If you have had a kidney stone, one of the most useful questions is what it was made of. This guide explains the five main stone types, how each forms and who is at risk, why doctors analyse stones and order blood and 24-hour urine tests, how prevention differs by type, and when symptoms need prompt medical care.
Medically reviewed by Prof. Dr. Samir Ahmed Al-Samarrai, consultant urologist. Last reviewed: · Last updated:
Why the type of kidney stone matters
Not all kidney stones are the same. They form from different substances for different reasons, so prevention depends on the type. According to NICE, the EAU and the AUA, knowing the stone type and checking blood and urine help doctors tailor prevention.
The main types of kidney stones
- Calcium oxalate: the most common type, according to MedlinePlus and the NHS. Calcium binds with oxalate, a substance found in foods such as spinach. MedlinePlus notes that diseases of the small intestine raise the risk, and that these stones are most likely in men aged 20 to 30.
- Calcium phosphate: the EAU guidelines link this type to an overactive parathyroid gland (hyperparathyroidism), a kidney acid-handling disorder (renal tubular acidosis) and urinary infection.
- Uric acid: forms when urine is too acidic, according to the NHS and the EAU. MedlinePlus says it is more common in men and can occur with gout or after chemotherapy for some cancers.
- Struvite (infection stones): form when certain bacteria that break down urea make the urine alkaline, as the EAU describes. MedlinePlus notes they are mostly seen with repeated urinary tract infections and can grow very large.
- Cystine: caused by an inherited condition (cystinuria) in which the kidneys leak the amino acid cystine into the urine. According to the EAU, cystine stones make up 1 to 2% of adult stones and 6 to 8% of stones in children.
Comparison of stone types
| Type | How it forms | Who is at higher risk | Main prevention focus (decided by the doctor) |
|---|---|---|---|
| Calcium oxalate | Calcium binds oxalate in concentrated urine | Low fluid intake, high calcium or oxalate in urine, bowel disease | Fluids, less salt, normal dietary calcium, limit high-oxalate foods if urine oxalate is high |
| Calcium phosphate | Forms in less acidic urine | Hyperparathyroidism, renal tubular acidosis, urinary infection | Fluids, less salt, treating the underlying condition |
| Uric acid | Crystallises in acidic urine | Gout, metabolic syndrome, after chemotherapy | Fluids, less animal protein, healthy weight; doctor may raise urine pH |
| Struvite | Urea-splitting bacteria make urine alkaline | Repeated urinary infections | Complete stone removal and treating and watching for infection |
| Cystine | Inherited leak of cystine into urine | People with cystinuria and their families | Very high fluid intake, less salt and protein, specialist medicines |
How the stone type is diagnosed: stone analysis
Stone analysis means sending a passed or removed stone to a laboratory. The EAU recommends analysing the stone of first-time stone formers with a validated method (infrared spectroscopy or X-ray diffraction) and repeating it if stones keep coming back despite treatment, recur early, or return after a long stone-free period, because composition can change. The AUA advises analysis at least once when a stone is available, and NICE asks doctors to consider it for adults. Fine crystals are discussed in our guide to kidney sand and urine crystals.
Blood and 24-hour urine tests (metabolic evaluation)
The EAU and the AUA describe a basic evaluation for every stone former: a medical and diet history, blood tests (such as creatinine, calcium and uric acid) and a urine test. NICE recommends measuring blood calcium in adults with stones, and the AUA advises a parathyroid hormone test if an overactive parathyroid gland is suspected.
A fuller assessment uses 24-hour urine collections, which measure volume, acidity, calcium, oxalate, uric acid, citrate and sodium. According to the EAU, this is for high-risk stone formers, usually as two collections. The AUA offers it to recurrent stone formers and to high-risk or interested first-time formers. High-risk groups in the EAU guidelines include:
- Stones in childhood or the teenage years, a family history or frequent recurrence
- Uric acid, infection or cystine stones, or brushite (a form of calcium phosphate)
- A single kidney or chronic kidney disease
- Hyperparathyroidism, metabolic syndrome or bowel disease
The EAU advises starting this work-up no earlier than 20 days, and preferably about three months, after an acute stone episode. NICE suggests referring children with stones to a paediatric kidney specialist (nephrologist) for metabolic tests; some adults with complex metabolic problems are also co-managed by a nephrologist.
