Diagnostic Test Characteristics of Ultrasound Based Hydronephrosis in Identifying Low Kidney Function in Young Patients with Spina Bifida: A Retrospective Cohort Study

PURPOSE: Kidney dysfunction in spina bifida is usually detected by low estimated glomerular filtration rate or ultrasound based hydronephrosis. We assessed the diagnostic test characteristics of hydronephrosis for detecting low estimated glomerular filtration rate, hypothesizing that hydronephrosis has low sensitivity compared to cystatin C based estimated glomerular filtration rate. MATERIALS AND METHODS: We conducted a single center, retrospective cohort study, including patients with spina bifida from 2012-2017 with 2 kidneys and complete data needed to calculate estimated glomerular filtration rate via multiple pediatric (age 1-17.9 years) or adult (age ≥18 years) estimating equations. We evaluated the association of hydronephrosis status (high grade, low grade or none) with estimated glomerular filtration rate, adjusting for small kidney size and scarring, and calculated diagnostic test characteristics of hydronephrosis for low estimated glomerular filtration rate. RESULTS: We analyzed 247 patients (176 children and 71 adults). Mean±SD age was 13.7±6.6 years, and 81% of patients had myelomeningocele. Hydronephrosis (77% low grade) was found in 35/176 children and 18/71 adults. Hydronephrosis was associated with low estimated glomerular filtration rate in stepwise fashion, independent of kidney size and scarring. However, across cystatin C based pediatric equations, any hydronephrosis (compared to none) had 23%-48% sensitivity, and high grade hydronephrosis (compared to none or low grade) had 4%-15% sensitivity for estimated glomerular filtration rate <90 ml/min/1.73 m2, which remained unchanged after excluding small kidneys and scarring. Across cystatin C based adult equations, any and high grade hydronephrosis had 55%-75% and 40%-100% sensitivity, respectively, for estimated glomerular filtration rate <90 ml/min/1.73 m2, although with wide confidence intervals. Specificity was higher with high grade vs any hydronephrosis. Sensitivities were higher for estimated glomerular filtration rate <60 ml/min/1.73 m2. CONCLUSIONS: Hydronephrosis was associated with low estimated glomerular filtration rate but had poor sensitivity for cystatin C based estimated glomerular filtration rate <90 ml/min/1.73 m2, especially among children with spina bifida.