Kidney stones, medically known as renal calculi or nephrolithiasis, are solid crystalline deposits that develop within the kidneys. Although we tend to talk about “a kidney stone” as if all stones are the same, there are several distinctly different types.

Knowing the composition of a kidney stone matters. Different stones have different causes, appearances, recurrence risks and, importantly, different strategies for prevention and treatment.

Modern management therefore involves more than simply removing the stone. The aim is to answer three questions:

What is the stone? Why did it form? And how can we prevent the next one?

What Are Kidney Stones Made Of?

The major types of urinary stones are:

1. Calcium oxalate stones
2. Calcium phosphate stones
3. Uric acid stones
4. Struvite or infection stones
5. Cystine stones
6. Rare metabolic and medication-related stones

Stones are not always chemically pure. Many contain a mixture of different crystalline components.

The European Association of Urology recommends reliable stone analysis and a basic metabolic evaluation in stone-forming patients, with more detailed metabolic investigation particularly important in patients at high risk of recurrence.

1. Calcium Oxalate Stones

The most common kidney stone

Calcium oxalate stones are the most frequently encountered urinary calculi.

They may consist predominantly of:

• Calcium oxalate monohydrate, known as whewellite
• Calcium oxalate dihydrate, known as weddellite

Calcium oxalate monohydrate stones can be particularly hard and resistant to fragmentation with shockwave lithotripsy.

Why do calcium oxalate stones form?

Contributing factors may include:

• Low urine volume or dehydration
• Excess urinary calcium
• Excess urinary oxalate
• Low urinary citrate
• High dietary sodium intake
• Excessive animal protein intake
• Certain bowel diseases or previous intestinal surgery
• Genetic predisposition
• Metabolic abnormalities

Importantly, calcium stones do not necessarily mean that a patient is eating too much calcium.

Severely restricting normal dietary calcium can actually be counterproductive because calcium within the intestine binds oxalate and reduces its absorption.

Management

Small asymptomatic stones may sometimes simply be monitored.

When treatment is required, options include:

• Shockwave lithotripsy (SWL)
• Flexible ureteroscopy and laser lithotripsy
• Percutaneous nephrolithotomy (PCNL)
• Observation in appropriately selected patients

Prevention depends on the underlying metabolic abnormality and may include increased fluid intake, dietary modification, reduction in excessive sodium intake, potassium citrate for selected patients, thiazide therapy for hypercalciuria and treatment of hyperoxaluria or hyperuricosuria when identified.

2. Calcium Phosphate Stones

Calcium phosphate stones are less common than calcium oxalate stones.

They may contain:

• Hydroxyapatite
• Carbonate apatite
• Brushite

Why do they form?

Calcium phosphate crystallisation is favoured by relatively alkaline urine.

They may be associated with:

• Hypercalciuria
• Renal tubular acidosis
• Hyperparathyroidism
• Urinary tract abnormalities
• Certain metabolic disorders

Brushite stones deserve particular attention. They can be extremely hard and may respond poorly to shockwave treatment.

Management therefore needs to consider not only stone size and location, but also stone density and previous stone composition.

3. Uric Acid Stones

Uric acid stones behave very differently from calcium stones.

They are strongly associated with persistently acidic urine and may occur in patients with:

• Gout
• Metabolic syndrome
• Diabetes
• Obesity
• High purine intake
• High animal-protein intake
• Chronic dehydration
• Excessive uric acid production or excretion

Uric acid stones account for approximately 10% of renal stones and all uric acid stone formers are considered at increased risk of recurrence.

Why are uric acid stones special?

Because unlike most kidney stones, uric acid stones can potentially be dissolved.

Treatment involves increasing urinary pH, usually using an alkalinising medication such as potassium citrate.

For active oral dissolution therapy, urinary pH needs to be carefully monitored. Current EAU guidance describes targeting approximately pH 7.0–7.2 during chemolysis, while avoiding excessive alkalinisation because this may encourage calcium phosphate crystallisation.

This creates one of the happier conversations in stone management:

“You have a kidney stone, but we may be able to make it disappear without an operation.”

Not every uric acid stone will dissolve successfully, and obstructed or infected kidneys require separate and sometimes urgent treatment.

4. Struvite Stones: The Infection Stones

Struvite stones contain magnesium ammonium phosphate and are closely associated with urinary infections caused by certain urease-producing bacteria.

These bacteria change the chemistry of the urine, producing an alkaline environment favourable for rapid stone formation.

Struvite stones can become very large and occasionally form a branching staghorn calculus, occupying much of the kidney’s collecting system.

Why are these stones important?

The stone and infection can maintain one another.

Leaving significant infected stone material behind may therefore contribute to:

• Recurrent urinary infection
• Rapid stone regrowth
• Kidney damage
• Sepsis

Management generally involves treating the infection and achieving as complete a stone clearance as reasonably possible. Large stones frequently require PCNL, sometimes performed in more than one stage.

Infection stone formers are regarded as being at high risk of recurrence.

