New Tool Predicts Risk of Experiencing Second Kidney Stone

Knowing which first-time kidney stone patients are likely to experience another symptomatic kidney stone could help physicians advise patients on whether to follow stone prevention diets or take medications. A new tool described in the Journal of the American Society of Nephrology accomplishes just that.

“We developed the Recurrence of Kidney Stone (ROKS) nomogram that uses 11 questions about the patient to calculate the probability of having another symptomatic kidney stone at 2, 5, or 10 years after the first stone,” said lead author Andrew Rule, MD, of the Mayo Clinic in Rochester, Minnesota. The nomogram is available on the Internet at http://www.qxmd.com/calculate-online/nephrology/recurrence-of-kidney-stone-roks.

Characteristics linked with recurrence

Although several studies have identified predictors for recurrence after a first stone episode, clinicians don’t have access to a formal prediction tool for routine use in patients. “Kidney stones are common and affect about 6 percent to 9 percent of the population. Of greatest concern to the patient after a symptomatic kidney stone is whether this excruciating painful event will ever happen again,” said Rule. To develop a tool that might address this concern, Rule and his colleagues performed a general population cohort study of all validated incident kidney stone formers in Olmsted County, Minnesota, from 1984 to 2012. As they followed up patients for a second episode the investigators hoped to develop a predictive tool that used only characteristics commonly available at the time of the first episode.

The team found that 4908 residents of Olmsted County received a new diagnosis of kidney stones during the study period. After reviewing the patients’ charts, the researchers excluded patients in the following categories: prevalent stone formers, asymptomatic only, suspected stone only, no evidence of kidney stone disease, age younger than 18 years, no research authorization, and never a resident of Olmsted County. This left 2239 patients as validated first-time symptomatic stone formers. These patients were followed up for a total of 20,548 person years, with a median follow-up time of 11.2 years from the time of stone diagnosis; 707 patients had a second symptomatic stone during this time. The symptomatic recurrence rates at 2, 5, 10, and 15 years were 11 percent, 20 percent, 31 percent, and 39 percent, respectively.

After following up the patients and recording their characteristics, the investigators found that factors contributing to a higher risk of symptomatic recurrence included younger age, male gender, white race, family history of kidney stones, blood seen in the urine, stone made of uric acid, obstructing stone in the kidney pelvis, any additional nonobstructing stone, and any past painful event attributed to a kidney stone even though a stone was never seen. Male sex, white race, and family history of kidney stones may relate to genetic factors that contribute to recurrence. Younger age may also reflect a genetic component that causes an earlier manifestation of stones and their recurrence. The investigators noted that the presence of a stone at the renal pelvis or lower pole suggests a predisposition to form a stone too large to be passed into the ureter, whereas patients who form smaller stones that pass to the ureterovesicular junction may not always experience symptoms with future stones. Finally, the chemistry and biology of uric acid stones differ from those of calcium stones and likely influence the risk of recurrence.

Individualized risk scores

As an example case, the researchers described a 30-year-old (82 points) white (30 points) woman who presented with flank pain from her first symptomatic kidney stone, along with gross hematuria (8 points). Her father also had kidney stones (48 points), and she had a similar pain episode 5 years ago that resolved on its own without a visible stone (70 points). A 10-mm renal pelvic stone was surgically removed (75 points + 15 points for the symptomatic stone not being at the ureterovesicular junction) that was 100 percent calcium oxalate. She also had an 8-mm upper-pole nonobstructing stone (54 points). The total for her nomogram was 382 points, and her risk of symptomatic recurrence at 2, 5, and 10 years was 49 percent, 75 percent, and 91 percent, respectively. For patients with such high risks, physicians may advise drinking lots of water and eating a low-salt, low–animal protein diet. Sometimes a low-oxalate diet is also recommended. Medications that may be prescribed to prevent kidney stones in high-risk patients include thiazide diuretics, which lower calcium in the urine, and potassium citrate, which increases citrate in the urine to help prevent stones from forming.

Although additional research is needed to see whether treatment decisions based specifically on the ROK nomogram will reduce symptomatic episodes, the nomogram may be useful in clinical trials that include high-risk stone formers.

In an accompanying editorial, Brian Eisner, MD, of Massachusetts General Hospital and Harvard Medical School, and David Goldfarb, MD, of the York Harbor VA Healthcare System and the NYU School of Medicine, noted that for the nomogram to demonstrate value, it now should be tested prospectively in additional populations of stone formers. “Whether additional variables can be included to increase the usefulness of this tool will be of interest in the future,” they wrote. The authors also pointed out that an interesting aspect of how the tool’s utility will extrapolate to other locations is the latitude of Olmsted County. The prevalence of kidney stones is higher in warmer climates, likely because higher temperatures lead to reduced urine volume and to higher concentrations of stone-forming salts. “Based on previously published data and the current nomogram, it follows that stone recurrence in Rochester, MN, where monthly average high temperatures are below 4.4 degrees Celsius for 7 months per year, may actually underestimate recurrence rates in lower latitudes and warmer climates,” they wrote.

The article is entitled “The ROKS Nomogram for Predicting a Second Symptomatic Stone Episode.” The editorial is entitled “A Nomogram for the Prediction of Kidney Stone Recurrence.”