Tests
1st tests to order
urinalysis
Test
Microhematuria is seen in the majority of patients with renal stones. Do not diagnose microhematuria solely on the results of macroscopic urinalysis via a urine dipstick test. Instead, urine microscopy should be performed to confirm microhematuria with a definition of three or more red blood cells per high-powered field.[50] A urine pH may be helpful for diagnosing the type of stone or underlying cause.
Result
may be normal; dipstick positive for leukocytes, nitrates, blood; microscopic analysis positive for WBCs, RBCs, or bacteria. pH >7 suggests presence of urea-splitting organisms, such as Proteus, Pseudomonas, or Klebsiella species, and struvite stones; pH <5.5 suggests uric acid stones.
CBC and differential
Test
An elevated WBC may suggest infection (pyelonephritis or urinary tract infection).
Result
variable
serum electrolytes, BUN, and creatinine
Test
These include sodium, potassium, chloride, bicarbonate, creatinine, BUN, calcium, uric acid, and phosphorus.
Hypercalcemia may suggest hyperparathyroidism as an underlying etiology; hyperuricemia may indicate gout.
Result
variable
urine pregnancy test
Test
Prior to exposure to ionizing radiation.
To exclude ectopic pregnancy.
Result
negative
noncontrast helical CT scan
Test
Noncontrast helical computed tomography scan (NCCT) is the preferred imaging modality for nephrolithiasis due to its high sensitivity and specificity, and should be ordered as soon as nephrolithiasis is suspected.[53]
A low-dose scan (<4 mSv) is preferred for patients with a body mass index (BMI) ≤30 kg/m², as this imaging study limits the potential radiation exposure while maintaining both sensitivity and specificity at 90% or higher. However, low-dose computed tomography (CT) is not recommended for those with a BMI >30 kg/m², owing to lower sensitivity and specificity in these patients.[54] A size-adjusted, reduced-dose CT protocol has been shown to be 96% sensitive for the detection of ureteral stones requiring intervention in all patients, regardless of BMI.[68] Do not order IV contrast-enhanced CT alongside NCCT to detect stones because there is no additional clinical benefit and there is an increased exposure to radiation.[53][55]
NCCT accurately determines presence, size, and location of stones; if negative, nephrolithiasis can be ruled out with high likelihood.
Radiation doses of <50 mGy have not been associated with increased risk of fetal anomalies or loss, therefore, low-dose protocol CT (<4 mGy) can be used as a last-line option in pregnant women after the first trimester to aid in difficult-to-diagnose cases.[48][54][62][63]
Do not routinely use CT to evaluate children with suspected nephrolithiasis because of the link between radiation exposure and increased risk of cancer. Instead, renal ultrasound should be ordered as the initial imaging test.[50]
Result
calcification seen in renal collecting system or ureter; hydronephrosis; perinephric stranding (indicative of inflammation or infection)
stone analysis
Test
Provides information on chemical composition and etiology. Stones are analyzed after they are extracted during surgery or when patients expel and collect them for analysis.
Result
stone composition
Tests to consider
plain abdominal radiograph (KUB)
Test
Plain abdominal film could be ordered initially along with computed tomography (CT) scan to determine whether stone is radiolucent. Calcium oxalate and calcium phosphate stones are radiopaque, whereas pure uric acid and indinavir stones are radiolucent and cystine stones are partially radiolucent.
A KUB x-ray should be performed if the stone is not visible on a CT scout, so that patients with stones identifiable on initial KUB x-ray or CT scout can be followed by KUB.[54]
Before definitive surgical therapy, a KUB should be ordered in an asymptomatic patient to ensure that patient has not already passed the stone.
Result
calcification seen within urinary tract
renal ultrasound
Test
In pregnancy, renal ultrasound is the first-line imaging modality. It should also be the modality of choice when there is a desire to reduce or eliminate radiation exposure, such as for evaluation of children.[50][64] Low-dose CT can be considered in children if renal ultrasound is nondiagnostic.[48][54][64] Point-of-care ultrasound (POCUS) may have a role in screening symptomatic patients. One systematic review and meta-analysis assessing POCUS performed in the emergency department studied 1773 patients, and showed sensitivity and specificity of 70% and 75% respectively for diagnosing nephrolithiasis.[61] Moderate to severe hydronephrosis was shown to be highly specific for the presence of stones; any hydronephrosis is suggestive of a large stone (>5 mm), in those presenting with renal colic.[61] POCUS was not shown to be associated with any increased risk to the patient.[61] POCUS may therefore have a role in sparing patients with a negative POCUS from further imaging, although those with a positive test will likely need definitive imaging to guide further management.
Result
calcification seen within urinary tract, along with dilation
intravenous pyelogram (IVP)
Test
This test has for the most part been replaced by the computed tomography (CT) scan (the new diagnostic standard) for the evaluation and diagnosis of renal stones; however, it is still useful to assess renal function and collecting system drainage.
Result
calcification seen within urinary tract or a filling defect seen when dye is passing through the kidney and down the ureter
magnetic resonance imaging (MRI)
Test
Although conferring no radiation to the patient, MRI is a second-line imaging modality because stones are not directly visible on MRI and only seen as a filling defect in the collecting system. It can help to define the level of urinary tract obstruction.[48]
Result
filling defect seen in the collecting system
24-hour urine monitoring
Test
Helps in determining underlying metabolic cause or etiology for nephrolithiasis. Should be ordered once the patient is stone free.
Basic measurements should include volume, pH, creatinine, sodium, calcium, oxalate, uric acid, and citrate.
Patients with recurrent renal stones should have subsequent periodic 24-hour urine monitoring.
Result
increased or decreased values for urinary electrolytes; reduced urine volume
spot urine for cystine
Test
A urine screen for cystine should be considered if the diagnosis of cystinuria is not excluded by stone analysis.[2]
Result
cystinuria
Emerging tests
dual-energy CT
Test
Dual-energy CT is able to detect uric acid stones without needing stone analysis.[65] It may be able to replace stone analysis as the diagnostic test for these types of stones, and therefore potentially facilitate earlier initiation of treatment with urinary alkalinization.[65]
Result
visualization of uric acid stones
Use of this content is subject to our disclaimer