BJUI. Seasonal variation in the acute presentation of urinary calculi over 8 years in Auckland, New Zealand

. JOURNAL COMPILATION 2009 BJU INTERNATIONAL Upper Urinary Tract SEASONAL VARIATION IN ACUTE URINARY CALCULI IN NEW ZEALAND LO et al. BJUI BJU INTERNATIONAL Seasonal variation in the acute presentation of urinary calculi over 8 years in Auckland, New Zealand Sum Sum Lo, Richard Johnston, Ahmed Al Sameraaii, Patricia A. Metcalf*, Michael L. Rice and Jonathan G. Masters Department of Urology, Auckland City Hospital, and *Department of Statistics, University of Auckland, Auckland, New Zealand Accepted for publication 13 August 2009 Study Type Symptom prevalence (retrospective cohort) Level of Evidence 2b OBJECTIVE To determine the incidence of acute presentation of urinary calculi (UC) in Auckland, New Zealand, during the period 1999 2007, and whether there was any significant seasonal variation. PATIENTS AND METHODS The details of all UC within the population presenting acutely to public hospitals in Auckland between 1999 and 2007 were collected using clinical coding searches International Classification of Disease 10th revision (Australian Modification) N132 and N20. Climatic variables for the Auckland region were obtained from the National Institute of Water and Atmospheric Research, New Zealand. The mean atmospheric temperature, hours of sunshine and humidity data were calculated monthly for this period. RESULTS During the study there were 7668 acute presentations of UC in the Auckland region. A Poisson regression model showed that the number of presentations was significantly related to temperature (P < 0.001) and hours of sunshine (P = 0.004) but not humidity (P = 0.14). For each degree increase in temperature the number of presentations increased by 2.8% (95% confidence interval 1.3 4.3%). For each 1-h increase in sunshine, the number of presentations increased by 0.2% (0.06 0.33)%. CONCLUSION The acute presentation of UC in Auckland, New Zealand, varies significantly with temperature and hours of sunshine. Humidity was not a significant factor. KEYWORDS acute, calculi, seasonal variation INTRODUCTION Demographic variations in the acute presentation of urinary calculi (UC) have been reported for many years; rates of hospitalization for calculi vary considerably, not only between countries [1,2] with higher rates in industrialized nations [3], but also regionally within countries [2,4,5]. Seasonal variation in stone presentation is well described[2 7]. The occurrence of UC is associated with increased average temperature [2,8 10] and greater sun exposure [11]. Increased exposure to sunlight has been shown to cause hypercalciuria [11], and geographical or regional variability in the prevalence of UC [6,12 14], and increased UC rates, have been reported during the warmer months [10,15 18]. Furthermore, urinary calcium excretion has been shown to be higher in summer than in winter [17,19,20], causing Robertson et al. [17] to speculate that as the seasonal changes in urinary calcium follow the monthly pattern of sunshine hours, vitamin D might cause the summer peak of calcium excretion through its effect on the intestinal absorption of calcium. Whether increased urinary calcium level accelerates the mechanism of stone growth to an extent that it cause symptoms in the summer is only an assumption. Previous epidemiological investigations of UC have shown that the disease is more common in males than in females [2,7,21]. Europeans are more affected than Asians [22] or Africans [12]. The disease is uncommon before the age of 20 years and the incidence increases between 20 and 30 years and then remains steady until the age of 70 years [23]. It has been estimated that for age 30 70 years, the annual incidence of first urinary calculus varies between 100 and 300 per 100 000 in men and 50 100/100 000 in women [23]. The lifetime prevalence of UC is 12% in men and 7% in women in the USA [5] and for the rest of the western world is 6 9% in men and 3 4% in women [23]. The primary aim of the present study was to determine if there was any seasonal variation in the acute presentation of UC in the population of Auckland, New Zealand (NZ) over 1999 2007. Secondary aims were to evaluate if this seasonal variation was consistent between all ages, gender and ethnic subgroups. PATIENTS AND METHODS The details of all patients with UC presenting acutely to public hospitals in Auckland between 1999 and 2007 were collected using clinical coding searches for International 96 JOURNAL COMPILATION 2009 BJU INTERNATIONAL 106, 96 101 doi:10.1111/j.1464-410x.2009.09012.x