5. Cystine Stones

Cystine stones are uncommon and are caused by cystinuria, an inherited disorder affecting the transport of certain amino acids through the kidneys.

Cystine is relatively insoluble in urine and can crystallise to form stones.

These patients may start developing stones at a young age and can experience repeated stone episodes throughout life.

Treatment and prevention

Prevention is particularly important and may involve:

• Very high fluid intake
• Reduced dietary sodium
• Urinary alkalinisation
• Potassium citrate
• Specialist medication such as tiopronin in selected recurrent cases

Current EAU guidance recommends aiming for a urine volume greater than 3 litres per day in adults with cystinuria and maintaining urinary pH above approximately 7.5 to improve cystine solubility.

Cystine stones are also relatively hard, which can make shockwave treatment less successful. Ureteroscopy with laser treatment or PCNL may therefore be required depending on stone burden.

6. Rare Kidney Stones

Much less commonly, stones may consist of substances such as:

• Xanthine
• 2,8-dihydroxyadenine
• Ammonium urate
• Matrix material
• Medication-related crystalline material

These stones can sometimes provide the first clue to an underlying metabolic, genetic or medication-related disorder.

Recurrent unusual stones therefore deserve specialist investigation rather than simply repeated stone removal.

How Do Kidney Stones Present?

Some kidney stones sit quietly within the kidney and are discovered incidentally during an ultrasound or CT scan.

Others announce their arrival with considerably less subtlety.

A stone entering and obstructing the ureter may cause renal colic, producing severe pain from the loin toward the abdomen or groin.

Other symptoms can include:

• Blood in the urine
• Nausea and vomiting
• Urinary urgency or frequency
• Burning during urination
• Recurrent urinary infection
• Fever or chills
• Intermittent loin discomfort

Fever plus an obstructed kidney is an emergency

An infected obstructed urinary system can progress rapidly to urosepsis.

Urgent drainage with a ureteric stent or nephrostomy tube, together with appropriate antibiotics and supportive treatment, may be required. Definitive stone treatment is generally delayed until the infection has been controlled.

How Are Kidney Stones Investigated?

CT Scan

A non-contrast CT scan of the kidneys, ureters and bladder provides detailed information regarding:

• Stone size
• Number of stones
• Exact location
• Degree of obstruction
• Kidney anatomy
• Stone density measured in Hounsfield units

CT density may also provide clues about stone composition and the likelihood of successful shockwave fragmentation.

Ultrasound

Ultrasound avoids radiation and is particularly useful for:

• Surveillance
• Detecting hydronephrosis
• Monitoring known renal stones
• Selected younger patients
• Pregnancy

However, very small stones and ureteric stones can sometimes be difficult to identify accurately.

Plain X-ray

A KUB X-ray may be useful for monitoring certain radiopaque stones.

Calcium-containing stones are usually radiopaque, whereas uric acid stones are typically radiolucent on plain X-ray.

Do All Kidney Stones Need Treatment?

No.

A small, non-obstructing and asymptomatic kidney stone may sometimes be monitored with periodic imaging.

Treatment becomes more appropriate when there is:

• Stone growth
• Recurrent pain
• Haematuria
• Urinary obstruction
• Recurrent infection
• Declining renal function
• Significant stone burden
• High risk of future complications
• Occupational or travel considerations
• Patient preference

These factors are reflected in contemporary EAU recommendations.

Treatment Options for Kidney Stones

1. Active Surveillance

Small asymptomatic renal stones can sometimes be observed.

Follow-up may include ultrasound, X-ray or CT depending on the type, size and visibility of the stone.

The advantage is obvious: no procedure unless one becomes necessary.

The disadvantage is equally obvious: stones have not signed a contract promising to remain where they are.

They may enlarge, migrate into the ureter, cause obstruction or become symptomatic.

2. Shockwave Lithotripsy

Extracorporeal shockwave lithotripsy (SWL) uses externally generated shockwaves focused onto the stone.

The aim is to fragment the calculus into smaller pieces that can subsequently pass through the urinary tract.

Advantages

• Non-invasive
• Usually rapid recovery
• No incision
• Useful for appropriately selected renal stones

Limitations

Success depends on:

• Stone size
• Location
• Density
• Composition
• Skin-to-stone distance
• Renal anatomy

Hard stones such as calcium oxalate monohydrate, brushite and cystine stones may be less responsive.

More than one treatment session may be required.

3. Flexible Ureteroscopy and Laser Lithotripsy

A fine flexible telescope is passed through the urethra and bladder, up the ureter and into the kidney.

There are no external incisions.

The stone can then be fragmented or dusted using a laser, with larger fragments removed using tiny retrieval baskets.

Advantages

• Minimally invasive
• High stone clearance rates for appropriately selected stones
• Can treat stones resistant to shockwave therapy
• Allows direct visualisation
• Suitable for many locations within the kidney

Possible disadvantages

• Requires anaesthesia
• Temporary ureteric stenting may be required
• Stent discomfort
• Bleeding or infection
• Ureteric injury is uncommon but possible
• Occasionally a second procedure is required

4. Percutaneous Nephrolithotomy

PCNL involves creating a small tract through the skin directly into the kidney.

