When should a renal ultrasound be ordered?

Indications for Renal Ultrasound

Renal ultrasound should be ordered in cases of suspected urinary tract obstruction, renal trauma with macroscopic hematuria, suspected renal masses, unexplained renal insufficiency, and for monitoring patients with conditions that require regular kidney assessment such as primary hyperoxaluria and Bartter syndrome. 1

Specific Clinical Indications

Acute Presentations

• Suspected urinary tract obstruction

  • Patients with flank pain and symptoms suggestive of renal colic
  • When hydronephrosis is suspected 1
  • To identify the level and cause of obstruction when present

• Renal trauma

  • Mandatory in cases with macroscopic hematuria after trauma 1
  • To identify evidence and extent of renal or urinary tract injury
  • To assess for perinephric hematoma or urinoma

• Painful hematuria

  • To evaluate for stones, masses, or other structural abnormalities
  • To assess for hydronephrosis which may indicate obstruction 1

• Unexplained renal insufficiency

  • To differentiate between pre-renal, intrinsic renal, and post-renal causes 2
  • To evaluate kidney size and echogenicity which may indicate chronic kidney disease
  • To rule out obstruction as a reversible cause of acute kidney injury

Monitoring and Follow-up

• Primary hyperoxaluria

  • At least yearly in patients who have not yet reached stage 5D CKD 1
  • To assess for stones and nephrocalcinosis
  • More frequent follow-up may be required depending on baseline findings

• Bartter syndrome

  • Every 12-24 months to monitor for nephrocalcinosis, kidney stones, and signs of obstructive uropathy 1

• Moderate to severe renal trauma

  • Follow-up imaging within 48 hours for AAST IV-V injuries 1
  • Ultrasound and contrast-enhanced US are preferred for pediatric patients

Clinical Scenarios Where Ultrasound Is Not Indicated

• Isolated microscopic hematuria without proteinuria

  • Patients without proteinuria or dysmorphic red blood cells are unlikely to have clinically significant renal disease 1
  • No imaging is indicated in these cases, as studies have shown no clinically significant findings

• Asymptomatic microscopic hematuria in children

  • Feld et al. evaluated 325 patients with microscopic hematuria; 87% had renal ultrasound with no clinically significant findings 1

Benefits of Renal Ultrasound

• Non-invasive and does not use ionizing radiation
• Real-time imaging capability
• No need for contrast material injection
• Cost-effective and widely available 3
• Can differentiate between multiple causes of flank pain
• Allows assessment of both structural (form and size) and functional (perfusion and patency) aspects of kidneys 4

Limitations and Pitfalls

• False negatives

  • Absence of hydronephrosis does not rule out ureteral stones, especially small ones 1
  • Dehydration may mask the presence of obstruction 1
  • Early stages of medical renal disease may appear normal on ultrasound 5

• False positives

  • Medullary pyramids may mimic hydronephrosis, especially in young patients 1
  • Renal sinus cysts and dilated renal vasculature can be mistaken for hydronephrosis 1
  • Normal anatomic variations (reduplicated collection systems, ectopic kidneys) may be mistaken for pathology 1

• Technical limitations

  • Renal stones smaller than 3 mm are usually not identified by current sonographic equipment 1
  • Obesity may limit visualization
  • Bowel gas can obscure portions of the kidneys

Special Considerations

• Pediatric patients

  • Ultrasound should be the first choice in both early and delayed follow-up phases 1
  • If cross-sectional imaging is required, magnetic resonance should be preferred to minimize radiation exposure

• Pregnancy

  • Ultrasound is the safest imaging modality during pregnancy
  • Can detect hydronephrosis which is common in pregnancy due to physiologic dilation of the collecting system

• Chronic kidney disease monitoring

  • Small echogenic kidneys suggest advanced stage chronic kidney disease 2
  • Echogenic kidneys with normal or enlarged size indicate parenchymal renal disease

Remember that both kidneys and the bladder should be imaged during renal ultrasound examination, as many conditions affecting the kidneys may also involve the bladder 1.