DALLAS—Data from separate studies presented at the National Kidney Foundation's 2015 Spring Clinical Meetings may help improve clinicians' ability to manage kidney stones.

In a meta-analysis, researchers found that high fluid intake is effective and appears to be safe for the prevention of incident and recurrent kidney stones. The other study, in which investigators analyzed 24-hour urine specimens from non-Hispanic whites, demonstrated that age and sex influence the excretion of key urinary factors related to kidney stone risk.

Both studies were led by John C. Lieske, MD, of Mayo Clinic in Rochester, Minn. The presenting authors, also of Mayo, were Wisit Cheungpasitporn, MD, and Majuran Perinpam, BSc, respectively.

The meta-analysis included 9 studies: 2 randomized controlled trials (RCTs) with 269 patients and 7 observational studies with 273,685 patients. High fluid intake— defined as intake sufficient to achieve a minimal urine volume of 2.0–2.5 L/day—was significantly associated with a 60% decreased risk of incident kidney stones in RCTs and a 51% decreased risk in observational studies. In addition, high fluid intake was significantly associated with a 60% decreased risk of recurrent stones in RCTs and an 80% decreased risk in observational studies.

In 2014, the American College of Physicians (ACP) released a new guideline for preventing recurrent kidney stones in adults. For patients who have had 1 or more prior kidney stone episodes, the guideline recommends increased fluid intake spread throughout the day to achieve a urine volume of at least 2 L/day.

Kidney stone specialist David S. Goldfarb, MD, clinical chief of nephrology at New York University Langone Medical Center in New York, pointed out the ACP's guidelines suggest that evidence for the efficacy of increasing fluid intake is weak and leave an impression that increasing fluid intake is not an important prescription for patients with kidney stones. “We know that it is an extremely important prescription, not just because it's efficacious, but because it's inexpensive and because it's safe,” Dr. Goldfarb said.

Randomized controlled trials have already provided convincing evidence of the safety and efficacy of increasing fluid intake, he said. Adding in what is understood about urine chemistry, “we know that increasing fluid intake leads to a reduction in supersaturation.”

The notion that more studies are needed to confirm that increasing fluid intake can prevent kidney stones is incorrect, said Dr. Goldfarb, who is president of the ROCK [Research on Calculus Kinetics] Society and director of kidney stone prevention and treatment programs at New York Harbor VA Healthcare System.

The new study provides “additional confirmatory evidence” of the safety and efficacy of high fluid intake for preventing kidney stones, he told Renal & Urology News.

Dr. Goldfarb's usual fluid prescription for kidney stone prevention is 96 ounces per day, assuming the weather is not very hot and the patient is not exercising too much or experiencing increased bowel losses of water. He typically instructs patients to think of 96 ounces in terms of 8 × 12 (8 12-ounce or 12 8-ounce portions). “You need a way to visualize this. “It's not enough to say to people, ‘drink a lot,' you have to say what a lot means.”

The other study by Dr. Lieske and colleagues, which was presented by Majuran Perinpam, BSc, included 416 female and 293 male subjects (mean age 64.6 and 66.5 years, respectively). Results showed that urinary calcium declined with age, and levels were higher in males than females. An increase in serum creatinine caused urine calcium to decrease. Urinary oxalate excretion was greater in males despite no difference in oxalate intake, “suggesting sex differences in metabolism or other food intake,” Dr. Lieske and his collaborators wrote in a poster presentation. Urinary uric acid excretion correlated positively with body mass index and estimated glomerular filtration rate (as calculated using cystatin C). Cystatin C correlations with uric acid may relate to hyperuricemia and inflammation, according to the researchers.

“Age and sex influence the excretion of key urinary factors related to kidney stone risk and should be taken into account when evaluating kidney stone parameters,” the authors concluded.

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