Kidney stones (called renal calculi in medical terminology, from Latin ren, renes, "kidney" and calculi, "pebbles"), are solid concretions (crystal aggregations) formed in the kidneys from dissolved urinary minerals. The terms nephrolithiasis and urolithiasis refer to the condition of having calculi in the kidneys and urinary tract, respectively. Bladder stones can form or pass into the urinary bladder. Ureterolithiasis is the condition of having a calculus in the ureter (the tube connecting the kidneys and the bladder).
Kidney stones typically leave the body by passage in the urine stream, and many stones are formed and passed without causing symptoms. If stones grow to sufficient size before passage—on the order of at least 2-3 millimeters—they can cause obstruction of the ureter. The resulting obstruction causes dilation or stretching of the upper ureter and renal pelvis (the part of the kidney where the urine collects before entering the ureter) as well as muscle spasm of the ureter, trying to move the stone. This leads to pain, most commonly felt in the flank, lower abdomen and groin (a condition called renal colic). Renal colic can be associated with nausea and vomiting. There can be blood in the urine, visible with the naked eye or under the microscope (macroscopic or microscopic hematuria) due to damage to the lining of the urinary tract.
There are several types of kidney stones based on the type of crystals of which they consist. The majority are calcium oxalate stones, followed by calcium phosphate stones. More rarely, struvite stones are produced by urea-splitting bacteria in people with urinary tract infections, and people with certain metabolic abnormalities may produce uric acid stones or cystine stones.
The diagnosis of a kidney stone can be confirmed by radiological studies or ultrasound examination; urine tests and blood tests are also commonly performed. When a stone causes no symptoms, watchful waiting is a valid option. In other cases, pain control is the first measure, using for example non-steroidal anti-inflammatory drugs or opioids. Using soundwaves, some stones can be shattered into smaller fragments (this is called extracorporeal shock wave lithotripsy). Sometimes a procedure is required, which can be through a tube into the urethra, bladder and ureter (ureteroscopy), or a keyhole or open surgical approach from the kidney's side. Sometimes, a metal tube may be left in the ureter (a ureteric stent) to prevent the recurrence of pain. Preventive measures are often advised such as drinking sufficient amounts of water, although the effect of many dietary interventions has not been rigorously studied.