The Northeastern Society of Plastic Surgeons

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Test Characteristics of a Limited versus Complete Urinary Metabolic Profile in Pediatric Stone Formers
Katherine H. Chan, MD, MPH1, Elizabeth Moser, MS2, Mark Cain, MD1, Benjamin Whittam, MD1, Rosalia Misseri, MD1, Amy Krambeck, MD1.
1Indiana University School of Medicine, Indianapolis, IN, USA, 2Indiana University School of Medicine and Richard M. Fairbanks School of Public Health, Indianapolis, IN, USA.

Background: AUA guidelines recommend a complete urinary metabolic evaluation in all pediatric stone patients due to a historically high incidence of metabolic abnormalities and risk of stone recurrence. In modern series, however, hypocitraturia and low urinary volume account for the vast majority of abnormalities. We hypothesized that a more limited evaluation would detect the majority of clinically significant metabolic abnormalities in pediatric stone-formers.
Methods: We performed a retrospective analysis of all children (<18 years of age) with renal and/or ureteral calculi evaluated at our institution between 2005 and 2015. We included those who had at least one 24-hour urinary metabolic profile after a stone-related clinical visit. We excluded children with bladder stones and those with under- or over-collections (normal Cr in pre-pubertal patients: 10-15 mg/kg/24 hours). We extracted demographics and the data from the first urinary metabolic profile. We defined low urine volume as <1ml/kg/hour, elevated 24-hour calcium as >4.0 mg/kg/24 hours and elevated 24-hour oxalate as <52 mg/1.73m2/24 hours. We defined low 24-hour citrate by age and gender criteria as previously described in the literature. We compared the sensitivity and positive predictive value of a limited urinary metabolic evaluation consisting of four parameters (24-hour calcium, 24-hour citrate, 24-hour oxalate and low urinary volume) compared to a complete urinary metabolic profile. We determined the number and type of metabolic abnormalities that we would have missed with this limited evaluation.
Results: Of 410 patients, 21 were excluded for age ≥18 years, 13 for bladder stones, 248 for over-collections and 38 for under-collections leaving 90 for analysis: median age 13 years (interquartile range 8-16), 61.1% female, 93.5% white, 19.5% obese and 13.0% overweight. In the complete evaluation, 64.4% had low 24-hour citrate, 55.6% had low urine volume and 20.0% had elevated 24-hour calcium. The sensitivity and positive predictive value of the limited metabolic evaluation were 87.8% and 100% respectively. Of the 90 patients, 11 had at least one abnormality missed by a limited metabolic evaluation (Table 1). The missed abnormalities were high pH (n=7), abnormal 24-hour phosphorus (low in 1 patient, high in 1 patient), low 24-hour magnesium (n=3), low 24-hour potassium (n=3) and high 24-hour sodium (n=5).
Conclusions: A limited urinary metabolic evaluation detects the vast majority of clinically significant metabolic abnormalities in pediatric stone-formers. Further studies are needed to compare the cost-effectiveness of the two strategies.
Table 1: Abnormalities missed by limited evaluation

Total Count of AbnormalitiesFrequencyPercent
1436.4%
2545.4%
3218.2%


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