A bladder scan is a quick, painless ultrasound check of how much urine is sitting in the bladder, done with a handheld probe on the lower belly. Its main job is to show whether someone is really holding urine, so a catheter isn’t inserted when it isn’t needed.1PubMed Central. Portable bladder ultrasound: an evidence-based analysis The technique comes down to a few moves done carefully, because the scanner reports a number, not a diagnosis, and that number is only as good as the aim behind it.
Getting the Person and Scanner Ready
Start by explaining what you’re about to do and asking for verbal agreement. Ask whether the person has a catheter, recent abdominal surgery, or a wound or dressing near the lower belly, since these affect where and whether you can scan. Tell them it uses sound waves, not radiation, and that it should not hurt. Mention that the gel will feel cool and that you will press on the lower belly, so the touch is not a surprise.
Make sure the person has privacy, and cover the pelvic area with a towel or sheet, since you’ll be working just above it.2Agency for Clinical Innovation. How to use a bladder scanner with adult patients: Clinician toolkit3POCUS 101. Bladder Ultrasound Made Easy: Step-By-Step Guide Have them lie flat on their back.4NYSORA. Bladder ultrasound Bending the knees can relax the belly muscles and make the probe easier to press in.5POCUS 101. Bladder Ultrasound Made Easy: Step-By-Step Guide Raise or lower the bed so you can reach the lower belly without stretching, because a hunched reach makes the probe wobble. If they can’t lie flat because of pain or breathing trouble, scan in the most level position they can tolerate and note it in the chart.
Timing matters too. To measure leftover urine, scan within about 10 to 15 minutes after the person pees. Waiting longer lets the bladder refill and makes the leftover look bigger than it is. Note whether the scan was taken before or after peeing, because the two numbers mean different things.
Before you walk into the room, check that the scanner is charged or plugged in and that the probe and cable are undamaged. Many automated scanners ask you to choose male or female before scanning, so the device doesn’t mistake the womb for the bladder. Female settings are for women who still have a womb. Male settings are used for men, for children in some devices, and for women who have had a hysterectomy.
Wipe off old gel and keep the probe face clean, because dried gel and debris cause poor contact. Then put a small amount of ultrasound gel on the lower belly just above the pubic hair line.6Phoenix Ultrasound. Bladder Scanning: Understanding the Procedure and Its Benefits Gel removes the air between the probe and skin, which lets the sound waves pass through and gives a clear picture.7Phoenix Ultrasound. Bladder Scanning: Understanding the Procedure and Its Benefits
Where to Place and Aim the Probe
Find the pubic bone, the hard ridge at the very bottom of the belly where the pelvis begins.8NYSORA. Bladder ultrasound Press gently with your fingertips on the lower belly and slide downward until you feel it. Measure from that ridge with your fingers rather than guessing from the pubic hair line, which varies a lot between people.
Place the probe on the midline of the belly, about 2.5 to 3 cm (roughly an inch) above that bone.9EchoNous. How to Perform a Bladder Scan Step by Step10Agency for Clinical Innovation. How to use a bladder scanner with adult patients: Clinician toolkit The midline is the imaginary vertical line down the center of the belly, and the bladder normally sits there. Point the probe’s orientation marker toward the person’s head.11POCUS 101. Bladder Ultrasound Made Easy: Step-By-Step Guide12EchoNous. How to Perform a Bladder Scan Step by Step
Now tilt the probe 15 to 20 degrees so the beam points slightly toward the feet.13EchoNous. How to Perform a Bladder Scan Step by Step Aiming down toward the tailbone brings the bladder into view.14Agency for Clinical Innovation. How to use a bladder scanner with adult patients: Clinician toolkit15POCUS 101. Bladder Ultrasound Made Easy: Step-By-Step Guide
The angle matters because sound waves do not pass through bone. A probe resting on the pubic bone shows a dark shadow instead of the bladder. Placing it too far up the belly can miss a small bladder, especially right after someone has peed, because it sits tucked down behind the bone. The downward tilt lets the beam slip under the lower edge of the bone to reach it.
