Kidney ultrasound: what the scan can show

What the scan looks at

An ultrasound machine sends high-frequency sound waves into the body through a small handheld probe. The waves bounce back from organs and are turned into a moving picture on a screen. The scan uses no radiation and no injected dye, so it is safe in pregnancy and for people with reduced kidney function.

A request may say "renal ultrasound", "KUB ultrasound" (kidneys, ureters and bladder) or "abdominal ultrasound". During the scan, the sonographer or radiologist checks:

• Size of each kidney. Measured in centimetres along its length. The two kidneys are compared.
• The cortex. This is the outer layer of the kidney, where filtering happens. Its thickness and brightness on the screen (called echogenicity) are noted.
• Hydronephrosis. Swelling of the inner drainage area of the kidney, which can happen when urine cannot drain freely, for example because of a stone or an enlarged prostate.
• Stones (calculi) in the kidney or bladder.
• Cysts (fluid-filled sacs) and any solid lumps.
• The bladder. Its wall, and in men the size of the prostate. A post-void residual is the amount of urine left in the bladder after you pass urine, measured with a second quick scan.

Some centres also use Doppler ultrasound, which shows blood flow in the kidney arteries. It is requested for specific questions, such as some types of difficult-to-control blood pressure.

Why it is ordered

A clinician may request a kidney ultrasound:

• when blood tests show reduced kidney function (a low eGFR), to look for a blockage or structural cause
• when there is blood in the urine, repeated urine infections or pain in the side or back
• when a stone is suspected
• when there is a family history of polycystic kidney disease, an inherited condition that causes many cysts
• when someone has difficulty passing urine or a weak stream
• to follow up a finding on an earlier scan

KDIGO 2024 guidance counts structural changes seen on imaging as one of the markers of kidney damage that, if they last more than 3 months, can support a diagnosis of chronic kidney disease (CKD) even when eGFR is 60 or above.

How the scan is done and how to prepare

You lie on a couch and a cool gel is spread on your skin. The probe is moved over your sides, back and lower abdomen. You may be asked to breathe in and hold your breath, or to turn onto your side. The scan is painless and usually takes 15 to 30 minutes.

Preparation depends on the centre, so follow the instructions you are given. Common advice includes:

• Arrive with a full bladder. You may be asked to drink several glasses of water about an hour before and not pass urine until after the scan. A full bladder is easier to see and helps assess it properly.
• Fasting for a few hours is sometimes requested if the liver and gallbladder are being scanned at the same time.
• Bring earlier scan reports and your recent blood and urine results, so any change can be compared.
• Wear loose clothing that is easy to move aside.

Ultrasound is available at many hospitals and imaging centres across Mombasa, including on the Island, in Nyali and in Changamwe. Waiting times and costs vary, and some centres need a written request from a clinician.

How results are reported

You usually receive a written report signed by a radiologist, a doctor who specialises in reading scans, sometimes with printed images. The report describes each kidney, often in this order: size in centimetres, cortical thickness and echogenicity, whether the outline of the cortex and medulla (the inner part) can be told apart clearly (called corticomedullary differentiation), presence or absence of hydronephrosis, stones or cysts, and the bladder findings. It ends with an "impression" or "conclusion" that summarises the main findings.

Kidney size depends on body size, age and sex, and the two kidneys are often slightly different. Reports usually comment when a kidney is smaller or larger than expected. Words such as "increased echogenicity" or "small kidneys" are descriptions, not diagnoses. Your clinician interprets them together with your blood and urine results.

A simple cyst is a common finding, especially in older adults, and on its own is usually not a concern. The report may use a grading system for cysts, and your clinician will say whether any follow-up scan is needed.

What can affect the scan

• Body build. A larger body size can make the kidneys harder to see clearly.
• Gas in the bowel can block the view of parts of the kidneys or bladder.
• An empty bladder limits what can be seen of the bladder.
• Operator skill and equipment. Ultrasound depends on the person doing the scan. Small stones in particular can be missed, and another type of scan may be suggested.

What ultrasound cannot tell you on its own

An ultrasound shows structure, not function. Kidneys can look normal on a scan and still filter poorly or leak albumin, and small kidneys can still be working at a level the blood tests describe. That is why an ultrasound is read together with a creatinine test, eGFR and a urine ACR. An ultrasound also cannot always tell what a solid lump is. Further imaging, such as a CT or MRI scan, may be needed for some findings.

Related tests

• eGFR: how well the kidneys filter.
• Urinalysis: blood, protein or infection in the urine.
• Kidney function tests: urea, electrolytes and creatinine.

When to discuss your result with a clinician

Take your report to the clinician who requested the scan. Ask for an explanation of any finding listed in the impression, whether it needs a repeat scan, and how it fits with your blood and urine tests. Findings such as hydronephrosis, small kidneys or multiple cysts usually lead to further assessment, and a clinician may discuss referral to a nephrologist (kidney specialist) or a urologist (a surgeon who treats the urinary tract). Our page on nephrologist or urologist explains the difference.

This page explains what a test or condition involves. It is not a diagnosis and does not replace advice from a doctor who knows your history. Your results should be interpreted together with your symptoms, other tests and a clinical assessment. Medical disclaimer.