A scheduled scan is not emergency care
Seek Urgent Assessment for Severe Urinary Symptoms
Go to an emergency service or seek immediate medical advice for:
- Fever or shaking chills with severe flank pain, vomiting, confusion or marked weakness.
- Inability to pass urine with a painful, increasingly swollen lower abdomen.
- Very little or no urine, especially with swelling, breathlessness, drowsiness or vomiting.
- Severe unrelenting flank pain or repeated vomiting, particularly during pregnancy or with a single kidney.
- Heavy visible blood in the urine, blood clots or difficulty passing urine because of clots.
- Flank or abdominal injury followed by blood in the urine, faintness, severe pain or weakness.
A blocked and infected kidney can deteriorate quickly. Do not wait for a routine ultrasound appointment; emergency evaluation may require blood and urine tests, urgent imaging, antibiotics or drainage.
What is a Renal and Urinary Tract Ultrasound?
A handheld probe sends high-frequency sound waves through water-based gel on the abdomen and flanks. The returning echoes create real-time images of the kidneys and bladder. The examination is external, non-invasive and does not use X-rays or CT radiation.
The scan is often called USG KUB, meaning kidneys, ureters and bladder. In practice, the kidneys and bladder are assessed in detail, but normal ureters are narrow and often cannot be seen along their full length. The referral determines whether prostate volume, pre-void bladder volume, post-void residual urine or selected Doppler observations are included.
Ultrasound Shows Structure—Not the Whole Story of Kidney Function
Kidney size and tissue appearance can suggest chronic disease, but a structurally normal kidney may still function poorly. Creatinine, estimated glomerular filtration rate (eGFR), urine protein and other tests assess kidney function and microscopic disease.
How is USG KUB Different From Other Tests?
Bring the exact referral. Similar scan names can describe different protocols, and the best test depends on the question your clinician needs answered.
Why Might the Scan Be Requested?
Other indications can include:
- Abnormal creatinine, eGFR or urine results requiring structural assessment.
- Suspected hydronephrosis, urinary obstruction or congenital urinary-tract abnormality.
- Chronic kidney disease, a small kidney or a single functioning kidney.
- Follow-up of a known renal cyst, stone, mass or bladder finding.
- Monitoring after selected operations, procedures, stent placement or treatment.
- Recurrent bed-wetting or urinary symptoms in a child when clinically indicated.
- Assessment before or after a trial without a urinary catheter.
- Guidance for a biopsy, drainage or another separately planned procedure.
Ultrasound is often an excellent first test, but it is not the only test for these symptoms. The clinician may also request urine culture, kidney-function tests, cystoscopy, CT, MRI or other imaging.
What Does the Radiologist Examine?
How Should I Prepare?
Follow the instructions issued with your appointment. A comfortably full bladder is usually needed because fluid improves the view of the bladder wall and helps with accurate volume measurements.
- Bring the referral, previous ultrasound/CT/MRI reports and recent relevant blood or urine results.
- Wear loose clothing that allows access to the abdomen, sides and lower back.
- Tell the team if you have a urinary catheter, stoma, recent surgery, dressings or severe mobility limitations.
- Contact the team before increasing fluids if you have a prescribed fluid restriction, heart failure or advanced kidney failure.
- Ask for individual instructions if you are pregnant, booking a child, cannot control urination or cannot hold a full bladder.
What Happens During the Scan?
The radiologist confirms the clinical question, symptoms, relevant history and whether bladder volume, post-void residual or prostate assessment is required.
Clothing is moved away from the abdomen and flanks while privacy is maintained. You usually begin on your back and may turn onto either side.
Gel is applied and the probe is moved across the upper abdomen, sides or back. You may be asked to breathe in, hold your breath briefly or change position for clearer views.
The probe moves over the lower abdomen to assess the bladder and estimate its volume. The prostate may be measured through the full bladder when requested.
Empty your bladder in the usual way and return promptly without drinking more water unless instructed. A second brief scan measures the post-void residual.
Measurements and representative images are saved. The radiologist prepares a formal report for the referring clinician and communicates urgent findings through the appropriate pathway.
The scan often takes about 15–30 minutes, including a post-void check when requested. It may take longer if the bladder is not sufficiently full, detailed Doppler is needed or views are technically difficult.
What Does a Post-Void Residual Measurement Mean?