Press firmly enough that the whole probe face touches the skin, but not so hard that it causes pain, particularly if the bladder is full and tender. In a larger belly, gentle extra pressure can help the beam get through soft tissue, as long as the person stays comfortable.
Taking the Scan and Checking the Aim
Press the scan button and hold the probe completely still until it beeps.16Agency for Healthcare Research and Quality. Appendix C. Sample Bladder Scan Policy The screen then shows a volume and an aiming display with crosshairs.17Agency for Healthcare Research and Quality. Appendix C. Sample Bladder Scan Policy The crosshairs show where the scanner thinks the bladder sits compared with the center of the beam. If the bladder is off-center, move the probe toward the side where the bladder appears and scan again.18PubMed Central. Operations & Maintenance Manual: BladderScan Prime Plus19Agency for Healthcare Research and Quality. Appendix C. Sample Bladder Scan Policy After each move, hold still and scan again, because every scan is a fresh reading and the old number does not update.
A “>” symbol next to the number means part of the bladder was outside the scan.20Agency for Healthcare Research and Quality. Appendix C. Sample Bladder Scan Policy The real volume is larger than the number shown.21Agency for Healthcare Research and Quality. Appendix C. Sample Bladder Scan Policy Reposition and rescan rather than recording that number.
Take at least two or three scans, since a single scan can be off if the aim was slightly wrong. Use the highest reading that was well centered, because poor aim tends to read low rather than high. Large differences between scans usually mean the aim is drifting, so settle your position before the next try.
When you’re done, wipe the gel off the person’s skin and help them tidy their clothing or bedding so they are comfortable and covered. Clean and disinfect the probe as your facility directs. Record the number, the time and whether the scan was taken before or after peeing in the person’s notes, and report any result that worries you.
Does Body Position or Size Change the Result
In children, scans of a full bladder taken lying down read about 45 mL lower than a detailed ultrasound, while standing scans matched it.22PubMed. Bladder scan accuracy in pediatric patients: Does patient position matter? Standing may be worth using when a child can do it and your facility allows it. These findings came from children, so they should not be assumed to apply the same way to adults.
After peeing, both standing and lying scans read slightly high, by about 20 to 30 mL.23PubMed. Bladder scan accuracy in pediatric patients: Does patient position matter? So a very low leftover reading is still reassuring.
Body size matters as well. After surgery, scans in people with obesity differed from the catheter-measured amount by a median of about 94 mL, versus about 35 mL in others.24PubMed Central. DEFINING THE RELIABILITY OF BLADDER SCAN IN OBESE PATIENTS WITH POSTOPERATIVE URINARY RETENTION In obesity the scan tended to read about 22% low, beyond the device’s own stated accuracy.25PubMed Central. DEFINING THE RELIABILITY OF BLADDER SCAN IN OBESE PATIENTS WITH POSTOPERATIVE URINARY RETENTION
With a larger person, take extra scans. Lift a belly fold gently if it covers the probe site, so the probe can sit in the right place on the midline. If symptoms and the number disagree, such as a very uncomfortable, swollen belly with a low reading, tell the nurse or doctor rather than relying on the number alone.
