01Purpose and principlesWhat the assessment is for and the core concepts behind it.
Urinary obstruction raises pressure proximal to a blockage and can dilate calyces, renal pelvis and ureter. Causes include calculus, stricture, tumour, blood clot, retroperitoneal fibrosis, external pelvic mass and bladder outflow obstruction. Imaging should locate the level, identify the cause, assess renal parenchyma and decide whether infection or renal failure makes drainage urgent.
Low-dose non-contrast CT is highly effective for adult renal colic because most calculi are visible without contrast and secondary signs show obstruction. It also identifies many alternative diagnoses. Ultrasound avoids radiation and shows hydronephrosis and some stones, so it is preferred in pregnancy and younger populations, but ureteric visualisation is incomplete.
CT urography is a contrast-enhanced examination tailored to renal parenchyma, collecting systems, ureters and urothelium, commonly including nephrographic and excretory information. It is used in selected haematuria and upper-tract tumour pathways. It is not the correct synonym for any contrast CT or for low-dose renal-colic imaging.
Key points
- For non-pregnant adults with suspected renal colic, NICE recommends urgent low-dose non-contrast CT within twenty-four hours.
- Use ultrasound first in pregnancy and in children or young people; persistent uncertainty in younger patients may justify low-dose CT.
- Non-contrast stone CT and CT urography are different examinations: one detects calculus, while the other characterises renal parenchyma and opacified urothelium.
- Hydronephrosis is a sign of collecting-system dilatation, not a complete diagnosis; define level, cause, renal function and infection.
- Sepsis, anuria, bilateral or solitary-kidney obstruction, acute kidney injury or uncontrolled symptoms requires urgent urological decompression planning.
- CT urography does not replace cystoscopy because small or flat bladder mucosal lesions may remain occult on imaging.
02Indications, selection and cautionsWhen it is useful, when urgency changes and important limitations.
Dilated calyces and renal pelvis suggest impaired drainage, but fullness can reflect hydration, pregnancy or an extrarenal pelvis.
A stone at a transition with proximal hydroureteronephrosis, renal enlargement or perinephric stranding supports acute obstruction.
Fever, pyuria, sepsis and obstructive imaging indicate a source-control emergency; gas or echogenic debris may heighten concern.
Filling defect, focal wall thickening or abnormal enhancement on urography raises tumour concern, but clot and inflammation can mimic it.
Distended bladder with bilateral upper-tract dilatation suggests distal obstruction and requires residual, renal and neurological assessment.
Anuria, bilateral disease, solitary kidney or rising creatinine increases urgency and may alter iodinated-contrast planning.
03Method and interpretationA systematic approach to the test and its findings.
Read from the initial assessment onwards. Tests may run in parallel in urgent care; first-line, preferred, confirmatory, definitive and gold-standard labels appear only when the chapter explicitly states them.
- 01
Low-dose non-contrast CTFirst step - Why
- Detect urinary calculi, level of obstruction and important alternative diagnoses in adults.
- Interpretation and limitations
- NICE recommends it urgently within twenty-four hours for suspected adult renal colic; it does not evaluate urothelial enhancement.
- 02
Renal tract ultrasound - Why
- Assess hydronephrosis, bladder volume and larger renal stones without radiation.
- Interpretation and limitations
- Use first in pregnancy and children; a negative study can miss ureteric stones and does not universally exclude obstruction.
- 03
CT urography - Why
- Evaluate renal parenchyma and opacified upper-tract urothelium for masses and filling defects.
- Interpretation and limitations
- Protocol uses contrast phases and renal excretion; check renal function and recognise that cystoscopy remains complementary.
- 04
Urinalysis and culture - Why
- Identify haematuria and infection in the obstructive presentation.
- Interpretation and limitations
- Pyuria with obstruction and systemic illness is urgent; a negative dipstick does not exclude every calculus.
- 05
Creatinine and electrolytes - Why
- Assess renal effect, bilateral disease and suitability for contrast or intervention.
- Interpretation and limitations
- A normal creatinine does not exclude unilateral obstruction, while rapid deterioration increases decompression urgency.
