Kidney Stones

Kidney stones are hard deposits that form when minerals and salts in the urine crystallise, most often within the kidney itself. They are extremely common, and while many stones are small enough to pass unnoticed, others cause some of the most severe pain a person can experience, particularly if the stone moves into the ureter, the tube connecting the kidney to the bladder, and becomes lodged there.

Kidney stones are also a genuinely recurring condition for many people: roughly half of those who form one stone will form another within five to ten years. This is an important starting point for understanding kidney stones properly — treating the current stone is only part of the picture, and for anyone who has had more than one episode, understanding why stones keep forming is just as relevant as treating the one causing symptoms today.

How Kidney Stones Form

Urine normally holds dissolved minerals and salts in solution, kept from crystallising by the balance of water and other substances within it. Stones form when urine becomes supersaturated, meaning it contains more of certain crystal-forming substances, such as calcium, oxalate, or uric acid, than the available fluid can keep dissolved. These substances then crystallise, and over time, additional layers build up around the initial crystal, gradually forming a solid stone that can range from smaller than a grain of sand to several centimetres across.

Types of Kidney Stones

• Calcium Oxalate Stones: The most common type by far, forming when calcium combines with oxalate, a substance found in many foods. Often associated with low fluid intake, high oxalate intake, or higher-than-normal calcium levels in the urine.
• Calcium Phosphate Stones: Less common than calcium oxalate stones, and more often associated with certain metabolic conditions or urine that is persistently more alkaline than usual.
• Uric Acid Stones: Form when urine is persistently too acidic, often related to a diet high in animal protein, obesity, or conditions such as gout. Uric acid stones are notable for sometimes being manageable with dietary and medical measures that make the urine less acidic, in addition to standard stone treatment.
• Struvite (Infection) Stones: Form as a direct result of certain urinary tract infections, and can grow rapidly, sometimes filling much of the kidney's internal collecting system (a “staghorn” stone). Treating and preventing recurrent infection is central to managing this type.
• Cystine Stones: A rare type caused by an inherited condition affecting how the kidneys handle a particular amino acid, typically presenting recurrently from a younger age and requiring specific long-term management.

Why Kidney Stones Are a Recurring Problem

For many patients, the most important question is not just how to treat the current stone, but why stones keep forming in the first place. With roughly half of first-time stone formers experiencing another episode within five to ten years, recurrence is the rule rather than the exception for a significant proportion of patients.

For anyone with recurrent stones, or a first stone alongside other risk factors, a metabolic evaluation, typically involving a 24-hour urine collection to measure levels of calcium, oxalate, uric acid, citrate, and other relevant substances, can identify a specific, correctable imbalance driving stone formation. Combined with knowing the type of stone previously passed or removed, this information allows prevention strategies to be tailored to the individual, rather than relying on generic advice alone.

When a Kidney Stone Becomes a Urologic Emergency

The vast majority of kidney stone episodes, however painful, are not immediately dangerous. The important exception is a stone that is both obstructing urine flow and associated with infection. When infected urine is trapped behind a blocked ureter with nowhere to drain, the infection can rapidly progress to a severe, systemic illness (urosepsis), which is a genuine medical emergency.

This is why fever accompanying stone-related pain is treated very differently from pain alone. An obstructing stone without infection can often be managed with pain control and monitoring, or scheduled treatment. An obstructing stone with fever generally requires urgent drainage of the kidney, often with a temporary stent or drainage tube placed before the stone itself is definitively treated, precisely because clearing the infection safely takes priority over removing the stone in that moment.

Symptoms of Kidney Stones

A stone sitting quietly within the kidney often causes no symptoms at all. Symptoms typically appear once a stone moves into the ureter and obstructs urine flow:

• Severe pain in the back or side, below the ribs, that may radiate to the lower abdomen and groin
• Pain or burning during urination
• Blood in the urine, which may be visible or only detected on testing
• Nausea and vomiting
• A persistent urge to urinate, sometimes passing only small amounts
• Cloudy or foul-smelling urine
• Fever and chills, if infection is also present

Causes of Kidney Stones

Kidney stones form when urine contains more stone-forming substances, such as calcium, oxalate, or uric acid, than the available fluid can dissolve. Several factors make this imbalance more likely:

• Chronic low fluid intake, resulting in more concentrated urine
• A diet high in sodium, animal protein, or oxalate-rich foods
• Underlying conditions such as gout, inflammatory bowel disease, or conditions affecting calcium metabolism
• Recurrent urinary tract infections, particularly for struvite stones
• Certain inherited metabolic conditions, relevant to cystine and some other rarer stone types

Risk Factors for Kidney Stones

• Family history: Having a close relative with kidney stones increases individual risk
• Dehydration: Insufficient fluid intake concentrates the urine, making crystal formation more likely
• Obesity: A higher body mass index is associated with increased kidney stone risk
• A high-sodium diet: Excess dietary salt increases the amount of calcium the kidneys must filter
• Previous kidney stones: The single strongest predictor of future stones
• Certain digestive conditions or prior bowel surgery, which can affect how the body absorbs stone-forming substances
• Hot climates or occupations involving heavy sweating, which increase fluid loss and urine concentration

How Kidney Stones are Diagnosed

• Urine Test: Checks for blood, signs of infection, and can assess urine chemistry relevant to stone formation.
• Blood Test: Assesses kidney function and levels of calcium, uric acid, and other substances that can contribute to stone formation.
• CT Scan: The most accurate imaging test for detecting kidney stones, determining their size and location precisely, and identifying whether a stone is causing obstruction — the primary imaging test used when a kidney stone is suspected.
• Ultrasound: A radiation-free imaging option, useful for detecting stones and signs of obstruction, and often preferred as a first-line test in pregnancy or when repeated imaging is anticipated.
• KUB (Kidney, Ureter, and Bladder) X-ray: Can detect certain types of stones and is sometimes used to monitor a previously identified stone over time, though it is less sensitive than CT for initial diagnosis.
• 24-Hour Urine Collection: Recommended for patients with recurrent stones, measuring levels of calcium, oxalate, uric acid, and other substances over a full day to identify a specific, correctable metabolic driver of stone formation.

Reducing Your Risk of Kidney Stones

• Drink plenty of water throughout the day, aiming for pale urine as a general guide to adequate hydration
• Limit sodium intake, since excess dietary salt increases urinary calcium
• Moderate animal protein intake, particularly if you have had uric acid or calcium stones
• Limit high-oxalate foods, such as spinach, nuts, and certain soy products, particularly if oxalate has been identified as a specific driver in your case
• Maintain a healthy weight through sustainable diet and activity changes
• Include adequate dietary calcium from food sources, since, counter to popular assumption, adequate dietary calcium can help reduce oxalate absorption and lower stone risk for many people
• If you have had more than one stone, ask about a metabolic evaluation to identify a specific, targeted prevention strategy rather than relying on general advice alone