SEASONAL VARIATION IN ACUTE URINARY CALCULI IN NEW ZEALAND Code N132 N136 N200 N201 N202 N209 Code description Hydronephrosis with renal and ureteric calculus obstruction Pyonephrosis Calculus of kidney Calculus of ureter Calculus of kidney with calculus of ureter Urinary calculus unspecified TABLE 1 Details of clinical coding in ICD-10-AM TABLE 2 Presentation of acute UC compared to the Auckland population by age and ethnic groups, for 2006, and the effect of temperature and ns Group Auckland population of UC n /100 000 population (% total) 1 C % change for increase of 1-h mean ns Gender Male 5385 (70.3) 3.0 0.20 Female 2275 (29.7) 2.1 0.20 Age group, years 15 44 603 429 518 85.8 3517 3.1 NS 45 64 293 265 450 153.4 3005 2.7 0.22 65 129 864 189 145.5 1146 NS NS Ethnic groups* European 754 749 746 98.8 4408 (57.5) 2.5 0.22 Maori + Pacific 273 309 220 80.5 1492 (19.5) 2.3 NS Asian 151 605 191 125.9 1113 (14.5) 3.7 NS Other 655 (8.5) NS NS NS, not significant; *Population divided into different ethnic groups include people aged <15 years. At the time of presentation patients completed an admission information sheet which collects patient demographics, including age, gender and ethnic group affiliation. Discussion with urologists working in private confirms that, in Auckland, private hospitals do not admit patients with acute renal or ureteric calculi and few calculi are treated acutely in private without first being initially investigated through the public hospital system. Climatic variables for the Auckland region from the National Institute of Water and Atmospheric Research NZ for the same period were obtained. There are 54 observation stations distributed around the Auckland region. The mean atmospheric temperature, numbers of hours of sunshine (ns) and humidity data were calculated from these stations each month [24]. Population data were obtained from Statistics NZ; a census is conducted every five years in NZ (most recent 2001 and 2006). Statistics NZ also calculates the estimated population and population subgroups each year [25]. The data were analysed statistically [26] and all tests were two-tailed with statistical significance set at P < 0.05. The variables were evaluated using Pearson s correlation coefficients and Poisson regression models. As sunshine and temperature were not independent, they were not added into the Poisson regression model together. FIG. 1. The variation in the total monthly rates for UC across the 8 years of the study. 140 130 120 110 100 90 80 70 60 50 40 01JAN98 01JAN00 01JAN02 Date 01JAN04 01JAN06 01JAN08 Classifications of Diagnosis 10th revision, Australian Modification (ICD-10-AM) codes N132 and N20 (Table 1). Following emergency department protocols, calculi were confirmed radiologically with CT in the vast majority, and by ultrasonography, plain abdominal X-ray or IVU, particularly in the earlier years when CT was not so readily available. Patients needed a FIG. 2. The seasonal trends in the rates showed a peak in February and March each year. Temperature, C 760 25 740 720 700 20 680 660 640 15 620 600 10 580 560 540 5 J A S O N D J F M A M J Temperature No. of presentations confirmed diagnosis of a stone to meet the coding criteria above. Calculi in the bladder were excluded from this study. Data on stone composition were not available. When a patient was readmitted within 7 days of the day of discharge from the first presentation, it was regarded as the same stone. Only the data from the first presentation was analysed here. RESULTS The greater Auckland region includes a population of >1.3 million, which represents nearly 37% of the total population of NZ (Table 2). Auckland has a mixed population of different ethnic groups, including Pacific, Asian, European, Maori and Middle Eastern groups. Over the 8-year study period 7668 cases of acute UC presented to public hospitals in Auckland. During the study the number of acute UC presented to public hospitals increased from 93/100 000 in 2001 to 113/100 000 in 2006, a 21.5% increase. The mean (range) age at presentation was 47.4 (15 94) years. The variation in the total monthly acute UC presentation rates over the 8 years of the study is shown in Fig. 1. The seasonal trends in the presentation rates showed a peak in February and March each year, as illustrated in Fig. 2. JOURNAL COMPILATION 2009 BJU INTERNATIONAL 97

