However, there are important caveats to the use of the renal ultrasound to effectively rule-in or rule-out obstructive nephropathy. For instance, false-negatives (e.g., the renal ultrasound does not show hydronephrosis, but there actually is) can occur in the following scenarios:
1. very early obstruction: truly acute obstruction (e.g., with a kidney stone, for instance) may take some time to develop enough distension of the collecting system to detect with our current imaging tools.
2. obstruction in the setting of pre-renal failure: volume-depleted kidneys may not demonstrate significant hydronephrosis until after volume resuscitation, which expands the collecting system.
3. large retroperitoneal tumors: tumors which encase the kidneys may cause obstruction but not allow expansion of the collecting system enough to see hydronephrosis.
4. retroperitoneal fibrosis: this can occur in patients with past extensive GU surgery or prior chemotherapy or radiation therapy and can cause mechanical obstruction without allowing expansion of the collecting system.
In addition, false-positives (e.g., the renal ultrasound is read as "hydronephrosis" but this is probably not pathologic) can occur as well, most notably in pregnancy. It is also common that hydronephrosis detected on ultrasound is a chronic, long-standing issue and not the main culprit for the present episode of AKI.
This post is summarized from some basic but well-written handouts by Dr. Joel Topf from Precious Bodily Fluids. Not a bad idea to print these out and go over cases with a medicine resident rotating on the nephrology consult service...