Tubular phosphate transport: a comparison between different methods of urine sample collection in FGF23-dependent hypophosphatemic syndromes

Objectives: Tubular maximum phosphate reabsorption per glomerular filtration rate (TmP/GFR) is used to evaluate renal phosphate reabsorption and it is a useful tool for the differential diagnosis of hypophosphatemic syndromes. TmP/GFR is typically calculated from fasting plasma and second morning void urine samples, obtained 2 h after the first void (TmP/GFR 2 h). The purpose of this study was to evaluate if TmP/GFR calculated from 24 h urine collection (TmP/GFR 24 h) can be used as an alternative for TmP/GFR 2 h in patients with urine phosphate wasting. Methods: We enrolled adult patients with X-linked hypophosphatemia (XLH) or tumor-induced osteomalacia (TIO). All patients underwent blood and urine sample collections, to calculate TmP/GFR 24 h and TmP/GFR 2 h. Results: Twenty patients (17 XLH and 3 TIO), aged 24-78 years, were included. All patients had low TmP/GFR 2 h (0.35 mmol/L, IQR 0.24-0.47 mmol/L) and TmP/GFR 24 h (0.31 mmol/L, IQR 0.22-0.43 mmol/L). The concordance correlation coefficient between TmP/GFR 2 h and TmP/GFR 24 h was 0.86 (95 % CI: 0.69-0.93), with a systematic bias of 0.05 mmol/L (95 % limits of agreement: -0.10 to 0.20). Furthermore, in 70 % (i.e., 14 patients out of 20) and 80 % (i.e., 16 patients out of 20) of cases the difference between TmP/GFR 2 h and TmP/GFR 24 h was within +/- 30 % and +/- 35 %, respectively. Conclusions: Despite TmP/GFR 2 and 24 h show a relatively suboptimal agreement, the difference between the two parameters appears to be small and not clinically significant in the setting of adult patients with FGF23-dependent urine phosphate wasting and secondary hypophosphatemia.