A computed tomography (CT) scan uses X-rays from multiple angles to create detailed cross-sectional images of the body. This technology is the standard method for evaluating patients suspected of having kidney stones (urolithiasis). While CT scans are highly reliable, they can potentially miss a kidney stone under specific, rare circumstances, requiring an understanding of the scan’s capabilities and limitations.
The High Reliability of CT for Stone Detection
Computed tomography is considered the gold-standard imaging technique for diagnosing kidney stones. Studies show that a CT scan correctly identifies kidney stones more than 95% of the time, providing high sensitivity and specificity because CT excels at identifying dense structures, such as mineralized stones, regardless of their chemical makeup.
The preferred method for this diagnosis is a non-contrast CT (NCCT) of the abdomen and pelvis, meaning no intravenous dye is injected. Non-contrast is important because contrast material can sometimes obscure smaller stones, making them harder to distinguish from surrounding tissues. The NCCT generates detailed images confirming the stone’s presence, exact location, size, and density.
Specific Scenarios Where Stones May Be Undetected
Despite its high reliability, a CT scan can occasionally fail to detect a kidney stone under specific conditions. One common factor is the stone’s size; extremely small stones, often referred to as microlithiasis, may fall below the resolution threshold of the scan. Stones smaller than 3 millimeters are the most likely to be missed.
The location of the stone can also present a challenge. Stones situated near dense bone, such as at the ureterovesical junction (UVJ) where the ureter meets the bladder, are harder to isolate from surrounding bony structures. Similarly, dense vascular calcifications or phleboliths (calcified pelvic veins) can also obscure a stone in the distal ureter, leading to image misinterpretation.
Technical factors related to the scanning protocol can also contribute to a missed diagnosis. Low-dose CT protocols are often employed to reduce the patient’s radiation exposure. While this is a benefit, the reduction in dose can slightly decrease image quality and reduce sensitivity for very small calculi, particularly in patients with a higher body mass index.
Rarely, a stone’s composition can make it difficult to visualize. Stones composed of pure matrix material or those associated with certain protease inhibitor medications may have a density similar to soft tissue, making them nearly invisible even on a non-contrast CT. This unusual density profile means the stone does not stand out clearly.
Comparing CT Scans with Other Diagnostic Tools
When a CT scan is unavailable, inconclusive, or contraindicated, other imaging modalities are utilized, each with its own advantages and limitations. Ultrasound is a common alternative, valued because it uses sound waves instead of ionizing radiation, making it the preferred initial choice for pregnant women and children. It is excellent at detecting hydronephrosis, which is the swelling of the kidney caused by an obstruction, but its sensitivity for small stones in the ureter is significantly lower than CT, often due to obstruction by bowel gas.
The Kidney, Ureter, and Bladder (KUB) X-ray has a limited role in initial diagnosis. Its major limitation is that it only visualizes radiopaque stones (those that contain calcium). Non-radiopaque stones, such as those made of uric acid, are completely invisible on this type of image. The KUB X-ray is primarily used for monitoring the movement or growth of a known stone rather than for initial detection.
What to Do If Symptoms Continue After a Clear Scan
Receiving a clear CT scan result while still experiencing the severe, classic pain of renal colic can be confusing and concerning. It is important to remember that a definitive diagnosis relies on correlating a patient’s clinical symptoms with the imaging results. A clear scan does not automatically rule out a problem if the symptoms are highly suggestive of a stone.
If symptoms persist, the physician may first consider the possibility that a very small stone passed spontaneously shortly before the scan was performed. Another possibility is that the initial scan was obtained during a period of reduced pain, and the stone has moved, or the symptoms are related to one of the rare instances where a stone was truly missed. In this situation, the medical provider may recommend a period of observation or serial imaging, which involves repeating the CT scan or performing an ultrasound after a few days to check for a developing obstruction.
It is also important to pursue a differential diagnosis for the persistent flank pain, as many other conditions can mimic the symptoms of a kidney stone. Various gynecological issues, musculoskeletal problems like muscle strain, or other abdominal pathologies such as appendicitis can cause similar discomfort. Therefore, if a high-quality CT scan is negative for a stone, further medical investigation is necessary to correctly identify the true cause of the patient’s ongoing symptoms.