How does a kidney stone form, and will it pass on its own?
A stone forms when urine is too concentrated: crystals stick together and grow. When small, it often passes on its own: 76% of 2 to 4 mm stones pass, versus 25% above 9 mm. Drinking enough to make 2 liters of urine a day lowers the risk of it coming back.
Set how much you drink: the crystals grow faster or slower. Then release the stone and watch it travel down to the bladder.
Chances based on 172 patients followed in the emergency room (Coll, 2002). The crystal speed is an illustration, not a measurement.
Why does a stone form?
Urine carries calcium, oxalate and uric acid. When there is too much of these salts for too little water, they crystallize, like sugar at the bottom of an over-sweetened coffee. In an Italian study, people who formed stones made about 1 liter of urine a day, versus 1.4 liters for the others (Borghi, 1996).
Who is affected?
In the United States, 8.8% of adults have had a stone: 10.6% of men and 7.1% of women (Scales, 2012). Heat raises the risk, because you lose water through sweat (Tasian, 2014).
Will a stone pass on its own?
Often, if it is small. Size matters, but so does where it is: the lower it already sits in the ureter, the better it passes.
| Size | Passes alone |
|---|---|
| 1 mm | 87 % |
| 2 to 4 mm | 76 % |
| 5 to 7 mm | 60 % |
| 7 to 9 mm | 48 % |
| Over 9 mm | 25 % |
By location: 48% in the upper ureter, 60% in the middle, 75% in the lower part, 79% at the entrance to the bladder (Coll, 2002).
Does drinking more really work?
Yes. In a 5-year trial, 199 people who had had a first stone were randomized. Those who drank enough to make at least 2 liters of urine a day had a recurrence in 12% of cases, versus 27% for the others (Borghi, 1996).
Frequently asked questions
Why does renal colic hurt so much?
The stone blocks the ureter: urine builds up behind it and the kidney swells. The ureter contracts to push the stone, which causes pain in waves, often in the back and side.
How much should I drink?
The benchmark in studies is to pass about 2 liters of urine a day, which often means drinking a bit more, especially in hot weather (Borghi, 1996). Pale urine is a good sign.
Should I stop dairy products?
No, in general. Calcium in a meal binds oxalate in the gut. A low-calcium diet can even raise the risk: in a 5-year trial of 120 men, it led to twice as many recurrences as a diet normal in calcium and low in salt and meat (Borghi, 2002). The best approach depends on the type of stone: ask your doctor.
When should I seek emergency care?
If you have fever with the pain, unbearable pain, vomiting that stops you from drinking, or if you have only one kidney. An infection behind a stone is an emergency.
See also
Vitamin D and sunlight Bones A workout
Sources
- Scales CD Jr, Smith AC, Hanley JM, Saigal CS. Prevalence of kidney stones in the United States. Eur Urol. 2012;62(1):160-5. PMID 22498635
- Coll DM, Varanelli MJ, Smith RC. Relationship of spontaneous passage of ureteral calculi to stone size and location as revealed by unenhanced helical CT. AJR Am J Roentgenol. 2002;178(1):101-3. PMID 11756098
- Borghi L, et al. Urinary volume, water and recurrences in idiopathic calcium nephrolithiasis: a 5-year randomized prospective study. J Urol. 1996;155(3):839-43. PMID 8583588
- Tasian GE, et al. Daily mean temperature and clinical kidney stone presentation in five U.S. metropolitan areas: a time-series analysis. Environ Health Perspect. 2014;122(10):1081-7. PMID 25009122
- Borghi L, et al. Comparison of two diets for the prevention of recurrent stones in idiopathic hypercalciuria. N Engl J Med. 2002;346(2):77-84. PMID 11784873