Kidney stones are small, hard mineral deposits that form within the kidneys and can travel through the urinary tract, causing intense pain. The ability of a stone to pass naturally, without medical intervention, is overwhelmingly dependent on its physical size. As a stone moves from the kidney into the narrow ureter—the tube connecting the kidney to the bladder—its diameter becomes the most important factor determining the outcome.
Critical Size Thresholds for Natural Passage
The likelihood of a kidney stone passing spontaneously is inversely proportional to its size, with specific millimeter measurements serving as clinical benchmarks. Stones less than 4 millimeters (mm) in diameter have the highest probability of passing, with spontaneous passage rates estimated to be around 80% to 98%. These small stones are often managed with increased fluid intake and pain medication, typically passing within a few weeks.
The chances of natural passage drop significantly for stones in the medium size range of 4 mm to 6 mm. A stone measuring 5 mm has an estimated passage rate of about 65%, while a 6 mm stone has only a 33% chance of passing on its own. For stones in this range, observation may be considered, but medical management with alpha-blockers is often initiated to relax the ureteral muscle and improve the passage rate.
Stones greater than 6 mm are generally considered unlikely to pass without assistance, making this a common threshold for recommending intervention. For stones measuring 6.5 mm or larger, the spontaneous passage rate plummets to less than 10%, making intervention almost always necessary. While stones up to 10 mm may occasionally pass, 10 mm is the practical ceiling for observation, with most stones exceeding this diameter requiring a planned medical procedure.
Factors Influencing Stone Passage Beyond Size
While size is the primary concern, the stone’s location within the urinary tract is another major determinant of its ability to pass. The ureter, the narrow tube leading from the kidney, has three natural constrictions where a stone is most likely to become lodged. Stones closer to the bladder, in the lower (distal) ureter, have a better chance of passing than those stuck higher up near the kidney or in the middle section.
The physical characteristics of the stone also play a role in its mobility. Stones with irregular, jagged, or rough surfaces tend to cause more trauma and are more likely to snag on the ureteral lining than those with a smoother contour. The patient’s individual anatomy, such as the natural width of their ureter or the presence of anatomical variations, can also affect the passage of medium-sized stones.
The presence of significant hydronephrosis, which is kidney swelling due to blocked urine flow, also influences the treatment decision. Even a relatively small stone causing a complete obstruction may necessitate rapid intervention, overriding the typical size-based observation period. Treatment is a function of size, location, and the functional impact of the obstruction on the kidney.
Diagnostic Imaging and Measurement
Accurately determining the stone’s size and location is a prerequisite for deciding on the appropriate management strategy. Non-Enhanced Computed Tomography (NECT) is the preferred diagnostic tool, as it provides precise three-dimensional measurements of the stone’s diameter and exact position. The clarity of the CT scan allows physicians to differentiate a 5 mm stone, which might pass, from a 7 mm stone, which almost certainly will not.
The measurement is important because a difference of just one or two millimeters can shift the patient from a conservative management plan to a scheduled surgical procedure. Ultrasound is frequently used, particularly to assess for hydronephrosis and to monitor stones in the kidney, though it is less accurate than CT for precise stone measurement. Abdominal X-rays may be used for follow-up, but they only detect certain types of stones and are not the standard for initial sizing and localization.
Medical Interventions for Larger Stones
When a kidney stone is measured at 6 mm or larger, or when a smaller stone causes severe symptoms or complications, medical intervention is typically required. Extracorporeal Shock Wave Lithotripsy (SWL) is a common non-invasive option that uses focused sound waves delivered from outside the body to break the stone into tiny fragments. These small pieces can then pass naturally in the urine, and SWL is typically reserved for stones up to 10 or 15 mm, depending on their location.
For stones too large for SWL, or those lodged in the ureter, Ureteroscopy (URS) is often the next step. This minimally invasive procedure involves inserting a thin, flexible scope through the urethra and bladder up into the ureter to reach the stone. The stone can then be retrieved with a small basket or fragmented into smaller pieces using a laser, a process called laser lithotripsy. URS is highly effective for stones in the 10 mm to 20 mm range throughout the ureter.
Percutaneous Nephrolithotomy (PCNL) is the procedure of choice for very large stones, typically those exceeding 2 centimeters (20 mm) in diameter. This method involves making a small incision in the back and creating a tunnel directly into the kidney to remove the stone. PCNL is necessary for managing complex stone burdens, such as staghorn calculi, which fill the entire renal pelvis, and offers the highest stone-free rates for the largest stone sizes.