LO ET AL. The correlation coefficient between the number of presentations () and the ns in that month was r = 0.37 (P = 0.007), but when the correlation coefficient was calculated between and the ns for the previous month, the correlation coefficient increased to r = 0.59 (P < 0.001). Similarly, the correlation coefficient between the and the ns in that month was 0.35 (P < 0.001), increasing to 0.40 (P < 0.001) when correlating with ns in the previous month. The relationships between the of acute UC and temperature in the previous month (Fig. 3), and ns in the previous month (Fig. 4) are also shown. However, humidity had no significant correlation with the (P = 0.14). Poisson regression models were constructed that used the variables temperature and ns, showing that the was significantly related to temperature (P < 0.001) and ns (P = 0.004). The model also showed that for each 1 C increase in temperature the increased by 2.8% (95% CI 1.3 4.3%). For each 1-h increase in ns per month, the increased by 0.2% (0.06 0.33)%. The acute UC presentation rates also had significant associations between temperature and ns across gender, age (15 44, 45 64 and >65 years) and ethnic (European, Maori, Pacific and Asian) groups (Table 2). Over the study period 5385 (70.3%) male and 2275 (29.7%) female patients presented (with eight in which gender was not specified and these were not included in the gender analysis). Males were more affected by an increase in temperature than were females (r = 0.39, P = 0.004 vs r = 0.21, P = 0.018, respectively) and were also more affected by increased ns (r = 0.34, P < 0.001 vs r = 0.22, P = 0.124, respectively). For each 1 C increase in temperature, the number of males and females presenting with acute UC increased by 3.0% (1.34 4.66)% and 2.1% (0.39 3.87)%, respectively (Table 2). For each 1-h increase in ns both male and female presentation rates increased by 0.2% (Table 2). There were 3517, 3005 and 1146 presentations from the 15 44-, 45 64- and >65-year age groups (Table 2). The presentations from those aged <65 were affected by temperature, but presentations in the oldest group were not affected by temperature increase or ns. Acute UC in the 15 44-year age group was related to temperature (P < 0.001) but not to ns (P = 0.07). A Poisson model showed that higher temperature increased the (P < 0.001) and for each 1 C increase in temperature the was predicted to increase by 3.06% (1.47 4.68)% (Table 2). The rate of the 45 64-year age group was also affected by both temperature (P = 0.006) and ns (P = 0.022). For each 1 C increase in temperature the is expected to increase by 2.69% (0.85 4.56)% (Table 2). However, for every 1-h increase in ns the was predicted to increase by 0.22% (0.04 0.40)% (Table 2). Rates of presentation for Europeans showed strong seasonal variation and were closely related to temperature (r = 0.33, P < 0.001) and ns (r = 0.40, P = 0.008). The Poisson model showed that temperature was related to the (P < 0.001) and that for each 1 C increase in temperature the increased by 2.55% (1.09 4.03)% (Table 2). The model also showed that ns was related to the (P < 0.002) and that for each 1-h increase in ns the increased by 0.22% (0.08 0.36)% (Table 2). The by Maori and Pacific peoples were affected by change in temperature (r = 0.21, P = 0.04), but not ns (r = 0.12, P = 0.39). The Poisson model showed that temperature was related to the (P = 0.036) and that each 1 C increase in temperature increased the by 2.3% (0.16 4.51)%. Similarly, the among Asians was only affected by temperature (r = 0.25, P = 0.013) and not by ns (P = 0.11). Unexpectedly, Asians had the highest per 100 000 amongst all the