What the Number Means
- Leftover urine after peeing, called post-void residual or PVR on charts, is normal below about 50 to 100 mL.26Point-of-Care Ultrasound Certification Academy. Urinary Retention Assessment with POCUS | POCUS Resources & Case Studies
- Repeated readings of 200 to 300 mL or more suggest the bladder isn’t emptying properly.27Point-of-Care Ultrasound Certification Academy. Urinary Retention Assessment with POCUS | POCUS Resources & Case Studies
- 500 mL or more often leads to a catheter in hospitals, especially with discomfort.28Point-of-Care Ultrasound Certification Academy. Urinary Retention Assessment with POCUS | POCUS Resources & Case Studies
Experts haven’t agreed on one number for “too high,” and studies use anywhere from 150 to 1,000 mL.29PubMed. Can we define the optimal postvoid residual volume at which intermittent catheterization should be recommended, and are there other measures that could guide an intermittent catheterization protocol: ICI-RS 202330PubMed Central. Postvoid Residual Thresholds Used to Define Chronic Urinary Retention: A Systematic Review Leftover amounts vary from one pee to the next, so one high reading isn’t the whole story.31PubMed. Can we define the optimal postvoid residual volume at which intermittent catheterization should be recommended, and are there other measures that could guide an intermittent catheterization protocol: ICI-RS 2023
Symptoms such as belly pain, a strong urge to pee, dribbling or a swollen lower belly matter as much as the number. A person with a middling number and obvious discomfort deserves more attention than a person with the same number and no symptoms. Note how well the person says they emptied, because feeling that the bladder is still full is useful information.
A high reading should usually be rechecked after another try at peeing. Helping the person get comfortable and private, for example sitting upright on the toilet, can improve emptying before the recheck. If the second reading is still high, report both numbers and the times rather than only the latest.
Facilities set their own cutoffs for when to call a nurse or doctor or to catheterize, so follow your workplace protocol. Chart the number with the time and the symptoms so whoever decides next has the whole picture.
Why a Scan Can Read Wrong
The scanner picks up any fluid in the lower belly, not only urine.32PubMed Central. False positive bladder scan in ascites with anuria33PubMed Central. Falsely Elevated Postvoid Residual Urine Volume in Uterine Myoma Fluid that collects in the belly from liver or heart problems sits in the same area the beam passes through, so the scanner has no way to separate it from urine. Ovarian or kidney cysts and some fibroids can be counted as bladder too.34PubMed Central. False positive bladder scan in ascites with anuria35PubMed Central. False Elevation of Volume Determined by Bladder Scanner Secondary to Bowel Obstruction A blocked bowel has also raised readings.36PubMed Central. False Elevation of Volume Determined by Bladder Scanner Secondary to Bowel Obstruction About 9% of scans that suggest retention turn out false.37PubMed Central. False positive bladder scan in ascites with anuria
A large reading in someone who has just peed comfortably is worth questioning, since it does not fit what you saw. If a catheter is placed and only a small amount of urine drains despite a huge scan number, other fluid was likely counted as bladder. Note known cysts, fibroids or belly swelling when reporting the result so the reader of your note can weigh it.
Readings can also come out too low, usually from poor aim, too little gel or the shadow of the bone. Rescan before accepting a surprisingly low number in someone with symptoms.
Scanner or Full Ultrasound
Automated scanners are easier to learn and use.38PubMed Central. Comparative Effectiveness of Two Models of Point-of-Care Ultrasound for Detection of Post-Void Residual Urine during Acute Ischemic Stroke: Preliminary Findings of Real-World Clinical Application39PubMed Central. Handheld Ultrasound Bladder Volume Assessment Compared to Standard Technique They are built for a quick answer at the bedside, which suits routine checks of leftover urine. Using scanners can also help reduce catheter infections by avoiding needless catheters.40PubMed. The effectiveness of the ultrasound bladder scanner in reducing urinary tract infections: a meta-analysis They can show larger measurement errors, though.41PubMed Central. Comparative Effectiveness of Two Models of Point-of-Care Ultrasound for Detection of Post-Void Residual Urine during Acute Ischemic Stroke: Preliminary Findings of Real-World Clinical Application Image-based ultrasound lets a trained person see the bladder and judge its shape and size directly, but it can underestimate a very full bladder.42PubMed Central. Comparative Effectiveness of Two Models of Point-of-Care Ultrasound for Detection of Post-Void Residual Urine during Acute Ischemic Stroke: Preliminary Findings of Real-World Clinical Application
Ask for imaging if readings keep conflicting with symptoms, such as repeated high numbers but little urine on catheterization. Image-based ultrasound lets staff see whether fluid is inside the bladder or is something else, such as a cyst.