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Try out PMC Labs and tell us what you think. Learn More. Kidney stones are heterogeneous but often grouped together. The potential effects of patient demographics and calendar month season on stone composition are not widely appreciated. Although calcium oxalate stones were the most common type of stone overall, hydroxyapatite stones were the second most common before age 55 years, whereas uric acid stones were the second most common after age 55 years.
It is well known that calcium oxalate stones are the most common stone type. However, age and sex have a marked influence on the type of stone formed. The higher of stones submitted by women compared with men between the ages of 10 and 29 years old and the change in composition among the elderly favoring uric acid have not been widely appreciated.
These data also suggest increases in stone risk during the summer, although this is restricted to calcium oxalate and uric acid stones. The prevalence of kidney stone disease seems to be rising in the United States. The reasons for this trend are not entirely clear. Factors such as obesity, diabetes, and diet have all been implicated. Other less common stone compositions include uric acid UA , struvite ST; magnesium ammonium phosphate , and cystine Cy. The Mayo Clinic Metals Laboratory performs compositional analysis of close to 50, kidney stones per year by infrared spectroscopy on samples submitted from across the United States.
In this study, we examined the distribution of stone types to determine if reports regarding the distribution of stone composition applied to this relatively large cohort in Because this sample includes comparable s of men and women of diverse ages, we also examined the effects of demographics and calendar month season on stone composition. suggest that the aggregate percentages are similar to those ly reported. However, age and sex both influence the distribution of stone type in important ways. Finally, certain stone types are more commonly submitted in warm summer months CaOx and UA , whereas the others did not show this seasonal trend.
Kidney stone samples were predominantly referred to the Mayo Clinic Metals Laboratory for analysis by community hospitals representing community practices ranging from urologists to general medicine. It is likely but no data are available that most of stones submitted were referred from specialty practice. The laboratory did not receive information describing medications or whether stones were passed or collected at time of intervention.
All stones were analyzed in the Mayo Clinic Metals Laboratory using their standard operating procedure. Initially, each stone was weighed before a representative specimen approximately 1 mg was taken from all identifiable layers. The resulting spectrum was compared against a reference spectrum of all known kidney stone components, allowing for accurate analysis of complex crystal mixtures of each crystal type 4.
The percentage of constituents was determined by comparing the ratio of peak heights of the constituents within a given sample to the ratio of peak heights in a library of known quantities of mixed constituents. This is currently considered the gold standard method for routine clinical analysis of stone composition 5. Stones that were composed of medications, protein, or rare constituents e. Stone compositions of artifact, protein, or medications were shown in the graphs but excluded from the statistical analysis. For monthly trends, all values were normalized to a day month.
Each stone composition was compared with all other compositions e. Chi-squared tests were used to assess the effects of sex, month, and age group on stone type. For monthly trends, the one-sample chi-squared test was used with expected values adjusted for length of the month. Statistical analyses were performed using the SAS software, version 9. Stones were received from across the United States, including the northeast ; Thus, Of UA stones, In total, only 61 stones 0. Only one rare stone dihydroxyadenine was received during this calendar year.
Samples containing only protein were classified as such 0. Men and women were both much more likely to submit stones between the ages of 20 and 79 years Table 2. Stone type varied by sex Figure 1. More women than men were likely to submit an HA General trends by age were similar between the two sexes, although there were some important differences Figure 3. Indeed, the percentage of HA stones was even greater than the percentage of CaOx stones for women between the ages of 20 and 29 years of age the only instance where CaOx was not the most common stone type.
In general, the sex differences in stone composition were much less prominent over age 70 years and most marked under age 30 years. Combined association of age and sex with stone type. A Men and B women are depicted separately. Monthly patterns within stone types did not vary ificantly by sex.