- 06
Bladder scan - Why
- Detect retention as a distal cause of bilateral upper-tract dilatation.
- Interpretation and limitations
- Interpret residual with voided volume, symptoms and neurological context; repeated large residuals require action.
04Clinical next stepsHow the result changes management or prompts escalation.
01Worked caseColic with hydronephrosisFirst stepA non-pregnant adult has acute flank-to-groin pain, microscopic haematuria and stable observations.+
- 1AlternativeContext: assess infection, renal function, solitary kidney, pregnancy possibility, analgesic need and alternative vascular or abdominal diagnoses.
- 2AlternativeReasoning: request urgent low-dose non-contrast CT to identify calculus, level, obstruction and an alternative cause.
- 3Outcome: uncomplicated small calculus follows the stone pathway; infection, renal impairment or uncontrolled pain prompts urgent urology.
- 4Verification: document symptom control, renal plan, return advice and responsibility for any incidental or obstructive finding.
02Haematuria routeImage the urotheliumVisible haematuria or another high-risk feature requires upper-tract tumour assessment.+
- 1Apply the current NICE referral criteria and assess infection, renal function, smoking, anticoagulation and prior urothelial cancer.
- 2AlternativeUse the specialist haematuria pathway to select CT urography or an alternative when contrast or radiation is unsuitable.
- 3Perform cystoscopy because CT urography cannot exclude every bladder mucosal lesion.
- 4DefinitiveTrack every filling defect, mass and indeterminate segment to definitive endoscopic, imaging or MDT resolution.
03Emergency obstructionDecompress the infected systemObstruction is accompanied by sepsis, anuria, acute kidney injury or a solitary threatened kidney.+
- 1Resuscitate, obtain cultures where feasible and start appropriate antibiotics without delaying source control.
- 2Contact urology immediately and identify the obstruction level with the fastest safe adequate imaging.
- 3Arrange ureteric stent or percutaneous nephrostomy according to anatomy, stability and specialist assessment.
- 4Confirm drainage, urine output, improving physiology and a later plan for the obstructing cause.
05Risks, monitoring and follow-upComplications, safety checks and further assessment.
- Trend temperature, pulse, blood pressure, urine output, creatinine, electrolytes and inflammatory markers in obstruction.
- After decompression, watch for post-obstructive diuresis and replace fluid or electrolytes according to clinical assessment.
- Ensure a stone, stricture, clot or tumour has a definitive follow-up plan after emergency drainage.
- Track haematuria CT, cystoscopy and cytology results to a final disposition rather than stopping after one normal test.
- Minimise repeated CT by comparing prior imaging and matching any surveillance to the defined disease pathway.
06Special situationsVariants, exceptions and circumstances that change the usual approach.
Dilatation is not obstruction
Pregnancy, reflux, high urine flow and extrarenal pelvis can mimic hydronephrosis; symptoms, ureter and renal function refine interpretation.
Obstruction may lack dilatation
Very early obstruction, dehydration or retroperitoneal encasement can impair drainage before obvious hydronephrosis develops.
CT protocols answer different questions
Unenhanced low-dose stone CT, multiphasic renal mass CT and excretory CT urography should never be treated as interchangeable labels.
Cystoscopy sees mucosa
Direct bladder inspection complements upper-tract imaging because flat carcinoma in situ and small mucosal lesions may be occult on CT.
Infection changes urgency
An otherwise manageable calculus becomes a source-control emergency when it obstructs an infected collecting system.
Contrast excretion can fail
Poor renal function or severe obstruction may produce an unopacified ureter; non-opacification is a limitation requiring targeted interpretation.
07Common pitfallsFrequent interpretation and management errors.
- 01
Calling hydronephrosis the final diagnosis without locating a cause.
- 02
Ordering CT urography for routine renal colic.
- 03
Using a normal ultrasound to exclude a ureteric stone or urothelial tumour.
- 04
Delaying drainage of infected obstruction for complete elective imaging.
- 05
Stopping haematuria investigation after normal upper-tract imaging without the complementary bladder pathway.