Kidney Stones

What kidney stones are

Urine contains dissolved minerals such as calcium, oxalate, phosphate and uric acid. When there is more of these than the urine can hold in solution, or too little of the natural inhibitors such as citrate, crystals form and clump together. Stones can sit in the kidney silently for years or break loose and travel down the ureter, a tube only 3 to 4 mm wide, where they cause renal colic.

Stones range from grains of sand to several centimeters. Large branching staghorn stones can fill the kidney's collecting system.

Types of kidney stones

Causes and risk factors

• Dehydration and low fluid intake, hot climates and heavy sweating
• Diet: high salt (raises urine calcium), high animal protein, high sugar and fructose drinks, very high oxalate foods in some people
• Obesity, type 2 diabetes and metabolic syndrome
• Gout and high uric acid
• Bowel disease and surgery: Crohn's disease, chronic diarrhea and gastric bypass surgery increase oxalate absorption and reduce urine volume and citrate
• Medical conditions: hyperparathyroidism, renal tubular acidosis, medullary sponge kidney, polycystic kidney disease, sarcoidosis
• Recurrent urinary infections (struvite stones)
• Medicines: topiramate, acetazolamide, high-dose vitamin C supplements, some HIV drugs, excess calcium and vitamin D supplements
• Family history: having a parent or sibling with stones roughly doubles your risk
• Sex and age: most common between 30 and 60. Historically more common in men, but rates in women have risen

Low dietary calcium actually increases stone risk, because less calcium is available to bind oxalate in the gut.

Symptoms

Stones in the kidney often cause no symptoms. When a stone moves, it causes sudden severe flank pain in waves, spreading to the groin, with nausea, vomiting and blood in the urine. See kidney stone pain for a detailed guide to symptoms and when to seek emergency care.

Diagnosis

1. Imaging: non-contrast low-dose CT is the most accurate test. Ultrasound is used in pregnancy, children and often for follow-up. Plain X-ray tracks calcium stones over time.
2. Urine tests: blood, infection, crystals and pH.
3. Blood tests: creatinine (kidney function), calcium, uric acid, bicarbonate, and parathyroid hormone if calcium is high.
4. Stone analysis: a passed or removed stone is sent to the lab to identify its type, which guides prevention.
5. 24-hour urine collection: recommended for people with recurrent stones, children and high-risk patients. It measures volume, calcium, oxalate, citrate, uric acid, sodium and other factors.

Treatment

Passing a stone

Stones under about 10 mm with controlled pain and no infection are often managed with medical expulsive therapy: an NSAID for pain, fluids and an alpha-blocker such as tamsulosin to relax the ureter. Follow-up imaging confirms the stone has passed. Most people are given 4 to 6 weeks before a procedure is advised.

Emergency drainage

A stone causing blockage with infection is a urological emergency. The kidney is drained urgently with a ureteral stent (a thin tube from kidney to bladder) or a nephrostomy (a tube through the back), along with antibiotics. The stone is treated later once the infection has cleared.

Procedures to remove stones

Dissolving stones

Uric acid stones can often be dissolved by making the urine less acidic with potassium citrate. Cystine stones respond partly to alkalinization and specific medicines.

Complications

• Kidney infection and sepsis when a blocked kidney becomes infected
• Hydronephrosis (swelling of the kidney) and kidney damage from long-standing blockage
• Ureteral narrowing (stricture) after stones or procedures
• Chronic kidney disease with recurrent stones or certain stone types

Preventing kidney stones

Prevention works and is worth the effort, as recurrence is common.

For everyone with stones

• Drink more: aim for enough fluid to produce at least 2.5 liters of urine a day, usually around 3 liters of fluid. Urine should be pale yellow. Water is best. Citrus drinks such as lemon water add citrate.
• Cut salt: aim for under 2,300 mg of sodium a day. Processed foods are the main source.
• Keep normal calcium in your diet: about 1,000 to 1,200 mg a day from food. Do not cut out dairy unless told to.
• Limit animal protein: meat, poultry, seafood and eggs in moderation.
• Avoid sugary drinks and high-fructose corn syrup.
• Maintain a healthy weight.

Type-specific advice

• Calcium oxalate: eat calcium-containing foods with meals, limit very high oxalate foods such as spinach, rhubarb, nuts and beets if urine oxalate is high, and avoid high-dose vitamin C supplements
• Uric acid: reduce red meat, organ meats, shellfish and alcohol, especially beer
• Struvite: treat and prevent urinary infections, and remove all stone fragments

Medicines

Based on 24-hour urine results, doctors may prescribe thiazide diuretics (to reduce urine calcium), potassium citrate (to raise citrate and urine pH), allopurinol (for high uric acid) or tiopronin (for cystine stones).

Frequently asked questions

What causes kidney stones?

Stones form when urine contains more stone-forming minerals than it can dissolve, most often because of low fluid intake. High salt and animal protein diets, obesity, diabetes, gout, bowel disease, some medicines, certain kidney and parathyroid conditions, infections and family history all increase risk.

What is the treatment for kidney stones?

Small stones usually pass with pain relief, fluids and sometimes tamsulosin. Larger stones or those that do not pass are treated with shock wave lithotripsy, ureteroscopy with laser, or percutaneous nephrolithotomy. A blocked, infected kidney needs emergency drainage with a stent or tube.

What is the best diet to prevent kidney stones?

Drink plenty of fluids, keep salt low, eat normal amounts of calcium from food, limit animal protein and avoid sugary drinks. Those with oxalate stones may also limit very high oxalate foods. A 24-hour urine test helps tailor the diet to your stone type.

Will I get another kidney stone?

Without prevention, around half of people have another stone within 5 to 10 years. Drinking more fluids, dietary changes and, when needed, medicines based on a 24-hour urine test greatly reduce that risk.

Can kidney stones damage your kidneys?

Usually not, if they are treated in time. A stone blocking the kidney for weeks, repeated stones, infection behind a blockage, and some stone types such as cystine and struvite can cause lasting kidney damage, which is why follow-up matters.

Should I avoid calcium if I get kidney stones?

No. Cutting calcium from the diet usually increases stone risk, because dietary calcium binds oxalate in the gut. Aim for normal calcium intake from food. Calcium supplements are a different matter and should be discussed with your doctor.