Kidney stones, also called nephrolithiasis, urolithiasis, or renal calculi, can form in the kidneys, but also in the ureters, bladder, or the urethra.
They appear when solutes in the urine precipitate and crystalize. Depending on which solute precipitates to form the stone, there can be calcium oxalate, calcium phosphate, uric acid, cystine, and struvite stones.
Risk factors for developing kidney stones depend on their composition.
Risk factors for calcium oxalate stones include high urine calcium, high urine oxalate, low urine citrate, and dietary factors include low calcium, low potassium, and low fluid intake, as well as a high oxalate, and a high animal protein intake.
Calcium phosphate stones usually develop in individuals with renal tubular acidosis type I and II.
Uric acid stones can form when urine pH is persistently below 5.5, which can happen with chronic diarrhea or conditions like gout, diabetes, and obesity.
Cystine stones occur in the setting of cystinuria - a genetic condition where too much cystine is excreted.
Finally, struvite stones, also called staghorn calculi, are made up of magnesium ammonium phosphate, and the main risk factor is a urinary tract infection with a bacterium that produces urease - like Proteus and Klebsiella. These bacteria increase urine pH, making it a favorable environment for magnesium ammonium phosphate to precipitate.
Sometimes, kidney stones can be asymptomatic and discovered incidentally during an ultrasound or a CT-scan.