ethnic groups in 2006 (Table 2). DISCUSSION This study shows a clear seasonal variation in Auckland in the for acute UC. The seasonal trends in the presentation rates showed a peak in February and March, following the warmest months, January and February, in Auckland. February and March correspond with the peak in 25-hydroxyvitamin D levels found in New Zealanders, and August and September have the lowest levels [27]. The fewest cases presented in August and September, which were the corresponding cooler months. Studies with variable evidence of seasonal variation of acute UC have emerged over the last four decades. Boscolo-Berto et al. [28] reported an association between the onset of renal colic and exposure to hot and dry weather. Fujita et al. [8] reported that FIG. 3. Relationship between the of acute UC and temperature ( C). 140 120 100 80 60 40 10 r = 0.40, P < 0.001 12 14 16 18 20 22 Temperature, C, in previous month FIG. 4. Relationship between the acute UC and ns. 140 120 100 80 60 40 r = 0.59, P < 0.001 120 140 160 180 200 220 240 Sunshine (hours) previous month increased temperature was associated with a higher incidence of renal colic. People who reside in the more southern location in the USA have a greater risk of having urinary calculi, possibly due to greater exposure to both higher temperature and sunlight [4]. This was true for both men and women [4], which is consistent with the findings of the present study. Chen et al. [15], investigating the seasonal variation in UC in Taiwan, concluded that only ambient temperature had a consistent association with acute presentation of UC. The single Australasian review concluded there were more cases of renal colic over the hottest months of the year in Australia [21]. The effect of increased exposure to sunlight in the summer months causes increased production of 25-hydroxycholecalciferol in the skin which, after conversion to 1,25 dihydroxy-vitamin D by the kidneys, enhances the intestinal absorption and urinary excretion of calcium [29,30]. While increased dietary intake of calcium is not associated with a greater prevalence of renal calculi [31,32], lower calcium intake levels can stimulate 1,25-vitamin D-mediated absorption and increase bone resorption with increased urinary calcium excretion [33]. 98 JOURNAL COMPILATION 2009 BJU INTERNATIONAL

SEASONAL VARIATION IN ACUTE URINARY CALCULI IN NEW ZEALAND Although this explains the pathogenesis of stone formation, it is yet to be proven if the same factors could in some way accelerate stone dislodgement and passage. Chronic dehydration had been identified as a classic cause of UC for many years [34,35]. Chronic dehydration during hot weather, especially among outdoor workers, has been shown to increase the risk of developing UC [36]. Dehydration and inadequate fluid intake increase the urinary oxalate and calcium concentration, which contributes to the increase in UC [31,37]. Increasing fluid intake had also been shown to reduce the incidence of UC [31]. This helps to explain the stone clinic effect which has achieved a reduction from 0.7 to 0.01 episodes/patient-year in stone formation [37]. Dehydration is more likely to occur in the summer than in winter months, as there is an increase in perspiration, especially when there is an increase in the level of outdoor activity. Urinary crystal growth in vivo consists of deposition from saturated urine or of precipitation from a chemical reaction in urine, followed by deposition on the nucleus so formed [38]. Previous studies have found that whites are more affected than blacks [39], and Asians [22], with the prevalence in Hispanic and Asian men intermediate between that of whites and blacks [12]. The difference in renal absorption rates of dietary calcium and oxalate among South African Blacks had been postulated, as they seem to be relatively immune to urinary calculi compared to white Caucasians [7]. Furthermore, Australian Aborigines did not develop kidney stone disease, whilst the white population did [9]. In NZ, vitamin D levels have been shown to be higher in late summer than in early spring and there are regional differences, with levels higher in people living in the north [40 42]. Ethnic