Pediatr Nephrol (2004) 19:1153–1155 DOI 10.1007/s00467-004-1568-z ORIGINAL ARTICLE Michelle Stuart Hilgenfeld · Stephen Simon · Douglas Blowey · Wendy Richmond · Uri S. Alon Lack of seasonal variations in urinary calcium/creatinine ratio in school-age children Received: 1 April 2004 / Revised: 28 May 2004 / Accepted: 2 June 2004 / Published online: 7 August 2004  IPNA 2004 Abstract Seasonal variations in urinary calcium excretion have been observed in adults and are thought to be due to seasonal changes in sunlight exposure, which results in fluctuation of blood levels of 25-hydroxyvitamin D. The goal of the present study was to explore the possible effect of season on urinary calcium/creatinine ratio (UCa/Cr) in healthy school children. We studied 65 healthy Caucasian children aged 5.1–14.6 years (mean€SD 9.6€2.0, median 9.7 years). Random, non-fasting urine samples were col- lected from each participant between the 1st and 5th days of the month for 16 successive months and analyzed for UCa/Cr. In parallel, meteorological data, which includ- ed average monthly temperature and average monthly UV index, were collected for the whole study period. A mathematical model describing the monthly mean values of UCa/Cr as a sine function was used to determine the presence of seasonal variation. The observed maximum mean UCa/Cr, 0.092 mg/mg (0.260 mmol/mmol), oc- curred in the late summer months (August/September) and minimum mean UCa/Cr, 0.083 mg/mg (0.235 mmol/ mmol), occurred in mid-winter (January/February), re- flecting an infinitesimal and statistically insignificant change. Moreover, both values were well within the pre- viously established normal range for UCa/Cr of 0.20 mg/ mg (0.566 mmol/mmol). The pattern seen in the last 4 months of the study repeated that seen in the first 4 months. The mean temperature in July of the 1st year was 27.1€2.6, January 1.8€5.4, and July of the 2nd year 27.0€2.2C. The mean UV index in July of the 1st year (7.61€1.01) was not different from that in July of the 2nd year (7.73€0.86), and both were statistically much higher than that in January (1.37€0.44) ( P <0.001). Evidently, in spite of significant changes in seasonal meteorological indices of sunlight, no significant seasonal variations in UCa/Cr were observed in a cohort of healthy school children. Thus the current UCa/Cr value used to diagnose hypercalciuria does not require a seasonal adjustment. Keywords Hypercalciuria · Urinary calcium/creatinine ratio · 25-Hydroxyvitamin D · Season Introduction Calcium is an essential component of the human skeleton. A positive balance is necessary for the accretion of bone mass in children. Calcium balance is the end result of the difference between calcium intake and its losses. The components of the latter include the obligatory losses in sweat and feces and urinary calcium excretion. Calciuria is affected by short-term physiological effects, such as intake of calcium, sodium, potassium, protein, phospho- rus, and sucrose, and long-term physiological effects, such as age, gender, genetics, physical activity, calcium intake, and vitamin D status [1, 2, 3]. Hypercalciuria is a common pediatric problem known to be associated with hematuria, voiding dysfunction, ab- dominal pain, and urolithiasis [4, 5, 6, 7, 8]. Although the gold standard values used to diagnose hypercalciuria are based on 24-h urine collection, timed urine collections are difficult to obtain and are often incomplete [9]. In chil- dren, a random urine calcium/creatinine (Ca/Cr) ratio is easier to obtain and regarded as an excellent substitute for a timed urine collection [1, 7, 8, 10]. Reference values for Presented in part at the American Society of Nephrology Annual Meeting, San Diego, California, November 2003 M. Hilgenfeld · D. Blowey · W. Richmond · U. S. Alon Section of Nephrology, Children’s Mercy Hospital, Kansas City, Missouri, USA S. Simon Department of Medical Research, Children’s Mercy Hospital, Kansas City, Missouri, USA U. S. Alon ( ) ) Section of Nephrology, Children’s Mercy Hospital, 2401 Gillham Road, Kansas City, MO 64108, USA e-mail: ualon@cmh.edu Tel.: +1-816-2343010 Fax: +1-816-2343494