OR07-05 Is Urinary Calcium the Only Predictor of Nephrolithiasis in Patients with Asymptomatic Primary Hyperparathyroidism?

The 4(th) International Workshop for the management of asymptomatic PHPT included, among the criteria for parathyroidectomy, the presence of hypercalciuria (dUCa> 400 mg/day) and increased biochemical stone risk profile. The aim of the present study was to evaluate the biochemical stone risk profile in 176 consecutive patients (143 females and 33 males) with asymptomatic PHPT. We recorded clinical and biochemical data, including 24 hours urinary measurements of the following parameters: volume and pH, creatinine, calcium, magnesium, sodium, potassium, ammonium, uric acid, oxalate, citrate, phosphate, inorganic sulphate and chloride and kidney ultrasound. In our cohort dUCa> 400mg/day showed a low sensitivity and positive predictive value (PPV) for nephrolithiasis with high specificity (46.2, 32.7, 73.0% respectively), while hypercalciuria by 4 mg/kg/bw (d-UCa>4mg/kg) had a high sensibility, with low PPV and specificity (79.5, 27.7, 40.1%) Daily hypomagnesuria (d-HypoMg), but not any other urinary parameter, was an independent predictor of nephrolithiasis in the univariate (OR 2.97 CI 1.27-7.09 P=0.014) and multivariate analyses adjusting for age, sex, BMI, and eGFR (OR 3.13 CI 1.17-8.42 P=0.02). d-HypoMg was relatively lower in the regression analysis with urinary calcium in patients with nephrolithiasis compared with those without. The mean ratio between (dUCa) and (dUMg) was higher in patients with nephrolithiasis compared with those without (4.6±2.0 vs 3.3±4.1; P<0.001). In the univariate and multivariate analyses the dUCa/dUMg ratio was a significant predictor of nephrolithiasis [OR 4.9 (2.3-10.5); P<0.001; OR 5.3 (2.4-11.6), P<0.001, respectively]. The AUC using the dUCa/dUMg ratio as variables was 0.69 (CI 0.60-0.79; P<0.0001). The best cut-off value, set at the highest Youden index, was equal to 4.0, with a sensitivity of 59.0% and a specificity of 77.4%. In patients with hypercalciuria (>400 mg/24-hour) dUMg was positively correlated with dUCa in those without nephrolithiasis (r=0.50, β=0.2, P=0.002) but not in those with nephrolithiasis (r=0.05, β= 0.014; P=0.8). In patients without hypercalciuria we found that hypomagnesuria remained a predictor of nephrolithiasis using either 400 mg/die (P=0.002, OR 5.12 (1.84-14.24) or 4 mg/kg bw (P=0.014, OR 6.24 (1.45-26.8). Moreover, the OR for nephrolithiasis improved using the combination of d-HypoMg with d-UCa>4mg/kg (OR 8.12, CI 1.92-34.18, P=0.004), but not with dUCa> 400mg/day. The current urinary calcium threshold of >400 mg/24-hour has a low sensitivity in detecting nephrolithiasis; our data suggest that sensitivity, specificity and positive predictive value could be improved including dUMg, dUCa/dUMg ratio and the combination of d-HypoMg with d-UCa>4mg/kg in the stone risk evaluation.