Recently, Abate et al. [1] reported that patients with recurrent uric acid stones manifest clinical and metabolic abnormalities consistent with the metabolic syndrome. As the latter is often predictive of the development of diabetes mellitus type 2, one may hypothesize that diabetic patients may be at particular risk of developing uric acid nephrolithiasis. However, to the best of our knowledge, few studies have evaluated the prevalence of uric acid nephrolithiasis among the diabetic population and stone composition in diabetic stone formers. Meydan and co-workers [2] recently reported that 21% of diabetic patients were affected by urolithiasis as compared with only 8% in the non-diabetic population, but the chemical type of stones was not given. Pak et al. [3] reported that 33.9% of 59 stone-forming patients with type 2 diabetes had uric acid stones, as compared with only 6.2% among non-diabetic stone formers. Taking advantage of the large number of calculi analysed in our laboratory (more than 43 000 by June 2004), we compared the distribution of the main components of stones from 631 diabetic patients (438 males, 193 females; type 2 diabetes in 84%) and from 4087 non-diabetic stone formers (2883 males, 1254 females) matched for age [59.8±12.4 (SD) vs 59.7±11.9 years] and gender distribution (sex ratio 2.26 in both groups). The mean body mass index was higher in diabetic than in non-diabetic stone formers (28.8±6.3 vs 25.6±4.4 kg/m2; P<0.0001).
Stone composition was determined quantitatively using Fourier transform infrared spectroscopy. The distribution of the main components of stones in males and females in the two groups (diabetics and non-diabetics) is shown in Table 1. Calcium oxalate was most prevalent as the main component of stones in both genders, but to a lesser extent in women than in men and in non-diabetic than in diabetic stone formers. No significant difference was observed for calcium phosphates (CaP) or magnesium ammonium phosphate (MAP) between the two groups. In contrast, uric acid was found as the main component of stones in a significantly higher proportion of diabetic than non-diabetic patients (28.5 vs 13.0%; P<0.0001), the difference being more marked in females (36.8 vs 9.7%) than in males (24.9 vs 14.7%). When taking into consideration calculi containing any proportion of uric acid (i.e. pure uric acid or mixed uric acid/calcium stones), the prevalence of uric acid-containing calculi was even higher in diabetic patients, at 35.4% vs 16.8% (P<0.0001) (40.3% vs 12.3% in women; 33.3% vs 18.8% in men). Pure uric acid stones represented 41.8% and 34.8% of uric acid-containing stones in diabetic and non-diabetic stone formers, respectively (not significant). There was no significant difference in stone composition between type 1 and type 2 diabetes, with the proportion of uric acid stones being 31.6% and 28.1%, respectively (Table 2). Overall, the proportion of uric acid stones was 2.2 times higher in diabetic than in non-diabetic stone formers, but the difference was strikingly more marked in women than in men with a ratio of 3.8 vs 1.7 (P = 0.003).
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Conflict of interest statement. None declared.
1 Biochemistry A Laboratory2 Nephrology Department Necker Hospital Paris France Email: michel.daudon{at}nck.ap-hop-paris.fr
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