Urinary calcium excretion increases at night in healthy adults, suggesting that the timing of spot urine collections should be considered when interpreting calcium levels.
Spot urine calcium timing may affect clinical interpretation; leaves open whether nocturnal elevation alters diagnostic thresholds pending prospective studies.
Twenty-four-hour urinary calcium excretion is normally the equivalent of daily calcium intake, and varies between 200-300 mg/dL with a calcium/creatinine ratio of 0.07-0.15. In this study, we aimed to investigate the diurnal rhythm of calcium excretion in healthy individual. Forty subjects (30 male, 10 female) were involved into the study. The spot urine samples were taken at 08:00, 14:00, and 22:00 together with a 24-hour collection. Mean spot urinary calcium levels at 08:00, 14:00, and 22:00 were 12.39 +/- 8.19, 12.97 +/- 8.37, and 16.95 +/- 10.39 mg/dL, with calcium/creatinine ratios of 0.104 +/- 5.261, 0.119 +/- 7.85, and 0.133 +/- 8.17, respectively. Twenty-four-hour urinary calcium excretion was 12.74 +/- 7.31 mg/dL with a calcium/creatinine ratio of 0.111 +/- 5.41. The values at 08:00, 14:00, and of 24-hour collection were statistically similar (p > 0.05), but the nighttime values were significantly elevated (p < 0.05). In conclusion, calcium excretion is increased at night, and urinary calcium measurements should be interpreted accordingly.
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Topal et al. (2008) studied this question.
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