differences have also been observed with lower levels of vitamin D in Maori and Pacific children [40] and adults [41 43] than in Europeans, presumably due to their darker skin. Greater amounts of melanin in dark skin act as a natural sunscreen, reducing synthesis of vitamin D from 7-dehydrocholesterol [44]. These features might explain the lack of association in the Maori and Pacific People. The reason for the higher rate in Asians than in Caucasians, shown in Table 2, is unknown. Hospital discharges by gender from patient surveys showed that the frequency of acute UC discharges among men and women has almost equalised in 2001 in the USA [45]. Our study showed a strong bias for calculi in men (70.3% vs 29.7%), but the reason for this difference is not clear. Over the study period the male to female ratio in Auckland remained relatively stable at 49:51 in favour of females. The difference between the USA and NZ is unlikely to be due to metabolic syndrome, as Maori and Pacific people have a higher prevalence, but they were not affected by season. Scales et al. [45] showed that over a 5-year period in the USA, the male-to-female ratio had almost equalised from 1.7:1 to 1.3:1, which is 23.5% closer. This was attributed to morbid obesity, which is more common in females [46]. Women had a greater increase in obesity from 1960 to 2002, which coincided with an increase in UC in women [45]. It was calculated that obese men have a 1.27 times greater risk of forming UC than normalweight men, whereas obese women have a 2.09 times greater risk than normal-weight women [45]. The increase of acute UC over the 8 years in Auckland might be associated with the increase in the prevalence of diabetes and obesity in the NZ population [46,47]. Furthermore, diabetes mellitus has been shown to be associated with UC [35]. It has been predicted that climate changes will result in an increased prevalence of urinary calculi in the more northern states of the USA [5]. Between 1976 1980 and 1988 1994, there has been a 0.8% increase in the prevalence of UC in the USA, as well as an increase of 0.5 C in the mean annual temperature [5]. The 8 C difference between the South-east and the North-west of the USA might explain the former having twice the prevalence of calculi than the latter [5]. Globally, the average surface temperature has risen by 0.6 C over the 20th century [48]. There was an increase of 0.9 C over the last century in NZ [49]. In Auckland, our study showed that for each 1 C increase in temperature, the increased by 2.9%; this temperature increase could in part explain the increase in overall numbers of patients with UC during the study period. Our aim was to examine the relationship between acute UC and environmental factors. However, we actually examined the onset of acute renal colic, i.e. only symptomatic calculi were included in the study. Very few studies assess the formation rate of new calculi, due in part to the inherent difficulties associated with this, and because acute renal colic necessitates treatment, and the associated costs to patient and health care are generated from this point. Although logically stone formation and stone passage must in some way be linked, the actual temporal association is poorly understood. There might be factors, e.g. hypercalciuria and concentrated urine, which could accelerate stone dislodgement and passage. It was not the aim of this study to determine all risk factors for stone formation. However, hot weather and increased exposure to sunlight increase the acute presentation of renal colic. Further study is still required to explore what the other factors are and how they accelerate stone formation, dislodgement and passage. This will be the target of further research. The key application of our findings is in healthcare planning and stone prevention strategies to lower the overall costs to the healthcare sector. It has been projected that in the USA alone, cost increases associated with the rise in UC could be US $0.9 1.3 billion (NZ$ 1.71 2.47 billion) annually [6]. We suggest that leading up to and over the summer months, when the UC presentation risk is highest, awareness campaigns should be launched. These campaigns would highlight the