How to Diagnose Kidney Stones
Begin with ultrasound as the primary diagnostic imaging tool, followed by non-contrast CT scan if ultrasound is inconclusive or more detailed stone characterization is needed. 1, 2
Initial Clinical Assessment
Obtain a detailed medical history focusing on:
- Previous stone episodes, recurrence patterns, and past urological surgeries 1
- Family history of kidney stones (increases risk 2.57-fold) 3
- Dietary habits including fluid intake, sodium consumption, and calcium intake 1, 4
- Presence of risk factors: obesity, hypertension, diabetes, metabolic syndrome 3, 5
- Symptoms: renal colic, flank pain, hematuria, dysuria, urinary frequency 6, 5
- Age and sex (men over 40 are highest risk) 3
Immediate evaluation is mandatory for:
- Patients with solitary kidney 1
- Fever (suggests infection/obstruction) 1
- Uncertainty about diagnosis of renal colic 1
Diagnostic Imaging Algorithm
First-Line: Ultrasound
Ultrasound should be performed first and must not delay emergency care. 1, 2
- Sensitivity: 45% for detection 1, 2
- Specificity: 94% for ureteral stones, 88% for renal stones 1, 2
- Advantages: No radiation, identifies hydronephrosis, safe in pregnancy and children 1
Second-Line: Non-Contrast CT
Non-contrast CT is the standard follow-up modality after ultrasound for acute flank pain. 1, 2
- Sensitivity: 93.1%, Specificity: 96.6% 1, 2
- Use low-dose CT protocols to minimize radiation while maintaining diagnostic accuracy 1, 2
- Provides precise information on stone location, size, burden, density, and anatomy 1
- Can classify stones by composition based on density and structure 1
Adjunctive: KUB Radiography
Plain abdominal radiography (KUB) helps differentiate radiopaque from radiolucent stones and aids follow-up. 1
- Sensitivity: 44-77% 1
- Useful for tracking stone passage over time 1
- However, KUB alone has limited diagnostic value (appropriateness rating 3/10) 1
Special Populations
Pregnant patients: Use ultrasound first-line, then MRI second-line, and low-dose CT only as last resort 1, 2
Children: Ultrasound first-line, followed by KUB or low-dose CT if ultrasound insufficient 1, 2
Laboratory Evaluation
Initial Blood Tests (All Patients)
Every patient requires: 1
- Serum creatinine (assess renal function) 1, 2
- Ionized calcium (screen for hyperparathyroidism) 1, 2
- Uric acid 1, 2
- Sodium and potassium 1
- Complete blood count 1
- C-reactive protein (if infection suspected) 1
Obtain serum intact parathyroid hormone if primary hyperparathyroidism is suspected (high or high-normal calcium). 1, 2
Initial Urine Tests (All Patients)
Perform urinalysis with dipstick and microscopic examination: 1, 2
- Assess pH 2
- Identify infection indicators 2
- Detect crystals suggesting stone type 2
- Evaluate for hematuria 6
Obtain urine culture if infection is suspected. 2
Stone Analysis
Stone composition analysis should be performed at least once for all first-time stone formers, ideally for all stone formers. 1, 2, 4
- Identifies specific metabolic abnormalities (uric acid, cystine, struvite) 1
- Directly guides preventive therapy 4
- Instruct patients to strain urine and save passed stones 4
Advanced Metabolic Testing
Indications for 24-Hour Urine Collection
Perform metabolic testing in: 1, 2
- All recurrent stone formers 1, 2
- High-risk first-time stone formers 1, 2
- Interested first-time stone formers 1
- Patients with family history, one kidney, malabsorption, or intestinal disease 5
24-Hour Urine Parameters
Collect one or two 24-hour urine samples (two preferred) analyzing: 1, 2
- Total volume 1, 2
- pH 1, 2
- Calcium 1, 2
- Oxalate 1, 2
- Uric acid 1, 2
- Citrate 1, 2
- Sodium 1, 2
- Potassium 1, 2
- Creatinine 1, 2
- Cystine (if cystine stones known/suspected or family history of cystinuria) 1, 2
Suspect primary hyperoxaluria when urinary oxalate exceeds 75 mg/day in adults without bowel dysfunction. 1
Genetic Testing
Consider genetic testing (next-generation sequencing) for: 1, 2
- Children and adults aged ≤25 years 1, 2
- Adults >25 years with suspected inherited/metabolic disorders 1
- Patients with ≥2 stone episodes 1, 2
- Bilateral stone disease 1, 2
- Strong family history 1, 2
Genetic testing must be combined with metabolic evaluations and pre-test counseling. 1
If genetic diagnosis confirmed, offer family screening. 1
Stone Burden Quantification
Review imaging to quantify total stone burden: 1, 2
- Multiple or bilateral stones indicate higher recurrence risk 1, 2
- Nephrocalcinosis suggests underlying metabolic disorder (renal tubular acidosis, primary hyperparathyroidism, primary hyperoxaluria) or medullary sponge kidney 1, 2
Common Pitfalls
Do not restrict calcium intake - paradoxically increases stone risk; recommend 1,000-1,200 mg/day dietary calcium 1, 4
Do not rely solely on KUB radiography - insufficient sensitivity and may miss radiolucent stones 1
Do not skip stone analysis - composition directly determines prevention strategy 1, 4
Remember 26% of first-time stone formers recur within 5 years - emphasizing importance of identifying underlying causes 3