MON-532 Urinary Calcium Indices in Primary Hyperparathyroidism (PHPT) and Familial Hypocalciuric Hypercalcaemia (FHH): Is There a Need for Better Discrimination?

Introduction: Primary hyperparathyroidism (PHPT) has a prevalence of around 1% and requires surgery for definite treatment. On the other hand, familial hypocalciuric hypercalcaemia (FHH) is much rarer and does not need specific treatment. There may be considerable overlap in biochemical features in these conditions. Urine calcium indices are used to help with the differential diagnosis, but their reliability in making this distinction is not clear. The aim of this retrospective cohort study was to review urinary calcium values in patients with proven PHPT and those with genetically proven FHH. Methods: Over six years (2011-2016), 265 patients with PHPT underwent surgery. Of these, 14 patients with intermittent or persistent hypercalcaemia within 6 months after surgery were excluded leaving 251 patients (female to male ratio 5:1) for final analysis. In the FHH arm, only genetically proven cases (n = 10) were included. Results: In the PHPT arm, pre-operative data was available for 24 hr urine calcium excretion (UCE) and fractional excretion of calcium (FeCa) in 93 patients; 24 hr UCE only in 95 patients; and FeCa only in 43 patients. 24 hr UCE was low (i.e. <2.5 mmol/L/24 hr) in 8 of 188 patients (sensitivity for PHPT - 95.7%). FeCa was low (<0.01) in 16 of 136 patients (sensitivity for PHPT - 88.2%). Most of the false negative (for PHPT) readings (all of 24 hr UCE and 87.5% of FeCa) could be explained by low vitamin D levels (<50 nmol/L), chronic kidney disease 3b or worse and drugs such as indapamide. In the subgroup of 93 patients who had both 24hr UCE and FeCa, low FeCa was seen in 10 patients, low 24 hr UCE in 5 patients and only one patient had low levels of both tests. In the FHH arm, both 24 hr UCE and FeCa results were available for 6 patients; 24 hr UCE only for 2 patients and FeCa only for 2 patients. Five of eight patients with 24 hr UCE had at least one normal or high level of urinary calcium and three of eight patients with FeCa had levels consistent with a diagnosis of PHPT. Conclusions: Vitamin D deficiency and chronic kidney disease should be considered as causes of low urine calcium excretion in patients with features of primary hyperparathyroidism. A significant proportion of gene positive FHH patients may have normal or high urinary calcium excretion at some stage. Research into better discrimination between these conditions would be clinically useful.