Endocrine Abstracts

Background: Calculation of the fractional excretion of calcium (FECa) from 24-hour (24h) urine collections is recommended to identify patients with suspected familial hypocalciuric hypercalcemia (FHH) during the work-up of primary hyperparathyroidism (pHPT). The current study was designed to test whether calculating the FECa or calcium-to-creatinin clearance obtained from 24h urine specimen can be substituted by results obtained from second morning or spontaneous void urine samples.

Methods: Ninety-one patients subsequently referred for the workup of PTH-dependent hypercalcemia (albumin-corrected and/or ionized calcium ≥ 2.6/1.3 mmol/l, PTH ≥ 30 ng/l, 25-OH vitamin D3 ≥ 50 nmol/l) were included and provided two independent 24h urine collections (groups D1 and D2), one second morning urine (MU) and one spontaneous void urine sample (SU). FECa (%) was calculated from paired blood and urine samples. Patients with a FECa <1% were categorized as suspicious for FHH (sFHH), the remaining considered to have pHPT. Descriptive statistics (median, IQR), Kruskal Wallis and McMemar tests were used to test whether FECa results or the proportion of sFHH patients were different across the four urine collections.

Results: The median FECa in the 4 samples were D1: 2.1% (1.2, 2.7), D2: 2.2% (1.6, 2.8), MU: 1.6% (1.0, 2.4) and SU: 2.1% (1.2, 2.7). FECa derived from the MU was significantly lower when compared to D1 and D2 (P=0.04 and <0.001, respectively). The proportion of patients assigned to the sFHH category were D1: 0.23 (95%CI 0.14–0.32), D2: 0.10 (95%CI 0.03–0.16), MU: 0.24 (95%CI 0.15–0.33), SU: 0.21 (95%CI 0.13–0.30). The proportions of patients assigned to sFHH were significantly different between D1 and D2 (P=0.027), D2 and MU (P=0.006) and D2 and SU (P=0.027). 7/91 subjects had a FECa<1% in all samples and 3 of them received a genetically confirmed diagnosis of FHH.

Conclusion: Considering the inherent variability of FECa derived from 24h urine collections using spontaneous void urine samples may be an adequate alternative to identify patients with possible FHH. Based on our preliminary data, we hypothesize that repeated testing of FECa in spontaneous void urine samples rather than relying on single 24h urine collection may aid to reduce the number of patients in whom genetic testing is required to rule out FHH.