Thus, the seasonal trends in these two stone types did not seem to vary ificantly by area of the country. Association of calendar month with stone s submitted. Submissions by month are depicted for four major stone types: A calcium oxalate, B uric acid, C struvite, and D apatite. Our study identified important demographic and seasonal features that are associated with the type of stone that a given patient is likely to form. Younger women are more susceptible to HA stones, whereas UA stone composition increases markedly in both sexes after the age of 50 years. CaOx stones are the most common stones across the age and sex spectra, but they are particularly common in middle-aged men.
The most recent large series of a central laboratory for Veterans Administration facilities across the United States 6 found several important trends, including an increasing likelihood of calcium phosphate stones in a given individual as the of stone events increased 6. Importantly, our data do suggest an increasing proportion of apatite and decreasing proportion of CaOx stones in the elderly, which is consistent with these prior findings of a shift toward apatite stones with time.
Men are at higher risk for kidney stones overall 7 , possibly because of a greater tendency for urine that is oversaturated for CaOx 8 and diet tendencies that raise CaOx supersaturations e. In one report, the urinary supersaturation for both CaOx and UA increased in men during warmer months, whereas these levels remained relatively flat across seasons for women We found that women were particularly more likely than men to have HA stones.
Perhaps related, women stone formers are at increased risk of urinary tract infection 11 , which in turn, could raise urinary pH from infection with organisms that contain urease and favor HA supersaturations. Overall, this study highlights that the factors that drive stone formation in younger women likely differ the factors that drive stone formation in the older and more common stone formers men. Other published data also support the idea that HA stones are more common in women than men and that the average amount of calcium phosphate in mixed CaOx stones is increasing over recent decades In this prior study from a large referral stone clinic, calcium phosphate stones began at a younger age In a separate report from the same referral stone clinic, urine pH was also noted to be higher in a group of both men and women who transformed from CaOx to calcium phosphate Hormonal status might play some role in the observed sex differences in stone composition.
Older women, likely to be postmenopausal i. Analysis of two large cohorts of women suggests that kidney stone risk may increase after menopause 14 , In general, younger women have been observed to have higher urinary pH and citrate, lower urinary calcium, and higher calcium phosphate supersaturation. However, postmenopause urinary calcium tends to rise 16 , In two studies, urinary pH and citrate both tended to be higher in postmenopausal women on estrogen replacement than women not on estrogen replacement 16 , These observations suggest that postmenopausal changes make older women more similar to men in their risk of stones, and this may explain the lack of sex differences in stone composition in the older age ranges.
Indeed, by age 90 years, UA stones are almost as common as CaOx stones. Two large stone analysis laboratories in Germany 18 and France 19 have reported a similar trend toward formation of UA stones in the elderly, and population-based data from Olmsted County, Minnesota 20 reported similar findings. Possibly, this observation has to do with changes in kidney function associated with aging. CKD is often associated with type 4 renal tubular acidosis, which in turn, is characterized by decreased renal ammoniagenesis and more acidic urine 21 that favors UA supersaturation Furthermore, urinary calcium excretion is reduced among those with declining GFR, perhaps making calcium stones less likely Obesity, insulin resistance, and diabetes, which are also more common with aging, are associated with lower urinary pH and UA stones 24 , It has been widely accepted that kidney stones are more common among adult men.
Our data suggest that, not only is the composition of stones different in younger women, containing relatively more HA, but the formed is also greater among adolescent girls 10—19 years old and young adult women 20—29 years old as opposed to men. Thus, the weight of evidence suggests that more research is needed to understand the factor s that mediate kidney stone risk in younger women and that these factors likely differ from those in older men and perhaps, older women.
Finally, our data suggest that the environment has effects on certain stone types, but perhaps not others. Patients were more likely to submit a CaOx or UA stone during the summer months. Increased urinary concentration because of insensible water loss in warm weather will increase urinary supersaturations for both UA and CaOx.
Alternatively, other stone compositions did not vary with calendar month. ST stones are caused by infection with a urease-positive organism, and one might speculate that seasonal effects on urinary concentration would be less important.Sex women in Stone
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The kidney-ureter stone sexual paradox: a possible explanation