The core question renal POCUS answers in the ED is whether the collecting system is obstructed. The exam is both kidneys in two planes plus the bladder — because unilateral hydronephrosis means one thing, bilateral hydronephrosis with a distended bladder means another.
Always scan both kidneys and always include the bladder. The comparison is the interpretation.
Curated from the POCUS 101 Renal Ultrasound Made Easy: Step-By-Step Guide, including its hydronephrosis grading images — the side-by-side grading examples there are the fastest way to calibrate your eye.
Abdominal preset. Phased array is an acceptable alternative through rib spaces.
Probe in the right mid- to posterior-axillary line around the 10th–11th intercostal space, indicator cephalad, using the liver as the window.
The left kidney sits more cephalad and posterior — posterior-axillary line, knuckles toward the bed, using the spleen as the window.
Indicator cephalad · rotate slightly oblique to align with the kidney's axis · fan through completely
Obtain the full bean-shaped long axis: bright renal capsule, hypoechoic cortex, echogenic central sinus fat. Fan anterior to posterior through the entire kidney — a single midline slice misses polar pathology.
Rotate 90° · sweep superior pole to inferior pole
In cross-section the kidney appears as a "C" of cortex around the sinus. Sweep pole to pole. Deep inspiration lowers the kidney a rib space and often rescues an obscured view.
Anechoic urine separating the central sinus · grade by extent
Suprapubic, transverse and sagittal · measure volume if retention is the question
Assess bladder filling and symmetry. Bladder volume ≈ length × width × height × 0.52 (in cm, yielding mL). Ureteral jets, when seen with color Doppler, indicate at least intermittent flow from that side.
| Finding | Appearance | Means |
|---|---|---|
| Unilateral hydro | Anechoic collecting-system dilation on one side | Obstruction distal to that kidney — in the right context, supports ureteral stone |
| Bilateral hydro | Both collecting systems dilated | Think distal/bladder outlet obstruction; check the bladder first |
| Simple cyst | Round, anechoic, thin-walled, posterior enhancement, usually peripheral | Common and benign-appearing; does not connect to the collecting system |
| Extrarenal pelvis | Central anechoic area without calyceal dilation | Normal variant that mimics mild hydronephrosis |
| Stone (if seen) | Echogenic focus with posterior shadowing, typically at the UPJ or UVJ | Direct visualization is the exception, not the rule |
| Post-void residual | > ~100–150 mL | Supports retention; interpret with symptoms and context |
A very distended bladder can produce mild bilateral collecting-system prominence. If the question matters, have the patient void or drain the bladder and rescan before finalizing the read.
Parapelvic cysts sit centrally and mimic a dilated pelvis. True hydronephrosis is a branching, interconnected system; cysts are discrete spheres that do not communicate.
Volume depletion can collapse a genuinely obstructed system to near-normal. In a convincing renal colic story with a normal scan, the scan has not excluded the stone.
The contralateral kidney is the built-in control for sinus appearance, cortical thickness, and size. Asymmetry is often more informative than any absolute measurement.
A held deep inspiration displaces the kidney caudally out from under the ribs. It is the simplest fix for the shadowed, intercostal, unreadable view.
Moderate or severe hydronephrosis, a solitary kidney, infection, or renal failure changes disposition. Mild unilateral hydronephrosis in an otherwise well patient often supports outpatient stone management — per your local pathway.
A starting template for the POCUS note. Text in brackets [ ] and pick-lists in { } are fields to complete. Save it as a SmartPhrase, then edit every field to match the actual study.
POINT-OF-CARE ULTRASOUND: RENAL / BLADDER
Indication: {flank pain : AKI : retention : hematuria} [***]
Exam type: Focused / limited, bedside
Operator: @ME@ Interpretation: independent, real-time
Probe: curvilinear
FINDINGS
- Right kidney: {no hydronephrosis : mild : moderate : severe hydronephrosis}
- Left kidney: {no hydronephrosis : mild : moderate : severe hydronephrosis}
- Bladder: {normal : distended : ***}; post-void residual *** mL (if measured)
- Other (cyst / mass / stone): {none : ***}
IMAGE QUALITY: adequate / limited by [***]
Images saved and archived to [PACS / Qpath / ***].
IMPRESSION: {no hydronephrosis : [side ***] hydronephrosis, grade *** : ***}.
I personally performed and interpreted this study.
Electronically signed: @ME@ @TD@ @NOW@
Sample template only — not a validated institutional SmartPhrase. Verify wording, image-archival, and attestation requirements against your department's POCUS credentialing, coding, and compliance policies before clinical use. Documentation supports, but does not replace, the medical record and QA workflow.