|A sagittal ultrasound image of the right kidney shows a normal-sized kidney with normal parenchymal echogenicity in a patient with acute renal failure.|
What clinicians want to know is whether elevated serum creatinine "acute or chronic"
- This information is important to narrow differential diagnosis, urgency of investigation and treatment.
- Chronicity of renal dysfunction can be determined with 1) a search for previous measures of renal function (i.e., old labs), 2) clinical history (i.e., recent onset of acute illness, oliguria that would suggest acute renal failure), 3) daily deterioration of renal function (suggestive of acute renal failure - ARF), 4) ultrasound
- ARF can be prerenal, renal or postrenal
- Ultrasound can help determining the kidney size. Small renal size (less than 8 cm in adults) is suggestive of CRF
- Demonstrate hydronephrosis, suggesting a postrenal cause. Note that to produce ARF, both kidneys must be affected (i.e., bilateral ureteric obstruction or bladder outlet obstruction, or hydronephrosis of a single functioning kidney)
- To exclude obstruction, US should be done as early as possible. But US is not necessary if there are "clear reversible causes on initial assessment + Rx instituted + clear evidence of prompt response with return to normal renal function within a few days"
- Increased echogenicity of renal cortex is not a sensitive measure of renal function. When present, it is more commonly seen in tubulointerstitial disease rather than glomerular disease
Limitations of Ultrasound
- No hydronephrosis is not equal to no obstruction. False negative study can occur 1) in the first few days of obstruction because the collecting system is relatively noncompliant, 2) if ureters and collecting systems are encased by tumor or fibrosis
- Detection of hydronephrosis can be difficult in patients with cystic kidney disease
Baxter GM, Sidhu PS. Ultrasound of the urogenital system, 2006.