of 1372 patients were included. The mean age was 56 $\pm$ 15.3 (53.5% male, 46.5% female). Cumulatively, 30.7% (421/1372) were obese, 32.9% (452/1372) overweight, 33.4% (458/1372) normal and 3.0% (41/1372) underweight. Overall, elevated urine sodium was the most common metabolic abnormality (52.8%). In obese stone formers, hypercalciuria (p = 0.027), hyperoxaluria (p < 0.001), elevated urine sodium (p < 0.001), hyperuricosuria (p < 0.001) low urine pH (p < 0.001) and high uric acid supersaturation (p < 0.001) were more likely compared to normal BMI stone formers. Underweight patients demonstrated greater likelihood of oliguria (p = 0.001), without adjustment for weight, and hypocitraturia (p = 0.001) compared to normal BMI stone formers.

Body weight differences are associated with different risk profiles on 24-hour urine collection. Obese, and to a letter degree, underweight patients are more likely to harbour metabolic derangements on a 24-hour urine analysis compared to patients with normal BMI, thus underscoring the importance of directed medical therapy for stone formers.