Prevention by stone type
For all types, fluid matters most. NICE advises adults to drink 2.5 to 3 litres of water a day, add fresh lemon juice to water, avoid fizzy drinks and keep salt to no more than 6 g a day, without cutting calcium. The AUA aims for a urine volume of about 2.5 litres daily. Beyond that, according to the AUA, the EAU and the NIDDK:
- Calcium stones: less salt, normal calcium from food, fewer high-oxalate foods if urine oxalate is high, more fruit and vegetables, and moderate animal protein.
- Uric acid stones: less non-dairy animal protein and weight loss if overweight; the doctor may use treatment to make urine less acidic.
- Struvite stones: complete removal of the stone and monitoring for repeat infection.
- Cystine stones: the EAU aims for a urine volume above 3 litres a day, with less salt; specialist medicines may be needed.
Any medicine, and whether it is needed, is decided by the doctor after testing. More practical tips are in our kidney stone prevention diet guide.
When to see a doctor
Seek medical help promptly, according to the NHS and the NIDDK, if you have:
- Severe pain in the side, back, lower abdomen or groin
- Fever, chills or shivering with pain (an infected, blocked kidney is an emergency)
- Blood in the urine, cloudy or foul-smelling urine
- Being unable to pass urine, or passing very little
For stones that move down into the ureter, see our guide to ureteric stones. Even without symptoms, book a review if you have had more than one stone, a stone at a young age, or a family history.
Frequently asked questions
What is the most common type of kidney stone?
According to MedlinePlus and the NHS, calcium stones are the most common type, and most of these are calcium oxalate. Only laboratory analysis of the stone confirms the exact type.
Should I keep a stone that I pass?
Yes. The AUA advises stone analysis at least once when a stone is available, and the EAU recommends it for first-time stone formers. Keep it dry and bring it to your appointment.
Who needs a 24-hour urine test?
According to the EAU, 24-hour urine collections are for high-risk stone formers, such as people with recurrent stones, stones at a young age, a family history, a single kidney, or uric acid, infection or cystine stones. Your doctor decides if you need it.
Should I stop eating calcium if I have calcium stones?
No. NICE advises not restricting calcium and keeping a normal intake, and the EAU says calcium should not be restricted without strong reasons. Calcium from food binds oxalate in the gut.
Are cystine stones inherited?
Yes. Cystine stones are caused by cystinuria, an inherited condition. Because it runs in families and may cause no symptoms at first, the EAU advises screening brothers and sisters of affected people.
Can infection stones come back?
They can. Struvite stones form with certain urinary infections, so the EAU stresses removing all stone material, and the AUA advises monitoring for reinfection. Leftover fragments can harbour bacteria and regrow.
Sources
- Kidney Stones: Medical Management Guideline (American Urological Association)
- Renal and ureteric stones: assessment and management – Recommendations (NICE NG118)
- EAU Guidelines on Urolithiasis: Metabolic Evaluation and Recurrence Prevention
- EAU Guidelines on Urolithiasis: Guidelines
- Eating, Diet, & Nutrition for Kidney Stones (NIDDK)
- Kidney stones (MedlinePlus Medical Encyclopedia)
- Kidney stones – Causes (NHS)
Book a consultation
Prof. Samir Al-Samarrai and Dr. Amer Mousa assess urological conditions at Prof. Al-Samarrai Medical Center, Dubai Healthcare City, and discuss the options that suit your case. Medical consultations are confidential.
Read more: Urologist in Dubai: doctors, services, insurance and booking
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