Nursing care
Bladder scanning: probe placement, false readings and how results guide catheterisation
Written and reviewed by Elijah Washington, BSN, RN, PCCN · 4 min read · Updated October 2026
Short answer
A bladder scan uses portable ultrasound to estimate urine volume, helping the nurse decide whether a client with suspected retention needs catheterisation. Apply gel to the probe, place it just above the symphysis pubis angled toward the bladder, centre the crosshairs and hold still. Readings can mislead with scars, pregnancy or nearby fluid, so interpret them alongside the client's symptoms.
When a bladder scan is useful
A bladder scan measures ultrasound reflections to tell the bladder apart from surrounding tissue and estimate its volume. It is noninvasive and quick. Typical uses include a client who has not voided after surgery or catheter removal, one with suprapubic fullness or discomfort, and measuring post-void residual urine in a client who voids small amounts.
Its main value is avoiding unnecessary catheters, which carry infection risk. Policies commonly list where it should not be used, such as over open skin or a wound in the suprapubic area, and in pregnancy, where the uterus and amniotic fluid make a bladder reading unreliable.
Probe placement and gel
Clean the scanner head with an approved disinfectant before and after use. Position the client supine if possible, since the most accurate readings come with the client resting quietly. Expose the lower abdomen and select the correct patient setting; some devices ask the user to choose the male setting for a female client who has had a hysterectomy.
Apply gel generously to the probe without air bubbles, and use more for very thin or obese clients or apply it directly to the skin over heavy hair. Place the probe about an inch above the symphysis pubis, pointing slightly downward toward the bladder with the head icon toward the client's head, and hold it still while scanning.
Readings that mislead and how to correct them
If the crosshairs are not centred on the bladder, reposition and rescan; an off-centre reading underestimates volume. Some devices show a greater-than symbol when the bladder is not fully in range, meaning the true volume is larger. Scar tissue, incisions, sutures or staples from pelvic surgery can distort the reading, and an indwelling catheter in the bladder affects accuracy.
Other fluid near the bladder, such as ascites or a cyst, may be counted as urine and inflate the result, and obesity can reduce image quality. When a number does not match the client's picture, for example a low reading in a client with a firm, tender suprapubic area who cannot void, rescan and report the discrepancy rather than relying on the figure.
Using post-void residual results to guide catheterisation
For post-void residual, ask the client to void, record the amount, then scan promptly. A small residual is expected; a large residual suggests incomplete emptying. Volume thresholds for action differ between facilities and ages, and one sample policy suggests intermittent straight catheterisation per order for a residual in the range of 300 to 500 mL rather than an indwelling catheter.
Consider this hypothetical scenario. A postoperative client has not voided for many hours and reports lower abdominal pressure. Options include inserting an indwelling catheter immediately, encouraging fluids and waiting, or scanning the bladder and following the protocol or order based on the result. Scanning first is correct, because it gives an objective volume and may avoid an unnecessary catheter.
Documentation and escalation
Document the time, the scanned volume, the amount voided before a post-void scan, the client's symptoms and the action taken. If straight catheterisation follows, record the amount drained so it can be compared with the scan estimate, which also helps judge how reliable the scanner was for that client.
Report repeated large residuals, retention with pain, signs of urinary tract infection, or a reading that conflicts with the clinical findings. Follow the facility's nurse-driven protocol where one exists, since some allow the nurse to catheterise at set volumes while others require a provider order each time.
Sources and further reading
AHRQ CAUTI Toolkit Implementation Guide, Appendix C: Sample Bladder Scan Policy. Purpose, contraindications (open suprapubic skin, pregnancy), gel and probe placement above the symphysis pubis, crosshair centering, greater-than symbol, scar and catheter effects, supine accuracy, PVR documentation and the sample 300 to 500 mL straight catheterisation threshold.
MSD Manual Professional Edition: Urinary Retention. Incomplete emptying diagnosed by post-void catheterisation or ultrasound showing elevated residual volume; retention predisposes to infection; catheterisation as treatment.
The next step on this is the same as on everything else here: answer questions and read the rationales. Our renal and genitourinary practice questions are the closest set to what this page covers.
Common questions
Where is the bladder scanner probe placed?
About an inch above the symphysis pubis, pointing slightly downward toward the bladder, with gel on the probe and the head icon facing the client's head. Center the crosshairs on the bladder before accepting the reading.
Why is a bladder scan avoided in pregnancy?
The pregnant uterus and amniotic fluid can be mistaken for bladder contents, making the volume unreliable. Sample policies list pregnancy as a situation where a bladder scan should not be used.
What is a post-void residual and how is it measured?
It is the urine left in the bladder after voiding. The client voids, the amount is recorded, and the bladder is scanned soon afterward or drained by straight catheter per order to measure what remains.
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