What are the recommended diagnostic tests and treatment options for patients suspected of having kidney stones?

Diagnostic Approach to Kidney Stones

Non-contrast CT (NCCT) of the abdomen and pelvis is the gold standard first-line imaging test for suspected kidney stones, with 95-97% sensitivity and 95% specificity, and should be performed using low-dose protocols (<3 mSv) to minimize radiation exposure. 1, 2, 3

Initial Imaging Strategy

First-Line Test: Non-Contrast CT

• NCCT is the reference standard because it detects virtually all renal calculi regardless of composition, provides precise stone size and location measurements, and identifies complications like hydronephrosis and perinephric inflammation 1, 2
• Low-dose protocols are mandatory and maintain pooled sensitivity of 97% and specificity of 95% while reducing radiation exposure to <3 mSv 2, 3
• Technical optimization requires thin (1-1.5 mm) axial slices, coronal reformations, bone window settings, and magnified views for accurate stone detection and measurement 2
• Stone size and location directly determine management: smaller, more proximal stones typically pass spontaneously, while larger distal stones require intervention 1, 2

Alternative First-Line: Ultrasound (Specific Populations)

• Ultrasound is the imaging modality of choice for pregnant patients due to radiation concerns, with reasonable sensitivity for hydronephrosis (up to 100%) though lower sensitivity (24-57%) for direct stone visualization 1, 3
• Combining ultrasound with plain radiography (KUB) achieves 79-90% sensitivity for detecting clinically significant stones, providing an acceptable alternative to CT in select patients 1, 3
• Color Doppler with twinkling artifact assessment can improve sensitivity for small renal stones 3

Common Pitfalls to Avoid

• Never use contrast-enhanced CT as first-line imaging because enhancing renal parenchyma obscures stones within the collecting system, potentially missing small calculi 2, 3
• Do not rely solely on ultrasound for small stones (<5 mm) as sensitivity decreases significantly for stones in this size range 3
• Avoid plain radiography (KUB) alone as it has limited sensitivity (29% overall, 72% for stones >5 mm in proximal ureter) and narrow capabilities for visualizing different etiologies 1, 3
• Do not order MRI for routine stone evaluation as it has limited utility in stone detection, though it can identify hydronephrosis and perinephric edema when radiation must be avoided 1

When Initial CT is Inconclusive

If NCCT cannot definitively distinguish a ureteral stone from mimics (phleboliths, arterial calcifications):

• CT urography (CTU) with IV contrast confirms ureteral location by opacifying the urinary tract and can better delineate the "soft tissue rim" sign to differentiate stones from phleboliths 1
• Contrast-enhanced CT provides additional diagnostic information in 5-18% of cases but changes management in only 2-3% 1
• MRI or MR urography without contrast can serve as an alternative when further radiation exposure must be avoided 1

Special Clinical Scenarios

Recurrent Stone Disease

• NCCT remains the test of choice but limit scanning to the area of interest or use ultra-low-dose protocols to reduce cumulative radiation exposure 1, 3
• Review previous imaging to track stone location and number 1
• Plain radiography (KUB) may be appropriate for tracking large radiopaque stones depending on composition and patient body habitus 1

Pregnant Patients

• Ultrasound of kidneys and bladder is the initial modality with appropriateness rating of 8 (usually appropriate) 1
• MR urography without IV contrast may be appropriate if ultrasound is inconclusive, though sensitivity for stone impaction (69%) is lower than CT (100%) 1
• Low-dose NCCT may be considered in second and third trimesters when ultrasound is non-diagnostic, though radiologists prefer CT over MRI (35% vs 20% in second trimester, 48% vs 18% in third trimester) 1

Metabolic Evaluation

• 24-hour urine assessment and metabolic testing are recommended for high-risk patients: those with family history of stones, solitary kidney, malabsorption, intestinal disease, or recurrent stones 4, 5
• Urine pH and stone composition analysis guide prevention strategies specific to stone type (calcium oxalate, calcium phosphate, uric acid, cystine, struvite) 5

Imaging Modalities to Avoid

• Intravenous urography (IVU) has lower sensitivity (87%) and specificity (94%) compared to NCCT and cannot guide toward alternative diagnoses if stones are ruled out 1, 3
• CT abdomen and pelvis without and with IV contrast offers no additional benefit over non-contrast CT alone for kidney stone evaluation 1, 2
• Abdominal radiography alone delivers smaller radiation than CT but has narrow diagnostic capabilities and appropriateness rating of 3 (usually not appropriate), potentially leading to repeat imaging 1