Kidney stones, or nephrolithiasis, are common formations of mineral and salt deposits that develop within the urinary tract. When a stone moves and causes blockage, it can result in acute, severe pain known as renal colic, prompting an urgent medical evaluation. The most reliable method for confirming the presence of a stone is through diagnostic imaging. While the computed tomography (CT) scan is the gold standard for this diagnosis, a CT scan can miss a kidney stone. This is uncommon, but technical limitations and stone characteristics mean a small percentage of stones may not be visible.
CT Scans as the Primary Diagnostic Tool
The non-contrast CT scan of the abdomen and pelvis is the definitive imaging test for suspected kidney stones. Its exceptional diagnostic accuracy includes a sensitivity of up to 97% and a specificity of around 96% for stone detection. The scan uses specialized X-ray technology to create cross-sectional images of the body, allowing for detailed visualization of calcified structures.
The non-contrast technique avoids intravenous dye, which could obscure small stones within the urinary system. It provides high spatial resolution, which is the ability to distinguish fine detail, allowing clinicians to accurately measure the size and determine the exact location of the stone. This precision is essential for deciding whether a stone is likely to pass on its own or requires intervention.
A primary advantage of the CT scan is its capability to identify virtually all stone compositions, including radiolucent stones that cannot be seen on a standard X-ray. Even uric acid stones, which lack high calcium content, are readily detectable on a CT scan because they still possess a higher density than surrounding soft tissue. Furthermore, the scan can reveal alternative causes of the patient’s pain, such as appendicitis or diverticulitis, which helps ensure a comprehensive diagnosis when no stone is found.
Specific Factors That Can Obscure Stone Detection
Despite the CT scan’s high accuracy, certain physical and technical factors can obscure a kidney stone, leading to a false-negative result. The size of the stone is a major determinant, as extremely small stones, often referred to as microlithiasis, may fall below the resolution threshold of the equipment. Stones smaller than three millimeters are the most likely to be missed, even though they can cause significant pain as they pass through the narrow ureter.
Technical settings influence visibility; thin slice thickness, typically between 1.0 and 1.5 millimeters, is superior for detection compared to thicker slices. Stone composition is another factor, as rare types such as those induced by certain antiretroviral drugs, like indinavir, or pure matrix stones, have such a low density that they blend in with soft tissues. In these unusual cases, the stone’s chemical makeup does not create enough contrast to be clearly visible on the scan.
The stone’s location can also cause detection difficulty, especially if it is situated adjacent to a dense bony structure or a metal implant. These dense materials create technical artifacts, appearing as streaks or distortions that can conceal a small stone in the immediate vicinity. Patient movement during the scan can also introduce motion artifacts that blur the images, making identification difficult.
Alternative Imaging Methods and Follow-Up
When a CT scan is negative but a patient’s symptoms, such as persistent flank pain or blood in the urine, strongly indicate a stone, alternative imaging or follow-up is necessary. Renal ultrasound is often employed because it uses no ionizing radiation and is the preferred initial choice for special populations, such as pregnant women. Ultrasound has a lower sensitivity for direct stone detection, especially in the ureter, but it is excellent at showing secondary signs like fluid buildup in the kidney (hydronephrosis).
The Kidney, Ureter, and Bladder (KUB) X-ray is primarily useful for tracking the movement of known radio-opaque stones. This method is ineffective for initial diagnosis because it misses radiolucent stones and can be obscured by bowel gas or overlying bone. Magnetic Resonance Imaging (MRI) is generally reserved for situations where radiation must be avoided and ultrasound is inconclusive, but it has limited utility for directly visualizing the stone itself.
If a patient continues to experience symptoms after a negative CT, the physician may recommend a repeat CT scan after a period of time to catch a stone that may have since moved or grown larger. Further evaluation often includes a comprehensive urinalysis and blood work to look for chemical markers or signs of infection that support the suspicion of a stone. This combination of clinical observation and selective secondary imaging ensures that a missed stone or an alternative diagnosis is not overlooked.