Should I order a kidney, ureter, and bladder (KUB) ultrasound or a computed tomography (CT) scan for a patient with a urinary tract infection (UTI) and suspected kidney stones?

From the Guidelines

For suspected urinary tract infection (UTI) and kidney stones, I recommend ordering a kidney and bladder US as the initial diagnostic test, reserving KUB for cases where US findings suggest the presence of radiopaque stones or when clinical suspicion remains high despite negative US findings. This approach is supported by the American College of Radiology's 2023 guidelines for acute onset flank pain-suspicion of stone disease (urolithiasis) 1. The use of US as the first-line imaging modality is justified by its high sensitivity (up to 100%) and specificity (up to 90%) for diagnosing ureteral obstruction, as well as its ability to detect hydronephrosis and perinephric fluid 1.

While US may have limited sensitivity for detecting smaller stones (<5mm) compared to noncontrast CT, the addition of color Doppler and assessment of twinkling artifact can improve its sensitivity, particularly for small renal stones 1. Furthermore, US avoids radiation exposure, making it a safer initial choice, especially for pregnant patients or those with concerns about radiation.

If the US findings are inconclusive or suggest the presence of radiopaque stones, a KUB X-ray can be ordered as a complementary test to improve diagnostic accuracy 1. The combination of US and radiography has been shown to yield a sensitivity of 90% and specificity of 68% for detecting stones, although detection rates may be decreased for stones <5mm 1. Ultimately, the choice of imaging modality should be guided by the clinical presentation and suspicion of stone disease, with noncontrast CT reserved for cases with high clinical suspicion and negative or inconclusive US and KUB findings.

Key benefits of this approach include:

• High sensitivity and specificity for ureteral obstruction and hydronephrosis
• Avoidance of radiation exposure with US as the initial test
• Complementary use of KUB for detecting radiopaque stones
• Reserved use of noncontrast CT for complex or high-suspicion cases.

From the Research

Imaging Options for UTI and Suspected Stones

• The decision to order a kidney and bladder ultrasound (US) or a Kidney, Ureter, and Bladder (KUB) X-ray for a patient with a urinary tract infection (UTI) and suspected stones depends on various factors, including the clinical presentation and the need to identify stone disease or exclude other pathology 2.
• A study published in 2009 found that both plain X-ray KUB and US should be performed in patients with suspected stone disease to identify stone disease and exclude other pathology 2.
• Another study published in 2004 compared the diagnostic accuracy of US and unenhanced CT for stone and obstruction in patients with renal failure, and found that US is sensitive and specific for renal stones, but its sensitivity for ureteric stones is low 3.
• The addition of X-ray KUB to US increases the sensitivity for ureteric stones to 77% 3.

Ultrasonography for Renal Stone Detection

• A study published in 2017 found that the sensitivity and specificity of US for detecting renal calculi were 54% and 91%, respectively, and that US significantly overestimated the size of stones in the 0-10 mm range 4.
• The use of US to guide clinical decision-making for residual or asymptomatic calculi is limited by low sensitivity and inability to size the stone accurately, and may lead to inappropriate counseling in up to 22% of patients 4.

Clinical Presentation and Diagnosis

• The diagnosis of uncomplicated UTI is usually made based on clinical presentation, including dysuria, frequency, and/or urgency without fever for cystitis, and fever and back pain/costovertebral angle tenderness for pyelonephritis 5.
• Pyuria is usually present with UTI, regardless of location, and its absence suggests that another condition may be causing the patient's symptoms 5.

Imaging Modalities

• Various imaging modalities are available for diagnosing and surveilling kidney stones, including renal bladder ultrasound, KUB plain film radiography, magnetic resonance imaging (MRI), and non-contrast computerized tomography (NCCT) 6.
• Each of these modalities has a unique role in the acute workup and management of suspected nephrolithiasis, and the choice of modality depends on the clinical setting and the need to identify stone disease or exclude other pathology 6.