correlation between temperature, hours of sunshine and UC occurrence, as well as encourage the public to increase fluid intake and reduce the hours of sun exposure. This information can also help with manpower and resource planning in the hospital sector. In conclusion, studies from different geographical locations, different cultures and different ambient temperatures have shown an association between rates of UC, temperature and ns. This study shows that seasonal variation exists and temperature increases affect all ethnic groups. Asians, Maori and Pacific People were not affected by ns. Young male Europeans were most affected by temperature rise and prolonged sun exposure in the summer months. CONFLICT OF INTEREST None declared. REFERENCES 1 Juuti M, Heinonen O. Incidence of urolithiasis leading to hospitalisation in JOURNAL COMPILATION 2009 BJU INTERNATIONAL 99

LO ET AL. Finland. Acta Med Scand 1979; 206: 397 403 2 Chauhan V, Eskin B, Allegra J, Conchrane D. Effect of season, age, and gender on renal colic incidence. Am J Emerg Med 2004; 22: 560 3 3 Andersen D. Historical and geographical differences in the pattern of incidence of urinary stones considered in relation to possible aetiological factors. In Proceedings of Renal Stone Research Symposium; 1968; Leeds. London: J & A Churchill Ltd, 1968: 7 32 4 Soucie J, Thun M, Coates R, McClellan W, Austin H. Demographic and geographic variability of kidney stones in the United States. Kidney Int 1994; 46: 893 9 5 Brikowski T, Lotan Y, Pearle M. Climaterelated increase in the prevalence of urolithiasis in the United States. Proc Natl Acad Sci USA 2008; 105: 9841 6 6 Barker D, Donnan S. Regional variations in the incidence of upper urinary tract stones in England and Wales. BMJ 1978; 1: 67 70 7 Rabanal L. Clinical epidemiology of urolithiasis in tropical areas. Available at: http://wwwuninetedu/cin2003/conf/ lreyes/lreyeshtml. Accessed October 2008 8 Fujita K. Epidemiology of urinary stone colic. Eur Urol 1979; 5: 26 8 9 Bateson E. Do Australian aborigines suffer from renal tract calculi? Aust NZ J Med 1977; 7: 380 1 10 Robertson W, Peacock M, Marshall R, Speed R, Nordin B. Seasonal variations in the composition of urine in relation to calcium stone-formation. Clin Sci Mol Med 1975; 49: 597 602 11 Parry E, Lister I. Sunlight and hypercalciuria. Lancet 1975; 305: 1063 5 12 Soucie J, Coates R, McClellan W, Austin H, Thun M. Relations between geographic variability in kidney stones prevalence and risk factors for stones. Am J Epidemiol 1996; 143: 487 95 13 Curhan G, Rimm E, Willett W, Stampfer M. Regional variation in nephrolithiasis incidence and prevalence among Unites States men. J Urol 1994; 151: 838 41 14 Pierce L, Bloom B. Observations on urolithiasis among American troops in a desert area. J Urol 1945; 54: 466 70 15 Chen Y-K, Line H-C, Chen C-S, Yeh S-D. Seasonal variations in urinary calculi attacks and their association with climate: a population based study. J Urol 2008; 179: 564 9 16 Prince C, Scardino PL, Wolan C. The effect of temperatures, humidity and dehydration on the formation of renal calculi. J Urol 1956; 75: 209 14 17 Robertson W, Gallagher J, Marshall D, Peacock M, Nordin B. Seasonal variation in urinary excretion of calcium. BMJ 1974; 4: 436 7 18 Frank M, devries A, Lazebnik J, Kochwa S. Epidemiologic investigation of urolithiasis in Israel. J Urol 1959; 81: 497 504 19 Morgan D, Rivlin R, Davis R. Seasonal changes in the urinary excretion of calcium. Am J Clin Nutr 1972; 25: 652 4 20 McCance R, Widdowson E. Seasonal and annual changes in the calcium metabolism in man. J Physiol 1943; 102: 42 9 21 Baker P, Coyle P, Bais R, Rofe A. Influence of season, age and sex on renal stone formation in South Australia. Med J Aust 1993; 159: 390 2 22 Hiatt R, Dales L, Friedman G, Hunkeler E. Frequency of urolithiasis in a prepaid medical care program. Am J Epidemiol 1982; 115: 255 65 23 Hughes P. Kidney stones epidemiology. Nephrology 2007; 12: 26 30 24 NIWA. The National Climate Database. Available at: http://cliflo.niwa.co.nz/. Accessed September 2008 25 Statistics New Zealand. 