Renal length z‐score for the detection of dysfunction in children with solitary functioning kidney

Aim To evaluate whether renal length z ‐scores predict renal dysfunction in children with a solitary functioning kidney (SFK). Methods In a single‐centre retrospective cohort of children with SFK, we correlated body mass index z ‐scores, extracellular volume and lean body mass to renal length z ‐scores. We grouped these z ‐scores to other markers of renal dysfunction (proteinuria, hypertension, extracellular volume and abnormal estimated glomerular function rate [eGFR]) and analysed renal length z ‐score with multivariate analysis, receiver‐operated characteristics (ROC) plots and Youden's index to determine an appropriate cut‐off. Results 111 children had a median follow‐up 5.08 years, eGFR 80.8 mL/min/1.73 m 2 , and age at last follow‐up 7.4 (3.8‐13.4 years). The median renal length z ‐scores of those without any renal dysfunction (n = 37, 25.1%) were greater (+3.66, interquartile range 3.02‐4.47) than those with renal dysfunction (median 3.11, interquartile range 1.76‐4.11, P  = .0107, Mann‐Whitney test). Using a cut‐off of z ‐score of >+1.911, the odds ratio for having no renal dysfunction was 0.07 (95% CI 0.002‐0.459, P  = .0010). However, accuracy of the renal length z ‐score was poor (ROC curve 0.6488). Conclusion In this cohort of children with SKF, using the renal length z ‐score as a biomarker of renal dysfunction at 7 years of age is not recommended.