Urinalysis, renal function and paediatric kidney assessment

Overview & Definition

A child passes urine, and you send blood to the laboratory. The two streams tell you different things, and the skill is making them speak to each other. Urinalysis tells you what is happening at the kidney surface and along the tubule right now. Renal function testing tells you how much filtration the kidney is performing. Together they separate pre-renal, intrinsic and post-renal problems, and they show whether a kidney is acutely injured, chronically scarred or working well. [2] [4]

Urinalysis means the macroscopic appearance of urine, the chemical dipstick, and the spun-urine microscopy. Each layer adds resolution. The dipstick is a screening tool; microscopy and culture confirm what it suggests. A dipstick is never the end of a diagnostic sentence in a sick child. [2] [3]

Renal function is the clearance work the kidney performs, summarised as the glomerular filtration rate. Direct measurement with iohexol or inulin clearance is the standard, but it is invasive and slow. At the bedside you estimate GFR from serum creatinine and height, because creatinine is freely filtered at the glomerulus and barely secreted or reabsorbed. [1] [11]

[2] [1] [9]

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1. [1]Schwartz GJ, et al New equations to estimate GFR in children with CKD J Am Soc Nephrol, 2009.PMID 19158356
2. [2]Simerville JA, et al Urinalysis: a comprehensive review Am Fam Physician, 2005.PMID 15791892
3. [3]Whiting P, et al Rapid tests and urine sampling techniques for the diagnosis of urinary tract infection (UTI) in children under five years: a systematic review BMC Pediatr, 2005.PMID 15811182
4. [4]Stevens PE, et al Evaluation and management of chronic kidney disease: synopsis of the kidney disease: improving global outcomes 2012 clinical practice guideline Ann Intern Med, 2013.PMID 23732715
5. [5]Pottel H, et al Estimating glomerular filtration rate at the transition from pediatric to adult care Kidney Int, 2019.PMID 30922665
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9. [9]Bogie AL, et al Is There a Difference in the Contamination Rates of Urine Samples Obtained by Bladder Catheterization and Clean-Catch Collection in Preschool Children? Pediatr Emerg Care, 2021.PMID 34772880
10. [10]Forster CS, et al Accuracy of NGAL as a Biomarker for Urinary Tract Infection in Young Febrile Children: An Individual Patient Data Meta-Analysis J Pediatr, 2023.PMID 37001635
11. [11]Salvador CL, et al Estimating glomerular filtration rate in children: evaluation of creatinine- and cystatin C-based equations Pediatr Nephrol, 2019.PMID 30171354
12. [12]Ng DK, et al Self-reported Race, Serum Creatinine, Cystatin C, and GFR in Children and Young Adults With Pediatric Kidney Diseases: A Report From the Chronic Kidney Disease in Children (CKiD) Study Am J Kidney Dis, 2022.PMID 34974031