The Journal of clinical endocrinology and metabolism · 2026
Challenges in Estimating Renal Function in X-linked Hypophosphatemia Because of Formula Overestimation and FGF23 Effects.
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Lemoine Sandrine, De Mul Aurélie, Perge Kevin, Derain-Dubourg Laurence, Chapurlat Roland, Vignot Emmanuelle, Legrand Mélanie Alice, Bacchetta Justine
Аннотация
INTRODUCTION: In X-linked hypophosphatemia (XLH), phosphate and vitamin D analogs increase the risk of nephrocalcinosis and renal impairment. Kidney function assessment is challenging, as creatinine-based estimated glomerular filtration rate (eGFR) may overestimate GFR because of reduced muscle mass. Cystatin C alternatively may be used. Moreover, fibroblast growth factor 23 (FGF23) has been suggested to potentially impact GFR, leading to hyperfiltration and complicating kidney function assessment.
METHODS: We conducted a single-center retrospective study on adults with XLH to compare eGFR formulas (CKD-EPIcreat, CKD-EPIcyst) with true measured GFR (mGFR). Bias, precision, and accuracy were calculated. In a subgroup, mGFR changes after burosumab initiation were assessed. Finally, we analyzed the FGF23/GFR relationship in a pediatric cohort in whom FGF23 was also hypersecreted (fibrous dysplasia) to support our hypothesis that FGF23 leads to hyperfiltration.
RESULTS: We included 20 patients (17 females, median age 35 years). None had nephrocalcinosis or hypercalciuria, but 8 had secondary hyperparathyroidism. Median mGFR was 90 (85-107) mL/min/1.73 m², significantly lower than CKD-EPIcreat (125 [114-121]) and CKD-EPIcyst (103 [118]) (respectively P < .0001 and P = .01). Bias was significantly greater with CKD-EPIcreat (-22 [-29 to -13]) than CKD-EPIcyst (-13 [-19 to -4]), with lower accuracy (25% vs 93%). In 5 patients with XLH, mGFR decreased from 98 to 84 mL/min/1.73 m² after burosumab introduction. FGF23 moderately correlated with eGFR in 14 fibrous dysplasia patients (R = 0.62, P = .02).
CONCLUSION: eGFR overestimates GFR in XLH, increasing CKD misclassification risk. Annual cystatin C measurement is recommended if CKD is suspected. FGF23 impact on GFR should also be considered in burosumab-treated patients.
METHODS: We conducted a single-center retrospective study on adults with XLH to compare eGFR formulas (CKD-EPIcreat, CKD-EPIcyst) with true measured GFR (mGFR). Bias, precision, and accuracy were calculated. In a subgroup, mGFR changes after burosumab initiation were assessed. Finally, we analyzed the FGF23/GFR relationship in a pediatric cohort in whom FGF23 was also hypersecreted (fibrous dysplasia) to support our hypothesis that FGF23 leads to hyperfiltration.
RESULTS: We included 20 patients (17 females, median age 35 years). None had nephrocalcinosis or hypercalciuria, but 8 had secondary hyperparathyroidism. Median mGFR was 90 (85-107) mL/min/1.73 m², significantly lower than CKD-EPIcreat (125 [114-121]) and CKD-EPIcyst (103 [118]) (respectively P < .0001 and P = .01). Bias was significantly greater with CKD-EPIcreat (-22 [-29 to -13]) than CKD-EPIcyst (-13 [-19 to -4]), with lower accuracy (25% vs 93%). In 5 patients with XLH, mGFR decreased from 98 to 84 mL/min/1.73 m² after burosumab introduction. FGF23 moderately correlated with eGFR in 14 fibrous dysplasia patients (R = 0.62, P = .02).
CONCLUSION: eGFR overestimates GFR in XLH, increasing CKD misclassification risk. Annual cystatin C measurement is recommended if CKD is suspected. FGF23 impact on GFR should also be considered in burosumab-treated patients.