Bone mineral density assessment in patients with cystinuria

Introduction Cystinuria is a rare genetic disease characterized by impaired tubular transport of cystine. Clinical features of cystinuria include mainly nephrolithiasis and its complications, although cystinuric patients may present with other comorbidities. There are currently no data on bone features of patients with cystinuria. Our aim is to characterize bone mineral density (BMD) in cystinuria. Methods Our study included adult cystinuric patients followed at 3 specialized outpatient clinics in Italy (Rome, Naples and Verona). Markers of bone turnover were analyzed in a centralized laboratory. Clinical, biochemical and dual-energy X-ray absorptiometry (DEXA) data were collected from September 2021 to December 2022. One sample t-tests were used to assess whether BMD Z-scores were significantly different from 0. Results 39 patients were included in the study. Mean (SD) age was 38 (15) years, 40% were women. Mean estimated glomerular filtration rate was 92 mL/min/1.73 m2. Serum parameters associated with bone remodeling (parathyroid hormone, FGF23, calcium and phosphate) were all in the normal range, with only 4 patients showing mild hypophosphatemia. Prevalence of low BMD, defined as T-score < ⁻1 at any site, was 65%, higher at the femur and lumbar spine. Average Z-scores were negative across most sites. Conclusions Our data show a high prevalence of low BMD in cystinuric patients, despite having normal (or moderately impaired) kidney function and being relatively young.