Fact-sheet: Urolithiasis - Renal colic


Updated on 06/25/2026 at 9:23 AM

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Definition

Urinary lithiasis (or urolithiasis) is defined by the presence of calculi at any point in the urinary tract (kidneys, ureters, bladder). Its worldwide prevalence is steadily increasing and now approaches 10%. The lifetime incidence is 5% in women and 12% in men.

Renal colic results from the migration of a calculus into the ureter, causing distension of the excretory tract.

Clinical features

  • Renal colic
  • Pain: Typically flank pain, sudden and severe. In children, it is often vaguer.
  • Hematuria: Very common, but may be absent in approximately 15% of cases.
  • Signs of severity: Fever (obstructive pyelonephritis), oligoanuria, or intractable pain.

Laboratory findings

  • Risk factors: Low fluid intake, malformations (horseshoe Kidney, duplex collecting system), infections with urease-producing bacteria (Proteus, Klebsiella).
  • Metabolic abnormalities: Hypercalciuria is the most common.
  • Influence of pH: Uric acid and cystine stones form in acidic urine (pH < 5), whereas struvite is associated with alkaline urine.

Radiography

Plain abdominal radiography looks for a radiopaque stone overlying the urinary tract.

  • Performance: Sensitivity estimated at approximately 60%.
  • Radiopacity by composition: Radiopaque: Calcium oxalate/phosphate, struvite (variable).
  • Faintly opaque: Cystine ("ground-glass" appearance).
  • Radiolucent: Uric acid, drug-induced stones (Indinavir), matrix stones.

Ultrasound

Ultrasound is often the first-line examination (children, pregnant women) despite overall poor sensitivity (~45% in general, but dropping to 24% compared with CT).

  • Direct signs: Echogenic focus with posterior acoustic shadowing and "twinkle artifact" on color Doppler (high velocity scale). Look for stones at the ureterovesical junction ++ or within the bladder
  • Indirect signs: Dilation of the pelvicalyceal system and ureter proximal to the stone.
  • Resistive index: Increased by 0.1 on the obstructed side compared with the contralateral side; an elevated RI may precede dilation.
  • Ureteral jets: Decreased or absent (< 1.5 jets/min on the affected side).
  • Perirenal fluid collection
  • Limitations: 75% of undetected stones are < 3 mm. Dilation may be delayed by 6 to 24 hours after the obstruction.

CT

Unenhanced helical CT (low-dose CT KUB) is the reference examination.

  • Performance: Detects 99% of stones, except for Indinavir (protease inhibitor) stones, which remain radiolucent.
  • Specialized low-dose protocol (0.5 to 3.5 mSv) versus 8-16 mSv for a standard protocol. Coverage from the diaphragm to the pubic symphysis.
  • Signs of obstruction: Asymmetric dilation of the pelvicalyceal system, perirenal fat stranding, and "rim sign" (edema of the ureteral wall around the stone).
  • Density (Hounsfield Units - HU): Uric acid: 100-200 HU (or 360-650 depending on the series).
  • Cystine: ~650-850 HU.
  • Calcium oxalate dihydrate: 1000-1450 HU.
  • Brushite: 1550-1950 HU (highly resistant to lithotripsy).
  • Dual-Energy CT: Enables characterization of chemical composition and detection of stones obscured by contrast material.

MRI

MRI is not the examination of choice for the stone itself (signal void).

It is useful for:

  • Differential diagnosis (characterization of atypical cysts according to Bosniak).
  • Assessing complications (Urinoma) or suspicious soft-tissue masses mimicking a matrix stone.

Vascular intervention

Percutaneous nephrostomy: Indicated emergently to divert urine in cases of complicated renal colic (septic or anuric) or failed retrograde ureteral stent placement.

X-Ray Angiography/Embolization: May be required emergently to treat rare complications of extracorporeal lithotripsy, such as a hemorrhagic splenic or hepatic Hematoma.

Nuclear medicine

Renal scintigraphy (MAG 3 with furosemide test) is used to confirm the obstructive nature of a dilatation, particularly in the setting of ureteropelvic junction syndrome associated with stones.

Management

Prognosis for spontaneous passage:

  • Stone < 5 mm: 68% to 90% chance of passage.
  • Stone > 8 mm: near-zero chance, urologic treatment required.

Strategy: low-dose CT as first-line, or KUB/ultrasound combination in children, pregnant women, or for follow-up.

Contrast injection: Indicated if unenhanced CT does not explain the symptoms (search for differential diagnoses) or in cases of febrile renal colic without oligo-anuria.

Classification

CLAFU (TNM classification of stones):

  • T: Topography/location.
  • N: Chemical nature.
  • M: Measurement (mm along the two largest axes).

Bosniak: Used to classify renal cysts, but cannot be applied in the setting of acute urologic conditions (trauma, hemorrhage).

Differential diagnosis

Non-lithiasic renal colic (20%): Clots, urothelial tumors, extrinsic compression (lymphadenopathy, endometriosis).

Abdominal calcifications:

  • Phleboliths (often showing a "comet tail" sign, unlike ureteral stones which show a "rim sign"),
  • Cholelithiasis and Gallbladder lithiasis,
  • Calcified mesenteric lymph nodes.

Extra-urinary emergencies:

  • Appendicitis,
  • Diverticulitis of colon,
  • Renal infarctus,
  • Gynecologic causes.