Clin Chem Lab Med 2012;50(1):147–151 © 2011 by Walter de Gruyter • Berlin • Boston. DOI 10.1515/CCLM.2011.880 Diurnal variability of total calcium during normal sleep and after an acute shift of sleep Peter Ridefelt 1, *, John Axelsson 2 and Anders Larsson 1 1 Section of Clinical Chemistry, Department of Medical Sciences, Uppsala University, Uppsala, Sweden 2 Section of Psychology and Osher Center for Integrative Medicine, Department of Clinical Neurosciences, Karolinska Institute, Stockholm, Sweden Abstract Background: Serum total calcium is becoming a widely used test when screening for hyperparathyroidism (HPT) and other causes of hypercalcemia, even if serum calcium is tightly regu- lated in the body it is unclear whether the reference values are correct for tests taken at different times of the day or for indi- viduals with altered sleep patterns. Thus, the aim was to inves- tigate how timing of testing and sleep affects serum calcium. Methods: The diurnal variation of total calcium in serum dur- ing night-time and day-time conditions was studied in seven healthy volunteers. Serum samples for calcium measurements were collected every hour (48 samples per individual) to eval- uate the effect of sampling times, sleep and food intake on the test results. Results: The median intra-individual coefficients of variations for calcium were 3.3% during night-time sleep and 2.8% during day-time sleep conditions. There was a clear diurnal variation in serum calcium, with a trough at 08.00 h in the morning after night-time sleep and a difference of approximately 0.07 mmol/L between trough and peak. Calcium was lower around the end of the sleep periods, for both night-time and day-time sleep. Food intake did not affect calcium concentrations. Conclusions: Evaluation of serum calcium results should take diurnal variation into account and allow slightly higher calcium values in the afternoon in comparison with samples collected in the morning. Keywords: calcium; circadian variation; human; laboratory test. Introduction Calcium has several important extracellular and intracellu- lar functions (1, 2). The extracellular calcium concentration is maintained within a narrow interval by parathyroid hor- mone (PTH) and 1,25-dihydroxyvitamin D. Serum calcium has a low within-subject coefficient of variation of 1.9% (3). Serum calcium is used to screen for, diagnose and monitor a range of conditions relating to endocrine organs, bone and kidneys. Hypercalcemia is a frequent disorder with a prev- alence exceeding 1% of the adult population, particularly affecting women and increasing with age (4). The physical manifestations of hypercalcemia are often uncharacteristic and the diagnosis mostly relies on laboratory tests. The intro- duction of automated methods for serum calcium determina- tion has increased the use of calcium assays dramatically and this has made sporadic primary hyperparathyroidism (HPT) a frequently diagnosed disorder (5). Serum calcium, with or without albumin modification, is the most widely used test for screening for HPT and other causes of hypercalcemia. In clinical practice, the diagnosis of HPT is usually based on measurements of serum calcium, phosphorus and parathyroid hormone (PTH) (6). In Scandinavia, the upper limit for the reference interval for serum-calcium used to be 2.60 mmol/L (10.4 mg/dL) (7, 8). The Nordic Reference Interval Project (NORIP) (9) showed in a large group of healthy individuals that this was too high and the upper limit of the reference interval was calculated to be 2.51 (90% confidence interval 2.50–2.52). This finding was confirmed by patient data from several of the participat- ing laboratories. Accordingly, most Scandinavian laboratories lowered their upper reference interval limits in response to the findings of NORIP. Considering the narrow interval for serum calcium the shift proposed by NORIP is surprisingly large. There may be sev- eral pre-analytical causes for the differences in reference inter- vals (10). One of these could be the sampling time if there is a diurnal variation. Both PTH and 1,25-dihydroxyvitamin D has been shown to have a diurnal variation (11, 12). As these hormones are the main regulators of extracellular calcium it seems likely that also calcium could exhibit a diurnal variabi- lity. There are a number of studies showing a diurnal variation for total calcium (13–16). However, there are also studies that failed to show any diurnal variation (17, 18). Several of the studies reporting diurnal variation report low values during late night/early morning hours. However, the number of sam- ples per 24 h period has often been limited, making judgment of the details of the circadian rhythm difficult (13, 14, 16). The aim of this study was to study the 24 h variation of serum calcium in a group of healthy individuals with normal night-time sleep as well as during acutely displaced sleep and with standardized meals. The study subjects were sampled every hour to provide data for a better resolution of the diur- nal variation of calcium. Frequent sampling from individuals *Corresponding author: Peter Ridefelt, Department of Medical Sciences, Uppsala University, 751 85 Uppsala, Sweden Phone: +46-18-6110000, Fax: +46-18-552562, E-mail: peter.ridefelt@akademiska.se Received July 15, 2011; accepted September 13, 2011; previously published online October 8, 2011 Brought to you by | Karolinska Institute Authenticated Download Date | 5/31/15 12:03 AM