Laboratory Tests and Ancillaries
Serum chemistries should include electrolytes (eg sodium, potassium, chloride, bicarbonate, and calcium), creatinine, and uric acid to uncover hypokalemia or RTA. Parathyroid hormone (PTH) level should be measured if there is high normal or elevated serum and urine calcium concentration. The level of 25-hydroxy vitamin D should also be investigated to rule out the possibility of vitamin D deficiency in patients with elevated PTH. Urinalysis should include dipstick and microscopic evaluation (urinary pH, indicators of infection, and identification of crystals that are pathognomonic of stone type). Urine culture is recommended before surgical intervention.
A 24-hour urine collection or metabolic testing is the cornerstone for which the therapeutic recommendations are based. This may be used to monitor treatment and is recommended for patients at high risk of stone formation, first-time or recurrent stone formers. At least two samples are collected while consuming their usual diet and volume of fluid. Metabolic testing should analyze total volume, pH, calcium, oxalate, uric acid, citrate, sodium, potassium, and creatinine.
Urinary potassium measured at baseline can be compared to urinary potassium obtained during follow-up to gauge compliance with medication regimens. Urinary cystine should additionally be measured in stone formers with known cystine stones or a family history of cystinuria or for those in whom cystinuria is suspected. Primary hyperoxaluria should be suspected when urinary oxalate excretion exceeds 75 mg/day in adults without bowel dysfunction. These patients should be considered for referral for genetic testing and/or specialized urine testing.
Genetic testing using a Next Generation Sequencing (NGS) gene panel should be considered in children and adults when an inherited or metabolic disorder is clinically suspected as the underlying cause of kidney stone disease. Clinical features such as recurrent episodes (≥ 2 discrete events), bilateral disease, or a strong family history should prompt genetic evaluation.
Basic Laboratory Analysis in Emergency Cases
Basic laboratory analysis in emergency cases includes urine (eg dipstick test of spot urine sample, urine microscopy and/or culture), serum blood sample (eg creatinine, uric acid, ionized calcium, sodium and potassium), blood cell count, C-reactive protein (CRP), and coagulation test, if intervention is likely or planned.
Basic Laboratory Analysis in Non-emergency Cases
Biochemical work-up is similar for all patients, but if there is no planned intervention, then sodium, potassium, CRP, and blood coagulation time can be omitted.
Imaging
The choice of imaging modality will depend on the clinical situation of the patient. This is indicated for patients with fever, solitary kidney, or when diagnosis is doubtful. This is used to differentiate ureteral stones from renal stones. This may be performed to inform the selection of surgical treatment.
Ultrasound (US)
Urolithiasis_Diagnostics
Ultrasound is used as the primary diagnostic tool. This is a first-line imaging tool in patients with symptoms suggestive of a bladder stone. This identifies the presence of stones in the calices, pelvis, pyeloureteric, and vesicoureteric junctions, and in patients with upper urinary tract dilatation. Sensitivity is 45% and 94% specificity for detecting ureteral stones and 45% sensitivity with 88% specificity for renal stones. The advantages of ultrasound include absence of radiation, contrast-free, inexpensive, reproducible, and readily available.
Kidneys, Ureter, and Bladder (KUB) Radiography
KUB radiography is used to evaluate adults with persistent symptoms suggestive of a bladder stone when ultrasound findings are negative. This is helpful in differentiating radiolucent and radiopaque stones. This demonstrates sensitivity and specificity values between 44% and 77%. This is used for comparison during follow-up. This should not be performed if a non-contrast computed tomography (NCCT) scan is being considered.
Cystoscopy
Cystoscopy is used to evaluate adults with persistent symptoms suggestive of a bladder stone when ultrasound findings are negative.
Evaluation of Patient with Acute Flank Pain
Non-contrast Computed Tomography (NCCT) Scan
A non-contrast computed tomography (NCCT) scan is the first choice in confirming stone diagnosis in patients with acute flank pain. This provides accurate determination of stone location, overall burden, and density. This is the most sensitive and specific imaging procedure to diagnose urolithiasis. This is significantly more accurate than intravenous urethrogram/intravenous pyelogram in evaluating patients with acute urolithiasis. An NCCT scan is used to determine the diameter, density, inner structure, skin-to-stone distance, and surrounding anatomy, which collectively guide treatment selection. This is used to evaluate adults with persistent symptoms suggestive of a bladder stone when ultrasound findings are negative. This can detect uric acid and xanthine stones that are radiolucent on plain films. This should be performed prior to performing percutaneous nephrolithotomy (PNL). This may be performed if recurrent. A low-dose NCCT scan can reduce radiation risk and is used to detect ureteric stones in patients with a body mass index (BMI) of <30.