What are the indications for renal ultrasound in patients with suspected kidney disease or injury?

Indications for Renal Ultrasound

Renal ultrasound should be performed as the first-line imaging modality to evaluate for hydronephrosis in patients with unexplained acute kidney injury or decreased renal function, to assess kidney size and echogenicity in chronic kidney disease, and as an alternative to CT in hemodynamically stable pediatric patients and pregnant women with suspected renal trauma. 1, 2

Primary Clinical Indications

Evaluation of Decreased Renal Function

• Renal ultrasound is the best screening modality for patients presenting with renal insufficiency to detect hydronephrosis, which indicates obstructive uropathy—a mechanical and reversible cause of acute renal failure. 1, 3
• Ultrasound can identify reduced renal size and increased echogenicity, which are features of chronic renal insufficiency/failure. 1
• Small kidneys suggest advanced stage chronic kidney disease, while echogenic kidneys indicate the presence of parenchymal renal disease. 3

Suspected Urinary Tract Obstruction

• Both kidneys and the bladder should be imaged in patients with suspected renal tract pathology, as hydronephrosis and urinary retention are frequently unsuspected causes of abdominal pain. 2
• If hydronephrosis is detected, further evaluation with CT, MRI, or renal scintigraphy may be needed to determine the specific cause and location of obstruction. 1

Hematuria Evaluation

Isolated Microscopic Hematuria (Nonpainful, Nontraumatic)

• No imaging is indicated for patients with isolated microscopic hematuria without proteinuria or dysmorphic red blood cells, as they are unlikely to have clinically significant renal disease. 2
• Ultrasound is the best modality to display kidney anatomy, size, and position when imaging is needed to assess for structural lesions or before renal biopsy. 2

Painful Hematuria (Suspected Urolithiasis)

• Ultrasound can be used in children with mild symptoms, minimal clinical findings, and hematuria <50 RBCs/HPF as an alternative to CT scanning. 2
• However, ultrasound has significant limitations: absence of hydronephrosis does not rule out a ureteral stone, and many ureteral stones (especially small ones) do not cause hydronephrosis. 2

Renal Trauma

Pediatric and Pregnant Patients

• Ultrasound, contrast-enhanced ultrasound, and Doppler can be used in pregnant women and the pediatric population as an alternative to CT in the presence of hemodynamic stability during immediate assessment and follow-up evaluations. 2, 4
• In children with mild symptoms, minimal clinical findings, and hematuria <50 RBCs/HPF without other indications for CT, ultrasound and/or contrast-enhanced ultrasound and/or Doppler associated with blood tests may be adopted for initial evaluation. 2

Adult Trauma Patients

• Ultrasound is generally NOT recommended as a diagnostic tool during the initial evaluation of adult patients with high-energy trauma when multiple injuries and/or injury to the urinary tract and collecting system are suspected. 2
• E-FAST is effective and rapid to detect intra-abdominal free fluid but has low sensitivity (22-67%) and may underestimate renal injuries in up to 30% of cases. 2
• CT with intravenous contrast and delayed urographic phase is the gold standard for hemodynamically stable or stabilized adults after blunt or penetrating trauma. 2, 4

Vascular Assessment

• Doppler ultrasound should be ordered when evaluating for renovascular hypertension, renal artery stenosis, renal vascular thrombosis, or when vascular pathology is suspected. 5, 1
• Doppler can differentiate vascular structures from dilated collecting systems and assess renal perfusion based on resistive indices. 5

Renal Infections

• Ultrasound assessment is indicated in complicated urinary tract infections, particularly in children, pregnant women, and patients with chronic kidney disease. 6
• Doppler and power-Doppler techniques increase sensitivity for detecting focal and multifocal acute pyelonephritis. 6

Pre-Procedural Planning

• Ultrasound is used to assess kidney anatomy, size, and position before renal biopsy. 2, 7
• It guides percutaneous procedures such as insertion of hemodialysis catheters. 7

Common Pitfalls and Caveats

• Ultrasound is less comprehensive than CT, MRI, or renal scintigraphy for evaluating the genitourinary system and is less useful for identifying the specific etiology of obstruction. 1
• Renal stones smaller than 3 mm are usually not identified by current sonographic equipment, and stones of all sizes may be missed. 2
• Presence of obstruction may be masked by dehydration. 2
• Increased renal echogenicity is a nonspecific finding that can be seen in various renal pathologies. 1
• Conditions that may mimic hydronephrosis include dilated renal vasculature, renal sinus cysts, bladder distension, and medullary pyramids (especially in young patients). 2
• Standard ultrasound cannot assess blood flow, potentially missing vascular pathologies—Doppler must be specifically ordered when vascular assessment is needed. 5
• Variations of renal anatomy (reduplicated collection systems, unilateral, bipartite, ectopic, and horseshoe kidney) are not uncommon and may be mistaken for pathologic conditions. 2