A renal stone, more commonly called a kidney stone, is a hard deposit of minerals and salts that forms inside your kidney. These stones develop when certain substances in your urine become so concentrated that they crystallize and stick together, gradually building into a solid mass. About 11% of men and 7% of women will develop at least one kidney stone in their lifetime, and once you’ve had one, there’s up to a 40% chance of forming another within five years.
How Kidney Stones Form
Your kidneys filter waste from your blood and send it out through urine. Normally, the water in your urine keeps minerals dissolved. But when urine becomes too concentrated, either because you’re not drinking enough fluid or because you’re excreting unusually high amounts of certain minerals, those substances can begin to crystallize. The crystals attract more material, bonding together layer by layer until they form a stone large enough to cause problems.
Calcium is the biggest culprit. When your body excretes excess calcium into the urine, it raises the concentration of both calcium oxalate and calcium phosphate, creating ideal conditions for crystals to form and grow. This process can take weeks or months, and many people carry small stones in their kidneys without knowing it until a stone shifts and begins to move.
Types of Kidney Stones
Not all kidney stones are made of the same material, and the type you form affects both treatment and prevention.
- Calcium oxalate stones are by far the most common. They form when calcium combines with oxalate, a compound found naturally in many foods. Calcium phosphate stones also exist but are much less frequent.
- Uric acid stones develop when urine is consistently acidic. They’re more common in people who eat large amounts of protein from red meat or poultry, since the body produces uric acid when it breaks down protein.
- Struvite stones are made of a mix of magnesium, ammonium, and phosphate. They typically form after urinary tract infections and can grow quickly.
- Cystine stones are rare and only occur in people with an inherited condition called cystinuria, which causes the kidneys to excrete too much of a specific amino acid.
What a Kidney Stone Feels Like
A stone sitting quietly in the kidney often causes no symptoms at all. The pain starts when a stone drops into the ureter, the narrow tube connecting the kidney to the bladder. This triggers renal colic: intense, wave-like pain that typically hits between the lower ribs and hip on the affected side. The pain can radiate to your back, groin, or lower abdomen, and it often shifts location as the stone moves downward.
Unlike a dull ache that stays constant, renal colic comes in surges. You may feel fine for a few minutes, then get hit with sharp pain that makes it impossible to sit still. Nausea and vomiting are common. You might also notice blood in your urine (pink, red, or brown), a persistent urge to urinate, or a burning sensation when you do. If a stone completely blocks urine flow, the kidney swells, which intensifies the pain significantly.
How Kidney Stones Are Diagnosed
A non-contrast CT scan is the most sensitive and specific way to detect a kidney stone. It can pinpoint a stone’s size, location, and whether it’s blocking urine flow, all without requiring any contrast dye. For follow-up imaging after a stone has been identified, an ultrasound combined with a standard X-ray offers a good balance of accuracy and lower radiation exposure, particularly for stones under 10 mm.
Stone size, measured in millimeters, is the single most important factor in deciding treatment. Stones under about 5 mm have a reasonable chance of passing on their own. Stones between 5 and 10 mm may pass but often need help. Stones larger than 10 mm rarely pass without intervention.
Passing a Stone Without Surgery
For smaller stones, the standard approach is watchful waiting with aggressive hydration and pain management. The goal is to flush the stone through the ureter and out with your urine. This can take anywhere from a few days to several weeks, and the pain during that time can range from mild to severe depending on where the stone sits and whether it’s moving.
Doctors sometimes prescribe medications designed to relax the smooth muscle in the ureter, making it easier for a stone to pass. Earlier analyses of multiple trials suggested these drugs increased the likelihood of spontaneous passage by roughly 45% compared to no treatment. However, a large, well-designed trial called SUSPEND found that the two most commonly used medications showed no clinically meaningful benefit in reducing the need for further treatment within four weeks. The effectiveness of this approach remains debated, and your doctor may or may not recommend it based on your specific situation.
Procedures for Larger Stones
When a stone won’t pass on its own, two main options exist.
Shock wave lithotripsy (SWL) uses focused sound waves from outside the body to break a stone into smaller fragments that can then pass through the urinary tract. It’s non-invasive and doesn’t require general anesthesia in all cases, but it works best on smaller stones. For stones 1 cm or smaller in the lower part of the kidney, SWL clears the stone about 37% of the time. For stones between 1 and 2 cm, that drops to around 29%. Because of these limitations, guidelines recommend against using SWL as a first-line treatment for stones larger than 1 cm.
Ureteroscopy (URS) involves threading a thin, flexible scope through the urethra and bladder up into the ureter or kidney. The surgeon can then grab the stone directly or break it apart with a laser. It’s more effective, clearing stones 1 cm or smaller about 59% of the time, but it’s also more invasive and carries a higher risk of complications.
For very large stones (generally over 2 cm), a procedure called percutaneous nephrolithotomy may be used, where a surgeon makes a small incision in the back to access the kidney directly.
What to Expect With a Ureteral Stent
After a procedure, your surgeon may place a ureteral stent, a thin flexible tube that holds the ureter open to allow urine (and any remaining stone fragments) to drain. Most stents stay in for a few days to a few weeks.
Stents are functional but not comfortable. Up to 80% of people with a stent experience side effects, including bladder irritation, frequent urination, blood in the urine, pain or burning while urinating, and bladder spasms. These symptoms are temporary and resolve once the stent is removed, which is typically a quick in-office procedure.
Preventing Recurrence
The single most effective prevention strategy is drinking enough fluid to produce more than 2.5 liters of urine per day. That’s roughly 10 to 12 cups of fluid, though the exact amount depends on your activity level, climate, and body size. The relationship between urine volume and stone risk is continuous: more dilute urine means less opportunity for crystals to form.
Dietary changes depend on the type of stone you formed, which is why saving a passed stone for analysis matters. For the most common calcium oxalate stones, the key adjustments are:
- Limit sodium to under 2,300 mg per day. High sodium intake forces your kidneys to excrete more calcium, raising your risk.
- Get enough calcium from food. This sounds counterintuitive, but dietary calcium binds to oxalate in your gut and prevents it from reaching your kidneys. Cutting calcium actually increases stone risk.
- Moderate high-oxalate foods. Spinach, rhubarb, nuts, and beets are especially high in oxalate. You don’t need to eliminate them, but eating them alongside calcium-rich foods helps.
- Watch protein intake. High animal protein consumption increases uric acid production and makes urine more acidic, promoting both uric acid and calcium stones.
For uric acid stones specifically, reducing red meat and shellfish and keeping urine less acidic are the primary strategies. Your doctor may also prescribe medication to adjust urine chemistry if dietary changes alone aren’t enough to prevent new stones from forming.