Kidney stones cause sudden, intense pain known as renal colic. The primary tool used to confirm the presence of a stone and determine its location is a computed tomography (CT) scan. While this technology is highly accurate, the question of whether a CT scan can miss a kidney stone is valid. Although rare, a stone can occasionally be missed, typically due to factors relating to the stone’s physical characteristics or the specific imaging technique used.
Why CT Scans Are the Standard Diagnostic Tool
Non-contrast computed tomography (NCCT) is the standard method for evaluating a patient with suspected kidney stones. This scan, often performed using a low-dose protocol to reduce radiation exposure, boasts a very high sensitivity, typically greater than 95% for stone detection.
CT scanning generates detailed cross-sectional images of the abdomen and pelvis, visualizing the entire urinary tract, including the kidneys, ureters, and bladder. Unlike older imaging methods, CT can detect stones regardless of their chemical composition. This ensures that even stones composed of materials like uric acid, which are often invisible on a standard X-ray, are clearly seen.
The scan also provides information about the stone’s size, location, and density, which are important factors for determining the best treatment plan. Furthermore, the comprehensive view allows physicians to identify other potential causes of acute abdominal pain, such as appendicitis or diverticulitis, that might be mimicking kidney stone symptoms.
Situations That Increase the Risk of a Stone Being Missed
Certain circumstances can increase the risk of a CT scan failing to detect a kidney stone. The most common reason for a missed diagnosis is the stone’s size. Stones that are extremely small, often referred to as microlithiasis, can be difficult to resolve on a standard image.
Studies suggest that while CT is highly sensitive for stones larger than 3 millimeters, its reliability decreases for stones measuring less than 2 millimeters. These tiny stones may be overlooked because they are near the limit of the scanner’s resolution. When a very low-dose CT protocol is used to minimize radiation exposure, the image quality is slightly compromised, which can further reduce the visibility of these minute calculi.
The specific technical settings of the CT scanner also play a role in detection. If the CT slices are too thick—for example, 5 millimeters instead of the preferred 1 to 1.5 millimeters—a small stone can be inadvertently skipped between the slices or its image detail blurred. This is especially true if the stone is moving or located in a complex anatomical area.
A CT scan performed with intravenous contrast dye may also obscure a stone. The contrast material enhances the surrounding soft tissues, making it difficult to distinguish a small stone nestled within the renal collecting system or the ureter. Additionally, patients with a large body size or significant abdominal fat may present a technical challenge, as the image quality can be degraded, making smaller stones harder to identify.
Other Imaging Options for Diagnosis and Follow-Up
If there are concerns about radiation exposure, other imaging options are used. Renal ultrasound is a common alternative, particularly for pregnant patients and children, because it uses sound waves instead of ionizing radiation. Ultrasound is very effective at detecting stones within the kidney itself and can also show signs of obstruction, such as swelling of the kidney (hydronephrosis).
However, ultrasound has a lower sensitivity than CT, especially for stones that have moved into the ureter. Stones lodged in the mid-ureter are particularly difficult to visualize because they can be obscured by overlying bowel gas.
Another option is the Kidney, Ureter, and Bladder (KUB) X-ray, though it is primarily used for monitoring known stones rather than initial diagnosis. A KUB X-ray relies on the stone’s ability to block X-rays, which only occurs with radiopaque stones, such as those made of calcium. Stones composed of materials like uric acid are radiolucent, meaning they do not show up on a KUB X-ray, which limits its diagnostic utility.