Non-contrast CT is the most sensitive test for stones, but management hinges on recognising the obstructed, infected kidney (an emergency) and — for recurrent or high-risk formers — a metabolic work-up built on stone analysis and the 24-hour urine.
Imaging
| Modality | Sensitivity | Specificity | Notes |
|---|---|---|---|
| Plain film (KUB) | 57% | 76% | Cheapest, low radiation (~0.7 mSv); misses small stones and underestimates >90% of stones >10 mm |
| Ultrasound | 84% | 53% | No radiation (~5× KUB cost); misses most ureteric stones; poor size correlation (underestimates <10 mm, overestimates >10 mm) |
| Non-contrast CT | 95% | 98% | Most sensitive; low-dose ~3 mSv, standard ~10 mSv (~10× KUB cost). Uric acid stones have much lower Hounsfield units than calcium |
| MRI | 82% | 98% | No radiation; stones appear as filling defects; most expensive (~30× KUB) |
Radiolucent stones (invisible on KUB): uric acid, matrix, and the medication stones (xanthine, triamterene, 2,8-dihydroxyadenine, indinavir). Radio-opaque: calcium oxalate and calcium phosphate (densest). Poorly radio-opaque: struvite and cystine.
Forniceal extravasation (usually from a small distal ureteric stone) is managed like any ureteric stone — intervene for fever, vomiting, or unrelenting pain, otherwise observe.
Nephrocalcinosis — diffuse renal calcium deposition, usually medullary (type 1 RTA, hyperparathyroidism, medullary sponge kidney, hypervitaminosis D, milk-alkali syndrome, sarcoidosis) or, less commonly, cortical (cortical necrosis, primary hyperoxaluria/oxalosis, Alport syndrome, chronic glomerulonephritis).
Acute Presentation and Management
New urgency/frequency suggests a stone at the UVJ irritating the bladder; sudden relief of flank pain suggests stone passage or forniceal rupture. Examine for costovertebral-angle tenderness and check urinalysis ± culture, CBC, and creatinine.
- Analgesia — ketorolac 30 mg IV, IV lidocaine 1.5 mg/kg (max 200 mg) over 10 minutes, acetaminophen 1000 mg PO, and a fluid bolus.
- Obstruction with suspected infection is an emergency — urgently drain the collecting system (ureteric stent or percutaneous nephrostomy) and defer definitive stone treatment until sepsis is controlled and antibiotics are completed.
Metabolic Evaluation
Screening evaluation (all newly diagnosed stones, AUA): history and physical, laboratory (urinalysis ± culture, serum electrolytes, calcium, creatinine, uric acid), and imaging to quantify stone burden. Obtain a stone analysis at least once — a calcium phosphate composition points to type 1 RTA, primary hyperparathyroidism, medullary sponge kidney, or carbonic anhydrase inhibitors.
Extended evaluation — one or two 24-hour urine collections, indicated for recurrent formers, family history, solitary kidney, malabsorptive intestinal disease, obesity, recurrent UTIs, predisposing conditions (RTA type 1, primary hyperparathyroidism, gout, diabetes), anatomic abnormalities, cystine/uric acid/struvite stones, and children. Analyse at minimum volume, pH, creatinine, sodium, potassium, calcium, oxalate, uric acid, and citrate (add cystine if suspected). First-time formers have a ~50% recurrence risk within 10 years.
- Collection adequacy is judged by 24-hour creatinine (males 20–25 mg/kg, females 15–20 mg/kg). Primary hyperoxaluria is suspected when urinary oxalate exceeds 75 mg/day without bowel dysfunction — refer for genetic testing.
- Urine pH — normal 5.8–6.2; >7.0 suggests infection lithiasis or RTA; <5.5 suggests uric acid stones / gouty diathesis.
- PTH — check if primary hyperparathyroidism is suspected (mid-range PTH with high/high-normal calcium, hypercalciuria, calcium phosphate stones); low vitamin D can mask it.
- Diet and drugs — assess fluid, protein, calcium, sodium, oxalate, and supplement intake; stone-provoking agents include triamterene, carbonic anhydrase inhibitors (topiramate, acetazolamide, zonisamide), probenecid, protease inhibitors, lipase inhibitors, chemotherapy, and high-dose vitamin C/D. Roux-en-Y bypass raises stone risk, whereas restrictive bariatric surgery (sleeve, band) does not.
Self-Test
1. What is the 10-year recurrence risk in first-time stone formers? About 50%.
2. What are the characteristic urine-crystal appearances of the common stones? Calcium oxalate — envelope or dumbbell; uric acid — rhomboid/rosette; cystine — hexagonal; struvite — coffin-lid; calcium phosphate — amorphous. (Filled from standard crystal morphology — see Corrections — as the source left this answer blank.)