It is particularly useful for:

• Large renal calculi
• Staghorn stones
• Complex stones
• Large-volume infection stones
• Stones unlikely to respond adequately to less invasive techniques

Current EAU guidance recommends PCNL as the first-line treatment for renal stones larger than 2 cm in most suitable patients.

PCNL generally provides excellent clearance of large stone burdens but is more invasive than ureteroscopy or shockwave treatment.

Potential complications include bleeding, infection, injury to surrounding structures and the need for additional procedures.

5. Dissolution Therapy

This option is mainly relevant to uric acid stones.

Urinary alkalinisation can gradually dissolve the stone, potentially avoiding surgery.

Treatment requires:

• Correct identification or strong suspicion of uric acid composition
• Regular urine pH monitoring
• Appropriate alkalinising medication
• Follow-up imaging
• Patient compliance

It is important not to assume that every radiolucent stone is uric acid, and treatment should be supervised appropriately.

Choosing the Right Treatment

There is no single “best” kidney stone operation.

The appropriate treatment depends on a combination of:

Stone factors

• Size
• Number
• Location
• Composition
• Density
• Previous growth

Kidney factors

• Anatomy
• Obstruction
• Infection
• Renal function

Patient factors

• Symptoms
• Medical conditions
• Anticoagulant therapy
• Previous stone procedures
• Occupation
• Travel requirements
• Personal preference

Two patients with apparently similar 10 mm stones may therefore receive quite different recommendations.

Preventing the Next Kidney Stone

Removing a stone solves today’s problem.

Preventing another stone solves tomorrow’s problem.

Patients with recurrent stones, multiple stones, bilateral stones, unusual stone composition, young age at presentation or other high-risk features may benefit from metabolic investigation.

This may include blood testing and 24-hour urine collections assessing factors such as:

• Urine volume
• Calcium
• Oxalate
• Citrate
• Uric acid
• Sodium
• Magnesium
• Urinary pH

Stone analysis is particularly valuable whenever a stone can be retrieved. Current EAU guidance recommends reliable stone analysis and basic metabolic evaluation for stone formers, with specific metabolic assessment for high-risk patients.

General Kidney Stone Prevention

Although prevention should ultimately be tailored to stone composition, several principles apply to many stone formers.

Drink more water

Maintaining generous urine output dilutes the substances responsible for crystal formation.

Water remains wonderfully unexciting and remarkably effective.

Reduce excessive salt intake

High sodium intake can increase urinary calcium excretion and contribute to calcium stone formation.

Maintain normal dietary calcium

Patients with calcium stones should not automatically eliminate calcium-containing foods.

A balanced dietary calcium intake may actually reduce intestinal oxalate absorption.

Moderate excessive animal protein

Large amounts of meat and other purine-rich foods may contribute to increased urinary uric acid and more acidic urine in susceptible individuals.

Maintain a healthy weight

Obesity and metabolic syndrome are particularly associated with uric acid stone formation.

Investigate recurrent stones

Repeatedly removing stones without investigating why they keep forming can become a rather expensive game of geological whack-a-mole.

Can Medication Prevent Kidney Stones?

Yes, in appropriately selected patients.

Depending on the metabolic abnormality and stone composition, preventive medication may include:

• Potassium citrate
• Thiazide or thiazide-like medication
• Allopurinol
• Urinary alkalinising therapy
• Tiopronin for selected cystinuria patients

Medication should ideally be guided by stone analysis, blood investigations and, when indicated, 24-hour urine testing rather than prescribed indiscriminately.

The Bottom Line

A kidney stone is not simply a kidney stone.

A calcium oxalate stone, uric acid stone, infection stone and cystine stone may look similar on a scan, but they can have very different causes and require very different long-term strategies.

Modern kidney stone management combines:

accurate imaging + appropriate stone removal + stone analysis + metabolic investigation + prevention.

Treatment may range from simple surveillance through to shockwave lithotripsy, flexible ureteroscopy and laser treatment, PCNL or, in selected uric acid stones, medical dissolution therapy.

The ultimate aim is not simply to leave the operating theatre stone-free.

It is to keep the patient stone-free.

When Should You See a Urologist?

Consider urological assessment if you have:

• Recurrent kidney stones
• Persistent loin or flank pain
• Blood in the urine
• Recurrent urinary infections
• A stone associated with urinary obstruction
• Increasing stone size on surveillance imaging
• Multiple or bilateral kidney stones
• A large renal calculus
• Previous complex stone surgery

Severe pain associated with fever, chills or feeling systemically unwell requires urgent medical assessment, as an infected obstructed kidney can be a medical emergency.

This information is intended for general patient education and does not replace individual medical assessment. The appropriate investigation, surveillance and treatment of kidney stones should be tailored to the individual patient.