Passing a kidney stone is among the worst pain in medicine. Most people who have had one will do nearly anything to avoid a second.
Here is the problem: most never find out why it happened. The stone is removed or passed, the pain ends, and nobody asks what the urine chemistry was doing. Roughly half of those patients form another stone within five to ten years.
The advice that makes it worse
If you were told to cut back on calcium, that advice was wrong, and it raised your risk.
It is the most common misconception in stone disease and it is intuitive — calcium stones, so less calcium. But dietary calcium binds oxalate in the gut and carries it out. Restrict the calcium and oxalate is freed for absorption, urinary oxalate climbs, and calcium oxalate stones become more likely.
Adequate dietary calcium is protective. The thing to restrict is sodium, which drives calcium into the urine, and most stone formers have never been told that either.
If you have had a stone and nobody ordered a 24-hour urine collection, the disease has never actually been evaluated.
That test is not optional and it is not routine. It measures volume, calcium, oxalate, citrate, uric acid, sodium, and pH — and nothing else identifies which mechanism is driving your stones. Without it, prevention is guesswork.
Who does what
When a stone obstructs or is too large to pass, a urologist removes it — ureteroscopy, lithotripsy, or percutaneous surgery. That is their procedure and we refer for it.
We determine why it formed. Removing a stone treats an episode. Understanding the chemistry treats the disease.
The asymmetry is the point. One of those prevents the next stone.
Composition decides everything
| Stone type | Share | What drives it | Key prevention |
|---|---|---|---|
| Calcium oxalate | ~80% | Low fluid, high urine oxalate, low citrate | More fluid, citrate, normal-calcium diet |
| Uric acid | ~9% | Acidic urine, high purine intake, gout | Alkalinize the urine, reduce purines |
| Struvite | ~7% | Urinary tract infections | Treat the infection; sometimes surgery |
| Cystine | ~1% | Inherited (cystinuria) | High fluids, alkalinize, specific medications |
Strain your urine and catch the stone, because its type dictates the entire strategy.
About four out of five are calcium oxalate, formed when urine runs low in volume or citrate, or high in calcium or oxalate. Calcium phosphate is less common but more revealing: it tends to point toward renal tubular acidosis or an overactive parathyroid, either of which can sit undiagnosed behind a stone for years.
The uric acid stone is the one worth catching early. It forms in urine that stays too acidic, and because of that it can often be dissolved outright by alkalinizing the urine — sometimes with no procedure at all. You will usually find gout, obesity, or metabolic syndrome somewhere in the background.
Two others round out the list. Struvite stones grow out of infection with urea-splitting bacteria and can get very large before anyone notices. Cystine stones come from an inherited defect in how the kidney handles the amino acid, and they tend to start young and recur hard.
The practical failure here is treating every stone as if it were calcium oxalate. Hand a uric acid stone former the oxalate playbook and you accomplish nothing — and that mismatch is one of the most common ones I see.
What the evaluation finds
Blood work for calcium, uric acid, parathyroid hormone, and electrolytes. Hyperparathyroidism and renal tubular acidosis both present as recurrent stones, and both are curable or specifically treatable once named.
The 24-hour urine, which is the centerpiece.
Stone composition analysis when we can get one. Imaging for what is already there.
What changes
Fluid first, always. Enough to make more than two and a half liters of urine daily — roughly three liters in. Most stone formers are chronically underhydrated and every other intervention is downstream of this.
Sodium restriction. Adequate dietary calcium. Oxalate moderation for the patients whose chemistry warrants it, not for everyone.
Then medication where the numbers call for it. Thiazides lower urinary calcium. Potassium citrate raises citrate and alkalinizes urine. Allopurinol lowers uric acid.
Each of those is chosen from the 24-hour urine. Without it, you are guessing.
The low-oxalate diet, done correctly
For a calcium oxalate stone former whose 24-hour urine shows elevated oxalate, restricting dietary oxalate is worth doing. Roughly eighty percent of stones are calcium oxalate, and urinary oxalate directly drives their formation.
Three things determine whether it works.
Take calcium with the oxalate. This is the part almost nobody is told. Calcium binds oxalate in the gut and carries it out in stool, where it cannot reach the kidney. Eaten together — in the same meal, not hours apart — dietary calcium is the most effective oxalate binder available.
Restricting oxalate while also restricting calcium raises urinary oxalate and makes stones more likely. That combination is common and is exactly backward.
Target the foods that actually matter. Oxalate content varies by orders of magnitude, and a flat list of thirty foods produces a patient who eats poorly and gains nothing.
The ones worth restricting: spinach and rhubarb, which are in a category of their own. Then almonds, cashews, and peanuts; beets and Swiss chard; soy products; potatoes and sweet potatoes; chocolate and cocoa; wheat bran; and black tea in large quantities.
Most other vegetables, most fruits, and most whole grains contribute little. Coffee is fine — in cohort studies it is associated with fewer stones, not more.
Understand where it ranks. Fluid volume matters more. Sodium restriction lowers urinary calcium more reliably than oxalate restriction lowers urinary oxalate. A patient who restricts spinach but drinks a liter a day and salts everything has not addressed their disease.
And a calcium oxalate stone former whose oxalate is normal gains little from this diet. If their citrate is low, potassium citrate is the intervention. If their urine calcium is high, sodium restriction and a thiazide are.
Which is why the 24-hour urine comes first. The stone type narrows the possibilities. The chemistry names the mechanism, and the mechanism selects the treatment.
Why nephrology
Stone disease is a disorder of urine chemistry, and urine chemistry is what a nephrologist does.
Recurrent stones scar the kidney over decades and some stone formers develop chronic kidney disease. A stone is also a window into hyperparathyroidism, renal tubular acidosis, and metabolic syndrome — all found by looking, and missed by not looking.
Come after your first stone, not your third. A metabolic evaluation is the difference between having had a stone and having stone disease.