How Do Kidney Stones Form?

For Patients

If you suspect you have stones, visit your primary care provider or urologist.

If you've been diagnosed with stones and would like to be seen in the Metabolic Stone Clinic, please call (608) 263-4757.

Why do Stones Form?
Everyone forms crystals in their urine, even people who never form kidney stones. Urine, which is produced in the kidneys, naturally contains substances that promote the development of crystals. These stone promoters include calcium, oxalate, sodium, phosphorus and uric acid (and also cystine, in the case of individuals with the inherited disorder known as cystinuria).
These stone-promoting compounds, which come from the bloodstream, are present in urine because the kidneys filter our blood and regulate whether and how much of these urinary constituents will be reabsorbed by the kidneys (put back into blood circulation) or eliminated as waste.
In addition to these stone promoters, urine also naturally contains proteins and other compounds that inhibit crystal formation. Examples of stone inhibitors in urine include total urine volume (the more urine produced, the less concentrated it is and less likely to become supersaturated), citrate, magnesium, pyrophosphate, phytate and proteins and other molecules derived from normal metabolism, collectively referred to as urinary macromolecules. These inhibitors aid the elimination of crystals before they attach to the kidney and grow into larger stones.
How do stones form?
In order for tiny crystals to grow into larger stones, the urine must first become supersaturated, either due to: 
  • Low urine volume
  • Abnormally high concentration of stone promoters
  • Abnormally low concentration of stone inhibitors
  • Any combination of the above
 Under normal circumstances, there is a balance of urinary stone promoters and inhibitors, leading to the painless elimination from the kidney of tiny crystals.
Thus, if the crystals remain small enough, and if the urine is dilute enough to avoid supersaturation, crystals will flow with urine through the ureters and bladder without being noticed or causing problems. However, this balance between stone promoters and inhibitors does not exist for everyone - either due to genetic, lifestyle or other factors - leading to the formation of kidney stones.