For suspected kidney stones, the most accurate test is a non-contrast low-dose CT of the urinary tract (CT KUB), which finds almost all stones and shows their size and location. Ultrasound is often used first for younger people and in pregnancy because it uses no radiation, though it can miss smaller stones. Your doctor chooses based on your age, symptoms and whether you're pregnant.
Key takeaways
- A low-dose non-contrast CT (CT KUB) is the most accurate test for kidney stones.
- Ultrasound is radiation-free and often used first in younger people and pregnancy.
- CT shows stone size and location; ultrasound can miss smaller stones.
- The choice depends on your age, symptoms and pregnancy status.
Kidney stones can cause sudden, severe pain in the back or side, and imaging is how they’re confirmed. Two scans do most of the work — and which one you get depends on your situation.
CT KUB — the most accurate test
The standard test for suspected kidney stones is a CT KUB (CT of the kidneys, ureters and bladder) — a non-contrast, low-dose CT of the urinary tract.[1] It finds almost all stones, and shows their exact size and location, which guides whether a stone is likely to pass on its own or needs treatment. No contrast injection is needed, and modern low-dose protocols keep the radiation well below a standard CT.[2]
Ultrasound — the radiation-free first look
Ultrasound uses no radiation, so it’s often the first test for younger people, children, and anyone who is pregnant.[3] It can show a stone in the kidney and any swelling (hydronephrosis) caused by a blockage. Its limitation is that it can miss smaller stones, especially in the ureter, so a CT may follow if the picture is unclear.
Which one you’ll get
| Situation | Usual first scan |
|---|---|
| Adult with typical severe stone pain | Low-dose CT KUB |
| Younger person / recurrent known stones | Ultrasound (± CT if unclear) |
| Pregnant | Ultrasound (CT avoided unless essential) |
| Child | Ultrasound |
A plain X-ray (KUB) is sometimes used to track a known stone that shows up on X-ray, but it isn’t the main diagnostic test.[2]
In Australia
A CT or ultrasound for suspected kidney stones needs a referral and usually attracts a Medicare rebate; some clinics bulk bill. In severe pain, most people are assessed in an emergency department, where a CT is done promptly. See our guides to CT with contrast and safety and ultrasound scans for what each is like.
Frequently asked questions
What’s the best scan for kidney stones?
A non-contrast low-dose CT (CT KUB) is the most accurate — it finds nearly all stones and shows their size and location. Ultrasound is a good radiation-free first test, especially in younger people and pregnancy.[1]
Do I need contrast dye for a kidney stone CT?
Usually no — a CT KUB is done without contrast, because stones show up clearly on their own.[2]
Can an ultrasound miss a kidney stone?
Yes — ultrasound can miss smaller stones, particularly in the ureter, so a CT may be needed if symptoms persist and the ultrasound is unclear.[3]
Sources
- RadiologyInfo.org (RSNA & ACR) — Computed Tomography (Body CT) — www.radiologyinfo.org/en/info/bodyct
- RANZCR / InsideRadiology — Computed tomography (CT) — www.insideradiology.com.au/computed-tomography/
- healthdirect (Australia) — Kidney stones — www.healthdirect.gov.au/kidney-stones
