From the Guidelines
To determine the type of kidney stone a patient has, the most direct approach is stone analysis after collection, and if this is not possible, a noncontrast CT scan of the abdomen and pelvis can provide valuable information about stone size, location, and density, which can suggest stone composition 1. When a patient presents with nephrolithiasis, it is essential to determine the type of stone they have to guide treatment and prevention of recurrence.
- Stone analysis after collection is the most direct method, where patients strain their urine using a fine mesh strainer during an active stone passage to collect the stone for laboratory analysis.
- If a stone has already been removed surgically or passed naturally, it should be sent for comprehensive analysis.
- Blood tests, including serum calcium, phosphorus, uric acid, electrolytes, BUN, and creatinine, can identify metabolic abnormalities that may contribute to stone formation 1.
- A 24-hour urine collection provides valuable information about urine volume, pH, calcium, oxalate, citrate, uric acid, sodium, and creatinine levels, helping identify risk factors for specific stone types.
- Imaging studies like CT scans can sometimes suggest stone composition based on density, with calcium stones appearing more dense than uric acid stones 1.
- Urine pH testing is particularly useful, as persistently acidic urine (pH <5.5) suggests uric acid stones, while alkaline urine (pH >7.0) may indicate struvite or calcium phosphate stones.
- The use of low-dose noncontrast CT for stone assessment has been shown to be effective, with a pooled sensitivity of 97% and a pooled specificity of 95% 1.
- It is also important to note that ultrasonography is the imaging tool of choice in pregnant patients 1.
- Understanding stone composition is crucial for preventing recurrence, as treatment strategies differ significantly between calcium oxalate, calcium phosphate, uric acid, struvite, and cystine stones.
From the Research
Testing for Nephrolithiasis
To determine the type of stone a patient has, several testing methods can be employed:
- Non-contrast CT of kidneys, ureters, and bladder (CT KUB) is considered the gold standard for detecting stones, with a sensitivity of over 99% 2
- Ultrasound KUB can be used as an alternative, especially when CT KUB is contraindicated or not available, but it has limited sensitivity, particularly for ureteric stones 2, 3
- Plain film KUB can be used for follow-up of radiopaque stones 2
Stone Size and Type Determination
The size of the stone can be determined using:
- CT KUB, which can accurately measure stone size 2
- Ultrasound KUB, but it may overestimate stone size, particularly for stones in the 0-10 mm range 3
- The type of stone can be inferred based on its size, location, and radiopacity, but further testing may be necessary to confirm the stone composition
Imaging Techniques for Nephrolithiasis
Different imaging techniques have their own advantages and limitations:
- Non-contrast CT is highly sensitive and accurate for detecting stones, but it involves radiation exposure 2, 4
- Ultrasound KUB is a non-invasive and low-cost option, but it has limited sensitivity and may not accurately measure stone size 2, 3, 4
- Plain abdominal film of kidney, ureter, and bladder (KUB) can be used for follow-up, but it has limited sensitivity and specificity 3
Special Considerations
In certain populations, such as pregnant women or children, special considerations must be taken into account: