What is the diagnostic approach for a patient suspected of having kidney stones?

Diagnostic Approach for Kidney Stones

Non-contrast CT of the abdomen and pelvis is the gold standard imaging test for suspected kidney stones, with sensitivity and specificity exceeding 95%, and should be performed after initial ultrasound screening. 1, 2

Initial Clinical Evaluation

Perform a focused history and physical examination immediately upon presentation:

• Assess for fever, which indicates infected obstructing stone or pyelonephritis requiring urgent urologic intervention 1, 3
• Check for costovertebral angle tenderness and flank pain characteristics 3
• Identify high-risk features: solitary kidney, anuria/oliguria, signs of sepsis (tachycardia, hypotension, altered mental status) 3
• Elicit stone history, family history, dietary habits (fluid intake, sodium, protein, calcium, oxalate-rich foods), and stone-provoking medications 1

Laboratory Workup

Order the following tests for all patients with suspected kidney stones:

• Urinalysis with dipstick and microscopy to assess pH, hematuria, crystalluria, leukocyte esterase, and nitrites 1, 3
• Serum creatinine, uric acid, ionized calcium, sodium, potassium 1, 2
• Complete blood count and C-reactive protein 1, 3
• Urine culture if infection is suspected based on urinalysis findings (positive leukocyte esterase, nitrites, or pyuria) 1, 3

Imaging Algorithm

First-Line Imaging

Ultrasound is the primary initial diagnostic tool but has limited sensitivity (45% for ureteral stones, higher for renal stones) with 94% specificity for ureteral stones and 88% for renal stones. 1, 3

• Ultrasound should not delay emergency care in unstable patients 1
• Ultrasound is mandatory as first-line in pregnant patients to avoid radiation 1
• For children, ultrasound is strongly recommended as first-line imaging 1

Second-Line Imaging (Definitive)

Non-contrast CT of the abdomen and pelvis is the standard modality after ultrasound for acute flank pain, providing:

• 97% sensitivity and 95% specificity for stone detection 2
• Precise stone size, location, density, and composition assessment 1, 2
• Detection of secondary signs: ureteral dilatation, perinephric stranding, periureteral inflammation 1, 2
• Identification of alternative diagnoses if stones are ruled out 1

Use low-dose CT protocols to reduce radiation exposure while maintaining 93.1% sensitivity and 96.6% specificity 1

Alternative Imaging Options

Plain radiography (KUB) has limited utility:

• 44-77% sensitivity, cannot reliably detect radiolucent stones (uric acid, cystine) 1
• Combining KUB with ultrasound improves sensitivity to 79-90%, providing an acceptable alternative to CT in select patients 1, 2
• KUB is useful for differentiating radioopaque from radiolucent stones and aids in follow-up of known stones 1

MRI/MR urography may be appropriate when:

• CT is contraindicated and ultrasound is inconclusive 1
• Pregnant patients require second-line imaging after ultrasound 1
• Poor accuracy for detecting small stones (<3mm) is a significant limitation 1

Special Populations

Pregnant Patients

Use ultrasound as first-line imaging (appropriateness rating 8/9) 1

• MRI without contrast is second-line if ultrasound is inconclusive 1
• Low-dose CT is last-line option only when diagnosis remains uncertain and management depends on imaging 1

Recurrent Stone Formers

Limit radiation exposure by:

• Using ultra-low-dose CT protocols 2
• Targeting CT to the area of clinical concern rather than full abdomen/pelvis 2
• Alternating with ultrasound for routine surveillance when clinical suspicion is low 2

High-Risk Patients Requiring Metabolic Evaluation

Consider genetic testing with next-generation sequencing for:

• Children and adults ≤25 years old 1
• Adults >25 years with suspected inherited/metabolic disorders 1
• Patients with recurrent stones (≥2 episodes), bilateral disease, or strong family history 1

Perform comprehensive 24-hour urine collection for metabolic evaluation in high-risk patients 1

Critical Pitfalls to Avoid

• Do not rely on ultrasound alone for definitive diagnosis—its 45% sensitivity for ureteral stones will miss significant pathology 1, 3
• Do not use KUB as a standalone test—it has poor sensitivity (53-62%) even for radioopaque stones 4
• Do not delay imaging in patients with fever, solitary kidney, or signs of sepsis—these require immediate evaluation 1, 3
• Do not skip stone composition analysis on first-time stone formers—this guides prevention strategies 1
• Do not order intravenous urography—this modality is obsolete and cannot guide alternative diagnoses 1

Stone Classification After Diagnosis

Classify stones by:

• Size: <5mm, 5-10mm, 10-20mm, >20mm (determines treatment approach) 1
• Location: upper/middle/lower calyx, renal pelvis, upper/middle/distal ureter, bladder 1
• Composition: determined by stone analysis (calcium oxalate, calcium phosphate, uric acid, struvite, cystine) 1
• Density on CT: helps predict composition and treatment response 1