Study: 24-Hour Urinalysis Key for Managing Stones

New findings show that urinary biochemical composition changes with age.
New findings show that urinary biochemical composition changes with age.

Urinary biochemical profiles change with age, and this needs to be considered in the medical management of stone disease, according to researchers.

In a study of 24-hour urine collections from 1,115 patients seen in a tertiary care stone clinic, Justin I. Friedlander, MD, of the University of Texas Southwestern Medical Center in Dallas, and colleagues found that increasing age was associated with significantly decreased mean urine pH, 24-hour calcium, uric acid, ammonium, creatinine, and supersaturation of calcium oxalate and calcium phosphate on univariate analysis.

Adjusted multivariate analysis showed that increasing age was associated with significantly increased 24-hour citrate and supersaturation (SS) of uric acid and decreased pH, 24-hour uric acid, creatinine, and SS of calcium oxalate and calcium phosphate.

“This highlights the importance of evaluating stone-forming patients of all ages with 24-hour urine collections because both the type and degree of metabolic abnormality may change with age,” the researchers concluded in a paper published online ahead of print in the Journal of Endourology.

The findings that urinary calcium and SS of calcium oxalate and calcium phosphate decrease with increasing age are consistent with a previous study by David S. Goldfarb, MD, and colleagues published in Clinics in Geriatric Medicine (1998;14:367-381).

Dr. Friedlander and his colleagues hypothesize that the decline in urinary calcium with increasing age is from age-related changes in calcium handling because of intestinal absorption, renal mechanisms, or both.

The decrease in SS of calcium oxalate and calcium phosphate with increasing age is likely driven by the decline in urinary calcium because the results demonstrate that both oxalate and phosphate increase with age, they noted. 

“When taken in the context of increasing incidence of stone disease and a change in the age of peak incidence of stones, our findings give credence to the idea that lower SS values in older stone-forming patients are likely sufficient to increase the risk of stone formation, and normal ranges may need to be age-adjusted to more appropriately counsel and treat patients,” they wrote.

Additionally, Dr. Friedlander's team reviewed the composition of 436 stones and found an increasing percentage of uric acid content with increasing age. They noted that a similar trend was found in a study by Amy Krambeck, MD, and colleagues published in The Journal of Urology (2013;189:158-164).

This study showed that uric acid and atypical stones were significantly associated with older age, whereas calcium phosphate stones were significantly associated with younger age.

Commenting on the paper by Dr. Friedlander's team, Dr. Goldfarb told Renal & Urology News: “This interesting study confirms that age is an important variable affecting urine chemistry. We know that increasing age is associated with declines in estimated glomerular filtration rate, and this in turn is likely associated with increases in parathyroid hormone. That effect may account for reductions in urine calcium excretion with age, seen in our 1998 study and in the current paper.”

The finding that increasing age is associated with reductions in urinary pH and, therefore, more uric acid stones, is consistent with the study by Dr. Krambeck and colleagues, said Dr. Goldfarb, professor of medicine and physiology at the New York University (NYU) School of Medicine and clinical chief of nephrology at NYU Langone Medical Center.

“The study [by Dr. Friedlander's group] also suggests that unmeasured variables, like urinary promoters and inhibitors, also contribute to the stone disease of older people, given their apparent tendency to form stones with lower supersaturations.”

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