Post-void residual (PVR) is the estimated amount of urine remaining in the bladder soon after you pass urine. It helps assess whether the bladder empties effectively. A raised value can occur with bladder-outlet obstruction, reduced bladder muscle contraction, medicines, nerve conditions, pain, constipation or an unfamiliar voiding situation.
The report may provide a volume without labelling it normal or abnormal. Your clinician interprets it in context; do not make treatment decisions from an isolated PVR number.
When is Doppler Used?
Colour Doppler may be switched on during a routine scan to assess selected blood-flow patterns or urine entering the bladder from the ureters. It does not require dye or an injection.
- Ureteric jets may be observed as urine enters the bladder, but their presence or absence alone does not prove or exclude obstruction.
- Blood flow within a kidney lesion may assist characterisation but does not provide the final tissue diagnosis.
- Selected renal perfusion observations may be made in infection, infarction, transplant or vascular questions.
- A dedicated renal artery Doppler is a separate study with specific preparation and measurements.
Colour on the screen does not mean that a full vascular study has been completed. The referral should state the suspected vascular problem so the correct protocol and appointment time can be arranged.
What Do Common Report Terms Mean?
What Can the Scan Not Determine Reliably?
- Kidney function, filtration or the cause of protein in urine without blood and urine tests.
- Every small kidney stone or most stones located within non-dilated ureters.
- Whether mild collecting-system dilatation represents true functional obstruction in every case.
- Microscopic kidney disease or early infection when no structural change is visible.
- Every small or flat bladder tumour, particularly when the bladder is poorly filled.
- The full extent or tissue type of an indeterminate kidney or bladder mass.
- Prostate cancer from prostate size or ultrasound appearance alone.
- Complete urinary-tract detail when bowel gas, body habitus, pain or limited mobility restrict views.
A normal ultrasound does not make persistent flank pain, haematuria, recurrent infection or abnormal kidney tests unimportant. The referring clinician decides whether cystoscopy, CT, MRI, repeat imaging, laboratory tests or specialist review is needed.
What Happens After the Scan?
You can empty your bladder and return to normal food, fluids, medicines and activity unless another test or your treating clinician requires restrictions. Wipe away the gel before dressing.
The radiologist reviews the saved images and issues a formal report to the clinician who requested the scan. Some findings need no action; others may lead to urine or blood tests, CT or MRI, cystoscopy, treatment for obstruction or infection, specialist review or planned follow-up imaging.
Ask how and when your result will be discussed. Seek earlier medical care if pain, fever, vomiting, visible bleeding, urinary difficulty or urine output worsens while you are waiting.
Myth vs Fact
Frequently Asked Questions
Do I need to fast for a renal or KUB ultrasound?
Usually not. A routine kidney-and-bladder scan generally requires a full bladder rather than fasting. If whole-abdomen imaging is booked at the same visit, different instructions may apply.
How full does my bladder need to be?
Follow the amount and timing in your appointment instructions. You should feel comfortably full, not be in severe pain. Tell reception if you urgently need to pass urine.
What if I cannot hold urine or cannot fill my bladder?
Tell the team when booking and again on arrival. Urgency, incontinence, retention, childhood, pregnancy and medical fluid restrictions all need individual preparation rather than forced drinking.
Can I have the scan with a urinary catheter?
Yes, but contact the imaging team beforehand. Catheter clamping, drainage and post-void assessment require specific clinical instructions; do not clamp or remove a catheter yourself.
Will the scan hurt?
It is usually painless. Probe pressure may be uncomfortable over a tender kidney or very full bladder. Tell the radiologist so pressure and position can be adjusted.
Can ultrasound find all kidney stones?
No. It can show many renal stones and signs of obstruction, but small stones and ureteric stones may not be visible. Non-contrast CT KUB is more sensitive when clinically appropriate.
Does a normal scan mean my kidneys are functioning normally?
No. Ultrasound can look normal despite reduced filtration or microscopic kidney disease. Kidney function is assessed with blood pressure, blood tests such as creatinine/eGFR and urine tests.
Why do I need to urinate and return for another image?
The second scan estimates post-void residual urine—how much remains after you empty your bladder. Return promptly because new urine continuously enters the bladder from the kidneys.
When will I receive the result?
The radiologist prepares a report for the referring clinician. Urgent findings are communicated through the appropriate pathway. Ask before leaving how and when the result will be discussed.