General Health · Kidneys

Flank Pain and Kidney Stones — Which Test Finds Them?

Renal colic is one of the most severe pains there is. Here is what causes it, when it becomes an emergency, and an honest account of where ultrasound sits against CT.

Person pressing a hand into the lower back and side in discomfort

Classic renal colic is unmistakable: severe pain starting in the loin and sweeping round towards the groin, coming in waves, with restlessness — people pace rather than lie still — and often nausea and blood in the urine. It happens when a stone lodges in the ureter and the kidney backs up behind it.

Stone pain with fever is an emergency

An obstructed kidney that becomes infected can deteriorate quickly and needs urgent hospital treatment. Go to A&E or call 999 if severe loin pain comes with fever or shaking chills, if you are vomiting and cannot keep fluids down, if you are passing little or no urine, or if you feel faint. Acute severe colic belongs in urgent care — not in a booked private appointment.

Differentials

Not all flank pain is a stone

What you notice Commonly
Waves of severe loin-to-groin pain, restlessness, blood in urine Ureteric stone
Loin pain with fever, burning and urinary frequency Kidney infection — needs prompt treatment
Ache worse with movement or certain positions, tender to press on the back Musculoskeletal
Sudden severe abdominal or back pain in an older adult with collapse Aortic emergency — call 999
One-sided pain with a known kidney cyst or previous stones Often recurrence — still assess
Pelvic pain in women, sometimes mistaken for renal Gynaecological — see our pelvic pain guide
Honestly

Ultrasound versus CT for stones

Being straight about this matters, because it is the one area where the test we sell is not the best test. A low-dose CT scan of the kidneys, ureters and bladder is the definitive test for stones — it finds them almost regardless of size or position, including small stones in the mid-ureter that ultrasound routinely misses. In acute colic, that is what an emergency department will do.

What ultrasound does well: it shows stones in the kidney itself, and — importantly — it shows hydronephrosis, the swelling of the collecting system that tells you a kidney is obstructed even when the stone itself cannot be seen. It also assesses kidney size and texture, finds cysts and masses, and looks at the bladder. It uses no radiation, which is why it is preferred in pregnancy and for repeated follow-up in people with recurrent stones.

So: acute severe colic goes to A&E, where CT is available. Ultrasound is well suited to non-acute assessment, to monitoring known stones over time, and to answering the kidney questions without another radiation dose. We do not offer CT.

At IUS London

When our scan is the right call

  1. 1
    Not during an acute attackSevere colic, especially with fever or vomiting, is an A&E matter today. We will say so if you call us mid-attack.
  2. 2
    Known stones being monitoredUltrasound follows stones over time without repeated radiation — a real advantage for anyone who forms them recurrently.
  3. 3
    Settled or recurring milder painOnce the acute episode has passed, a scan assesses the kidneys, checks for obstruction, and looks for other causes.
  4. 4
    The appointmentA comfortably full bladder helps. 30 minutes, no referral. Findings explained on screen as we scan, written report usually within two hours.
FAQs

Your questions answered

Can an ultrasound see kidney stones?
It sees stones within the kidney well, and it shows obstruction reliably. Small stones lodged in the mid-ureter are frequently missed — low-dose CT is the definitive test, and it is what an emergency department uses in acute colic.
Why choose ultrasound at all then?
No radiation, which matters for pregnancy and for anyone who needs repeated imaging over years. It also answers the question that changes management most urgently — whether the kidney is obstructed.
How do I know if a stone has passed?
Pain settling is a good sign but not proof. A follow-up scan can confirm the obstruction has resolved, which is a common and sensible reason to book once the acute episode is over.
What is hydronephrosis?
Swelling of the kidney’s collecting system because urine cannot drain past a blockage. Ultrasound shows it clearly, and its presence is often what determines whether urgent treatment is needed.
How can I reduce the chance of more stones?
Drinking enough fluid to keep urine pale is the single most effective measure for most people. Dietary changes depend on the stone type, so ask your GP — advice for one kind can be wrong for another.
Do I need a full bladder for the scan?
Comfortably full helps us assess the bladder and the lower ureters. We will confirm what to do when you book.
Related

Where to go from here

UrinaryBlood in Urine

Why every episode needs your GP, and what the scan covers.

Read the guide →AbdomenStomach Pain After Eating

What the timing suggests, and what a scan can find.

Read the guide →CompareUltrasound vs MRI vs CT

What each scan sees best, and which suits your question.

Read the guide →

Once the acute episode has settled

Both kidneys, the collecting system and the bladder assessed without radiation — obstruction, stones, cysts and emptying — explained on screen as we scan, with the report usually within two hours.

From £159
Urinary tract ultrasound scan

Urinary tract scan

5a Lucerne Mews
Kensington, London W8 4ED
3 mins from Notting Hill Gate

This article is general information about flank pain and kidney stones, not individual medical advice. Severe loin pain with fever, vomiting, reduced urine output or faintness needs emergency care — see NHS guidance on kidney stones and kidney infection. Low-dose CT is the definitive test for stones; we do not offer CT.

Written and clinically reviewed by the HCPC-registered sonographers who perform this scan at IUS London — a CQC-registered diagnostic ultrasound clinic (Provider ID 1-2775844974). Your own findings are explained to you at the scan and set out in your report.

Author: Yianni Kiromitis, Senior Sonographer, HCPC RA38415 — over 20 years’ experience in NHS and private ultrasound
Medically reviewed: 9 August 2026