What is the appropriate workup for a suspected kidney stone in an adult?

Workup for Suspected Kidney Stone

Begin with renal ultrasonography as the first-line imaging modality for all patients with suspected kidney stones, followed by low-dose non-contrast CT if ultrasound is inconclusive or clinical suspicion remains high. 1

Initial Imaging Strategy

First-Line: Renal Ultrasonography

• Ultrasound should be performed first in all patients with suspected nephrolithiasis, providing approximately 45% sensitivity and 94% specificity for ureteral stones and 88% specificity for renal stones. 1
• This approach minimizes radiation exposure while effectively detecting hydronephrosis and large stones (>5 mm with nearly 100% sensitivity). 2
• Ultrasound must not delay emergency care in unstable patients. 1

Second-Line: Low-Dose Non-Contrast CT

• If ultrasound is inconclusive or detailed anatomic information is required, proceed with low-dose non-contrast CT of the abdomen and pelvis, which is the reference standard with 93-97% sensitivity and 96.6% specificity. 2, 1
• Low-dose protocols maintain diagnostic accuracy (93.1% sensitivity, 96.6% specificity) while significantly reducing radiation exposure compared to conventional CT. 2, 1
• Non-contrast CT precisely measures stone size and location, which are critical for determining management (stones <5mm typically pass spontaneously). 2

Avoid These Imaging Pitfalls

• Do not use contrast-enhanced CT as first-line imaging—contrast obscures stones in the collecting system and provides no diagnostic advantage over non-contrast CT for stone detection. 1
• Plain KUB radiography has limited sensitivity (44-77%) and is more useful for follow-up of known radiopaque stones rather than initial diagnosis. 1

Laboratory Evaluation

Immediate Urinalysis

• Perform bedside dipstick urinalysis in all patients to detect hematuria, assess urine pH, identify signs of infection, and reveal crystals that may indicate stone type. 1
• Note that hematuria may be absent in up to 15% of patients with confirmed stones, so its absence does not exclude the diagnosis. 3

Serum Chemistry Panel

• Obtain basic serum biochemistry including creatinine, uric acid, ionized calcium, sodium, potassium, and complete blood count to identify metabolic abnormalities and assess for acute kidney injury. 1
• Add C-reactive protein if infection is suspected or if interventional procedures are planned. 1

Urine Culture

• Order urine culture when urinalysis suggests infection or the patient has recurrent UTI history, as infection with obstruction constitutes a urologic emergency requiring immediate decompression. 1

Stone Analysis

• Analyze the composition of any retrieved stone from a first-time stone former when material is available, as results direct preventive therapy and help identify underlying metabolic disorders. 1
• Instruct patients to strain their urine to capture passed stones for analysis. 1

Special Population Considerations

Pregnant Patients

• Use ultrasound as the initial imaging modality, MRI as second-line, and reserve low-dose CT only when other modalities are insufficient to minimize fetal radiation exposure. 2, 1
• Physiologic hydronephrosis occurs in >80% of pregnant patients (more commonly on the right), which can confound interpretation. 2

Pediatric Patients

• Begin with ultrasonography; if inadequate, proceed with KUB radiography or low-dose non-contrast CT. 1
• Obtain thorough family history including stone disease, hearing loss, and metabolic disorders. 2

High-Risk Patients Requiring Genetic Testing

• Offer genetic testing to patients ≤25 years old, those with suspected inherited disorders, recurrent stones (≥2 episodes), bilateral disease, or strong family history, combined with metabolic evaluation and preceded by genetic counseling. 1

Red-Flag Situations Requiring Emergency Intervention

Immediate Urologic Consultation Needed For:

• Fever or signs of infection with urinary obstruction—mandates urgent decompression via percutaneous nephrostomy or ureteral stent. 1
• Obstruction in a solitary kidney—requires immediate evaluation. 1
• Bilateral obstruction or anuria—constitutes a urologic emergency. 1
• Intractable pain or vomiting despite adequate analgesia—may require urgent intervention. 4

Management of Infected Obstructed Stone

• Collect urine for culture before and after decompression, start empiric antibiotics immediately, and adjust based on culture results. 1
• Definitive stone treatment must be postponed until sepsis is resolved. 1

Metabolic Workup for Recurrence Prevention

Indications for 24-Hour Urine Collection

• High-risk first-time stone formers (family history, young age, solitary kidney, occupational risk). 1
• All recurrent stone formers (≥2 episodes). 1
• Patients with bilateral or multiple stones. 1

Parameters to Analyze

• Total urine volume, urine pH, calcium, oxalate, uric acid, citrate, sodium, potassium, and creatinine. 1