After a child passes a kidney stone — or the stone is surgically removed –, urologists/nephrologist recommend a full metabolic stone evaluation (ME) to uncover the causes for stone formation.
Doctors order a full ME in addition to routine tests (i.e. blood chemistry) and the analysis of the actual stone.
The ME consists of a 24-hr urine collection sent to a specialized lab to determine total volume, pH, calcium, oxalate, uric acid, citrate, sodium, potassium, and creatinine.
Urologists/Nephrologist use the results to make diet recommendations, optimize water intake, or start a new medication. A subsequent 24- hr urine collection assesses for the effectiveness of the therapies recommended.
But is it worth it?
A 24-hr urine collection is a burden for the family. Parents collect the child’s urine for 24 hrs and then send it to the lab. With the results in hand, a follow-up appointment is made to discuss changes to the diet, drinking, and/or medications. Then the process is repeated periodically.
The above would all be fine if we could demonstrate that not doing the ME would increase the risk of recurrent stone formation by a significant amount.
What are the options?
The currently recommended course of action in pediatric stone formers is to always do a ME. An alternative course of action in patients with calcium stones would be to just drink enough water to void X amount daily and to make diet changes — fewer animal products, less salt, more citrus and plant products.
What do the studies show?
A recent study from the University of Iowa analyzed the 24-hr urine collection of 113 first time pediatric stone formers (mean age 11 years). The most common abnormality found in 89% of patients was low urine volume. All is needed for a parent to measure 24-hr urine volume is a plastic hat, paper, and pen: no need for fancy expensive specialized testing.
The second most common abnormality was low citrate (hypocitraturia) in 68% of the cohort. Hypercalciuria was seen in only 11% of the tests.
The Pareto principle (80/20) rule
The Pareto principle states that for many events, 80% of the effects come from 20% of the causes. In the case of pediatric kidney stones, not drinking enough and a diet poor in plants and citrus explains more than 80% of the stones. If we just focus on these when counseling pediatric stone formers, the great majority of them will stop having stones if they comply. The small percentage that continues to have stones despite drinking adequate amounts and having a diet high in citrus could benefit from the full ME.
My opinion is that ordering a full ME in first-time pediatric stone formers might not be necessary for most patients. Instead, we could have the families measure 24 hr urine volumes periodically and focus first on having the patient drink enough. Simplicity is king in life and also in medicine. If the patient is not able to accomplish the most basic task of drinking enough, bombarding them with medicines would be an option — but not one that would be best in the long term.
Once the patient is consistently achieving their 24-hr voiding goals, emphasis could be switched to the diet.
Periodic follow-ups with a nurse practitioner with a 24-hr urine volume can help reinforce the drinking habit without being too costly.
For patients following good diets and drinking enough who continue to get stones, referral to a pediatric nephrologist would be very reasonable.