2006 Census: Final Counts Tables. Available at: http:// www.stats.govt.nz/census/2006-censusdata/2006-census-reports/final-countstables/auckland-region.aspx. Accessed September 2008 26 SAS Institute Inc. SAS/STAT User s Guide Version 11.2. Cary NC: SAS Institute Inc., 2004 27 Bolland M, Chiu W, Davidson J et al. The effects of seasonal variation of 25- hydroxyvitamin D on diagnosis of vitamin D insufficiency. N Z Med J 2008; 121: 63 74 28 Boscolo-Berto R, Fabrizio D, Abate A, Arandjelovic G, Tosato F, Bassi P. Do weather conditions influence the onset of renal colic? A novel approach to analysis. Urol Int 2008; 80: 19 25 29 Lips P. Vitamin D deficiency and secondary hyperparathyroidism in the elderly: consequences for bone loss and fractures and therapeutic implications. Endocr Rev 2001; 22: 477 501 30 Gallagher J, Riggs B, Eisman J, Hamstra A, Arnaud S, Deluca H. Intestinal calcium absorption and serum vitamin D metabolites in normal subjects and osteoporotic patients: effect of age and dietary calcium. J Clin Invest 1979; 64: 729 36 31 Curhan G, Willett W, Rimm E, Stampfer M. A prospective study of dietary calcium and other nutrients and the risk of symptomatic kidney stones. N Engl J Med 1993; 328: 833 8 32 Sowers M, Jannausch M, Wood C, Pope S, Lachance L, Peterson B. Prevalence of renal stones in a population-based study with dietary calcium, oxalate, and medication exposures. Am J Epidemiol 1998; 147: 914 20 33 Strazzullo P, Mancini M. Hypertension, calcium metabolism, and nephrolithiasis. Am J Med Sci 1994; 307: 102 5 34 Olapade-Olaopa E, Agunloye A, Ogunlana D, Owoaje E, Marinho T. Chronic dehydration and symptomatic upper urinary tract stones in young adults in Ibadan, Nigeria. West Afr J Med 2004; 23: 146 50 35 Embon O, Rose G, Rosenbaum T. Chronic dehydration stone disease. Br J Urol 1990; 66: 257 62 36 Pin N, Ling N, Siang L. Dehydration from outdoor work and urinary stones in a tropical environment. Occup Med (Lond) 1992; 42: 30 2 37 Goldfarb S. Diet and nephrolithiasis. Annu Rev Med 1994; 45: 235 43 38 Lonsdale K. Human stones. Science 1968; 159: 1199 207 39 Sarmina I, Spirnak J, Resnick M. Urinary lithiasis in the black population: an epidemiological study and review of the literature. J Urol 1987; 138: 14 7 40 Rockell J, Green T, Skeaff C et al. Season and ethnicity are determinants of serum 25-hydroxyvitamin D concentrations in New Zealand children aged 5 14y. J Nutr 2005; 135: 2602 8 41 Rockell J, Skeaff C, Williams S, Green T. Serum 25-hydroxyvitamin D concentrations of New Zealanders aged 15 years and older. Osteoporos Int 2006; 17: 1382 9 42 Rockell J, Skeaff C, Venn B, Wiliams S, Green T. Vitamin D insufficiency in New Zealanders during the winter is associated with higher parathyroid hormone concentrations: implications for bone health. N Z Med J 2008; 121: 75 84 43 Scragg R, Holdaway I, Singh V, Metcalf P, Baker J, Dryson E. Serum 25- hydroxyvitamin D. 3 is related to physical activity and ethnicity but not obesity in a 100 JOURNAL COMPILATION 2009 BJU INTERNATIONAL

SEASONAL VARIATION IN ACUTE URINARY CALCULI IN NEW ZEALAND multicultural workforce. Aust NZ J Med 1995; 25: 218 23 44 Holick M. Vitamin D: importance in the prevention of cancers, type 1 diabetes, heart disease, and osteoporosis. Am J Clin Nutr 2004; 79: 362 71 45 Scales C, Curtis L, Norris R et al. Changing gender prevalence of stone disease. J Urol 2007; 177: 979 82 46 Duffey B, Pedro R, Kriedberg C et al. Lithogenic risk factors in the morbidly obese population. J Urol 2008; 179: 1401 6 47 Taylor E, Stampfer M, Curhan G. Diabetes mellitus and the risk of nephrolithiasis. Kidney Int 2005; 68: 1230 5 48 Bi P, Parton K. Effect of climate change on Australian rural and remote regions: what do we know and what do we need to know? Aust J Rural Health 2008; 16: 2 4 49 New Zealand s Climate Change Solutions. Available at: http:// www.climatechange.govt.nz/science/ index.html. Accessed 22 April 2009 Correspondence: Sum Sum Lo, Apt701, 47 Hobson Street, Auckland City, 1010, New Zealand. e-mail: losumsum@gmail.com Abbreviations: UC, urinary calculi; NZ, New Zealand; ICD-10-AM, International Statistical Classification of Diseases and Related Health Problems, Tenth Revision, Australian Modification;, number of presentations for UC; ns, number of sunshine hours. JOURNAL COMPILATION 2009 BJU INTERNATIONAL 101