BACKGROUND AND AIMS Calcium load is usually used in clinical practice to diagnose a normocalcaemic hyperparathyroidia in stone disease. However, the calcium load developed by Pak and colleagues has been described in very few patients. We aimed to provide data about calcium load in a bigger cohort of patients. METHOD We included retrospectively 115 patients who underwent a calcium load for stone disease or hypercalciuria with a basal increased PTH. At basal state, ionized calcium (iCa2+), PTH,

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... osphate urinary Calcium to creatinine ratio (UCa/creat), 25 OHvit D and 1–25 OHvit D were measured. Patients ingested 1g of calcium and iCa2+ PTH, phosphate and UCa/creat were measured after 2 h and 4. Calcium load was interpreted based on Pak's publications. Normocalcaemic hyperparathyroidia was defined by fasting urinary calcium/creatinine ratio >0.35 mmol/mmol associated with a non-adapted PTH to ionized calcium (iCa2+) after calcium load and absorptive hypercalciuria. Renal hypercalciuria is defined by fasting and absorptive hypercalciuria with an adapted decrease of PTH to iCa2+. RESULTS At a basal state, PTH was 88 (±19) ng/L, calcium was 2.36 (±0.14) mmol/L, iCa 1.23 (±0.08) mmol/L, calcium excretion fraction was 1.5 (0.85)% and phosphataemia was 0.88 (±0.14) mmol/L. The mean fasting Ca/creat ratio was 0.4 mmol/mmol. A total of 21% were hypercalcemic (iCa > 1.30 mmol/L). iCa2+ was 1.28 (±0.07) 2 h after calcium load (H2) and was 1.27 (±0.07) mmol/L 4 h after calcium load (H4). The mean decrease in PTH was 32% after H2 and 44% after H4. Urinary Ca/creat was 0.55 and 0.73 mmol/mmol after H2 and H4, respectively. A total of 27% of patients had a UCa/creat increase of >0.5 mmol/mmol after H2, and 47% after H4. A total of 72% had a calcium excretion fraction of >1%. Normocalcaemic hyperparathyroidia was diagnosed in 64% of these patients. In Normocalcaemic hyperparathyroidia, mean PTH was 91 (±21) ng/L, the mean decrease of PTH was 35% and 27% after H2 and H4, respectively. The mean iCa2+ increased from 1.24 (±0.07) to 1.28 (±0.07) mmol/L. UCa/creat increased from 0.45 (±1.26) to 0.71 (±1.26) and 0.81 (±1.26) after H2 and H4, respectively. The 1–25 OH vit D was 159 (±55) pmol/L. In renal hypercalciuria, mean PTH was 66 (±26) ng/L, 42 (±20) after H2 and 30 (±13) after H4. The mean decrease in PTH was 65% after H4. Mean iCa2+ increased from 1.24 (±0.07) to 1.27 (±0.07) mmol/L. UCa/creat increased from 0.33 (±0.19) to 0.37 (±0.25) and 0.68 (±0.44) mmol/mmol after H2 and H4, respectively. The 1–25 OH vit D was 130 (±67) pmol/L. Basal PTH is significantly higher in normocalcaemic hyperparathyroidia compared with renal hypercalciuria, but there is no difference in fasting UCa/creatinine ratio between both group. The 1–25 OHvit D level was significantly increased in normocalcaemic hyperparathyroidia. CONCLUSION Calcium load allows to discriminate patients with basal increased PTH between normocalcaemic hyperparathyroidia and renal hypercalciuria. We provide new data to help physicians in clinical practice.