Abstract Introduction Calcium load test (CLT) has been developed by Pak et al. in 1974 to better discriminate hypercalciuria. Absorptive hypercalciuria (AH) is defined by an increase of the difference between urinary calcium/creatinine ratio (ΔUCa/Cr) of more than 0.5mmol/mmol with a 4-hour (4h) calcium load test. In clinical practice and more recent studies, CLT is a 2-hour (2h) test. We hypothesized that the 4h timepoint is more efficient in AH diagnosis. Methods We report a single-center, retrospective study including all patients who underwent CLT because of hypercalciuria or hyperparathyroidism. After a 3 days low-calcium diet and a 12-hour fast, 24-hour urines were collected. Blood and urinary samples were done at arrival and after 2h and 4h of oral ingestion of one gram of calcium. AH was diagnosed by ΔUCa/Cr between baseline and 2h or 4h of more than 0.05 mmol/mmol. Results We included 328 patients. Baseline UCa/Cr ratio was 0.3 ± 0.2mmol/mmol and increased significantly after 2h and 4h (0.6 ± 0.3 and 0.8 ± 0.4mmol/mmol, p<0.001). ΔUCa/Cr was significantly different between baseline and 2h or 4h (0.2 ± 0.2 versus 0.5 ± 0.4, p<0.001). AH was diagnosed in 35 (10.7%) patients after 2h, 84 (25.6%) more were diagnosed at 4h (p<0.001). Conclusions The 4h CLT improves the diagnosis of AH with more than fifty percent of AH diagnosed within 4h of calcium ingestion. It seems that there are AH of later diagnosis with a similar clinical and biological profile depending on enteral absorption.
