RESUMO
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 2â¯h after the first void (TmP/GFR 2â¯h). The purpose of this study was to evaluate if TmP/GFR calculated from 24â¯h urine collection (TmP/GFR 24â¯h) can be used as an alternative for TmP/GFR 2â¯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 24â¯h and TmP/GFR 2â¯h. RESULTS: Twenty patients (17 XLH and 3 TIO), aged 24-78 years, were included. All patients had low TmP/GFR 2â¯h (0.35â¯mmol/L, IQR 0.24-0.47â¯mmol/L) and TmP/GFR 24â¯h (0.31â¯mmol/L, IQR 0.22-0.43â¯mmol/L). The concordance correlation coefficient between TmP/GFR 2â¯h and TmP/GFR 24â¯h was 0.86 (95â¯% CI: 0.69-0.93), with a systematic bias of 0.05â¯mmol/L (95â¯% limits of agreement: -0.10 to 0.20). Furthermore, in 70â¯% (i.e., 14 patients out of 20) and 80â¯% (i.e., 16 patients out of 20) of cases the difference between TmP/GFR 2â¯h and TmP/GFR 24â¯h was within ±30â¯% and ±35â¯%, respectively. CONCLUSIONS: Despite TmP/GFR 2 and 24â¯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.