From the Guidelines
The best test for diagnosing nephrolithiasis (kidney stones) is a non-contrast CT scan of the abdomen and pelvis. This imaging test provides detailed pictures of the kidneys, ureters, and bladder without requiring contrast dye, making it both safe and highly accurate for detecting stones, with a reported sensitivity as high as 97% 1. The use of non-contrast CT allows for the detection of even small stones, as well as secondary signs of urolithiasis and complications such as periureteral and perinephric inflammation and ureteral dilatation.
Some of the key benefits of non-contrast CT include:
- High spatial resolution and ability for multiplanar reformations
- Ability to accurately detect stones as small as 1mm
- Ability to determine the exact location of the stone
- Ability to assess whether the stone is likely to pass naturally
- Low-dose non-contrast CT options are available, which can reduce radiation exposure, especially in young patients 1
Alternative tests, such as ultrasound and X-rays (KUB), may be used in certain situations, but they have limitations. Ultrasound is radiation-free and good for pregnant women or children, but it is less sensitive than CT. X-rays can detect calcium-containing stones but may miss uric acid stones. Urinalysis and blood tests may also be performed alongside imaging to check for blood in urine, signs of infection, kidney function, and calcium levels.
In terms of specific techniques, the use of coronal reformations, viewing on bone window, and use of magnified views can improve the accuracy of stone measurements 1. Additionally, low-dose CT has been shown to yield equivalent stone measurements compared with standard-dose CT, and stone location and size can be accurately depicted at non-contrast CT 1.
From the Research
Diagnostic Tests for Nephrolithiasis
- Computed Tomography (CT) is considered the gold standard for diagnosing nephrolithiasis due to its high accuracy in detecting stones and providing information on stone composition, size, burden, location, collecting system, and renal parenchyma 2, 3, 4, 5.
- However, CT scans expose patients to ionizing radiation, which is a concern, especially for patients who may require multiple scans throughout their lifetime 3, 4, 5.
- Ultrasonography (US) is a cheaper and safer alternative to CT, with no radiation hazard, but it has limited sensitivity and specificity, particularly for small stones (<3 mm) and in nondilated systems 2, 5, 6.
- The sensitivity and specificity of US for detecting renal calculi are around 45-54% and 88-91%, respectively, with a significant association between sensitivity and stone size 2, 6.
- US can overestimate stone size, particularly for stones in the 0-10 mm range, which can lead to inappropriate counseling for patients 6.
- Low-dose CT protocols exist and can provide good detection rates for all types of stones, even small ones, with the exception of small uric acid fragments, which may require higher energy settings 3.
- Other imaging modalities, such as plain abdominal film of kidney, ureter, and bladder (KUB) and MRI, have their own advantages and limitations, but are not as widely used or recommended as CT or US for diagnosing nephrolithiasis 4, 5.
Choosing the Best Test
- The choice of diagnostic test for nephrolithiasis depends on various factors, including the patient's symptoms, medical history, and the suspected location and size of the stone 4, 5.
- CT is generally preferred for its high accuracy and ability to provide detailed information on stone composition and location, but US may be used as a first-line imaging modality due to its low cost, lack of radiation, and ease of use 2, 5.
- In some cases, a combination of imaging modalities, such as US and KUB, may be used to increase sensitivity and accuracy 6.