Calcium Kidney Stones

Why calcium stones form

Calcium stones are not caused simply by "too much calcium in the diet." They form when the balance of stone-promoting and stone-inhibiting substances in the urine tips the wrong way. Common contributors include:¹

Low urine volume - the single biggest risk factor.
High urine calcium (hypercalciuria) - usually driven by dietary sodium rather than dietary calcium.
High urine oxalate (hyperoxaluria) - from high-oxalate foods, gut disease (malabsorption after bariatric or bowel surgery), or very low dietary calcium.
Low urine citrate (hypocitraturia) - citrate normally binds calcium and inhibits crystal formation.
High urine uric acid - can seed calcium stone formation.
Hyperparathyroidism - a treatable endocrine cause of calcium stones; must be excluded when serum calcium is elevated.

Calcium oxalate vs calcium phosphate

Calcium oxalate

About 60-75% of all stones.
Hard, dense - often > 1000 HU on CT.
Can be resistant to shockwave lithotripsy; ureteroscopy with Holmium laser is often more reliable.
Two sub-forms: monohydrate (harder, darker) and dihydrate (softer, easier to break).

Calcium phosphate

About 10-15% of stones.
Forms in alkaline urine - often associated with distal renal tubular acidosis or primary hyperparathyroidism.
Responds reasonably well to most treatments.
Finding a phosphate stone should prompt a careful look for an underlying metabolic cause.¹

Diagnosis

Both subtypes show clearly on CT KUB. Calcium oxalate monohydrate is one of the densest stones - an HU value above about 1000 on CT is a useful clue that a stone is very hard and may be better treated with ureteroscopy than ESWL.² Definitive identification requires composition analysis of a retrieved stone.

Treatment

Treatment of a calcium stone follows the same general principles as for any stone - the choice of ESWL, ureteroscopy, or PCNL depends on size and location rather than composition, though density matters. See the treatments hub.

Prevention

Prevention is where stone type matters most. For calcium stones the strongest evidence supports:^{1, 3}

Fluid intake sufficient to produce at least 2.5 L of urine a day.
Normal dietary calcium (approximately 1,000-1,200 mg/day, ideally from food with meals). Low-calcium diets increase calcium oxalate stone risk by allowing more dietary oxalate to be absorbed.
Low-sodium diet - reducing salt lowers urinary calcium excretion.
Moderate animal protein - excess red meat, poultry, and fish increase urinary calcium and uric acid and reduce urinary citrate.
Moderating high-oxalate foods - spinach, rhubarb, beetroot, nuts, tea - only if dietary calcium is already adequate.
Citrate-rich drinks - home-made diluted lemon or lime, for some patients.

If a 24-hour urine study shows specific abnormalities, targeted medication may be added:

Thiazide diuretics for persistent hypercalciuria.
Potassium citrate for low urinary citrate or persistently acidic urine.
Allopurinol for hyperuricosuric calcium oxalate stone formers.

Dr Kooner works with renal physician colleagues for detailed metabolic assessment of recurrent calcium stone formers.

References

Skolarikos A, et al. EAU Guidelines on Urolithiasis. 2025.
Wiesenthal JD, et al. Stone density on non-contrast CT predicts shock wave lithotripsy success. Urologic Nursing. 2011;31(4):231-235.
Qaseem A, et al. Dietary and pharmacologic management to prevent recurrent nephrolithiasis: a systematic review. Annals of Internal Medicine. 2026.