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Urinary Calculi

(Nephrolithiasis; Stones; Urolithiasis)

By Glenn M. Preminger, MD

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Patient Education

Urinary calculi are solid particles in the urinary system. They may cause pain, nausea, vomiting, hematuria, and, possibly, chills and fever due to secondary infection. Diagnosis is based on urinalysis and radiologic imaging, usually noncontrast helical CT. Treatment is with analgesics, antibiotics for infection, medical expulsive therapy, and, sometimes, shock wave lithotripsy or endoscopic procedures.

About 1/1000 adults in the US is hospitalized annually because of urinary calculi, which are also found in about 1% of all autopsies. Up to 12% of men and 5% of women will develop a urinary calculus by age 70. Calculi vary from microscopic crystalline foci to calculi several centimeters in diameter. A large calculus, called a staghorn calculus, can fill an entire renal calyceal system.


About 85% of calculi in the US are composed of calcium, mainly calcium oxalate (see Table: Composition of Urinary Calculi); 10% are uric acid; 2% are cystine; most of the remainder are magnesium ammonium phosphate (struvite).

Composition of Urinary Calculi


Percentage of All Calculi

Common Causes

Calcium oxalate





Calcium phosphate





Renal tubular acidosis



Magnesium ammonium phosphate (struvite)


UTI caused by urea-splitting bacteria

Uric acid


Urine pH < 5.5

Occasionally hyperuricosuria

General risk factors include disorders that increase urinary salt concentration, either by increased excretion of calcium or uric acid salts, or by decreased excretion of urinary citrate.

For calcium calculi, risk factors vary by population. The main risk factor in the US is hypercalciuria, a hereditary condition present in 50% of men and 75% of women with calcium calculi; thus, patients with a family history of calculi are at increased risk of recurrent calculi. These patients have normal serum calcium, but urinary calcium is elevated > 250 mg/day (> 6.2 mmol/day) in men and > 200 mg/day (> 5.0 mmol/day) in women.

Hypocitruria (urinary citrate < 350 mg/day [1820 μmol/day]), present in about 40 to 50% of calcium calculi-formers, promotes calcium calculi formation because citrate normally binds urinary calcium and inhibits the crystallization of calcium salts.

About 5 to 8% of calculi are caused by renal tubular acidosis. About 1 to 2% of patients with calcium calculi have primary hyperparathyroidism. Rare causes of hypercalciuria are sarcoidosis, vitamin D intoxication, hyperthyroidism, multiple myeloma, metastatic cancer, and hyperoxaluria.

Hyperoxaluria (urinary oxalate > 40 mg/day [> 440 μmol/day]) can be primary or caused by excess ingestion of oxalate-containing foods (eg, rhubarb, spinach, cocoa, nuts, pepper, tea) or by excess oxalate absorption due to various enteric diseases (eg, bacterial overgrowth syndromes, chronic pancreatic or biliary disease) or ileojejunal (eg, bariatric) surgery.

Other risk factors include taking high doses of vitamin C (ie, > 2000 mg/day), a calcium-restricted diet (possibly because dietary calcium binds dietary oxalate), and mild hyperuricosuria. Mild hyperuricosuria, defined as urinary uric acid > 800 mg/day (> 5 mmol/day) in men or > 750 mg/day (> 4 mmol/day) in women, is almost always caused by excess intake of purine (in proteins, usually from meat, fish, and poultry); it may cause calcium oxalate calculus formation (hyperuricosuric calcium oxalate nephrolithiasis).

Uric acid calculi most commonly develop as a result of increased urine acidity (urine pH < 5.5), or rarely with severe hyperuricosuria (urinary uric acid > 1500 mg/day [> 9 mmol/day]), which crystallizes undissociated uric acid. Uric acid crystals may comprise the entire calculus or, more commonly, provide a nidus on which calcium or mixed calcium and uric acid calculi can form.

Cystine calculi occur only in the presence of cystinuria.

Magnesium ammonium phosphate calculi (struvite, infection calculi) indicate the presence of a UTI caused by urea-splitting bacteria (eg, Proteus sp, Klebsiella sp). The calculi must be treated as infected foreign bodies and removed in their entirety. Unlike other types of calculi, magnesium ammonium phosphate calculi occur 3 times more frequently in women.

Rare causes of urinary calculi include indinavir, melamine, triamterene, and xanthine.


Urinary calculi may remain within the renal parenchyma or renal collecting system or be passed into the ureter and bladder. During passage, calculi may irritate the ureter and may become lodged, obstructing urine flow and causing hydroureter and sometimes hydronephrosis. Common areas of lodgment include the following:

  • Ureteropelvic junction

  • Distal ureter (at the level of the iliac vessels)

  • Ureterovesical junction

Larger calculi are more likely to become lodged. Typically, a calculus must have a diameter > 5 mm to become lodged. Calculi 5 mm are likely to pass spontaneously.

Even partial obstruction causes decreased glomerular filtration, which may persist briefly after the calculus has passed. With hydronephrosis and elevated glomerular pressure, renal blood flow declines, further worsening renal function. Generally, however, in the absence of infection, permanent renal dysfunction occurs only after about 28 days of complete obstruction.

Secondary infection can occur with long-standing obstruction, but most patients with calcium-containing calculi do not have infected urine.

Symptoms and Signs

Large calculi remaining in the renal parenchyma or renal collecting system are often asymptomatic unless they cause obstruction and/or infection. Severe pain, often accompanied by nausea and vomiting, usually occurs when calculi pass into the ureter and cause acute obstruction. Sometimes gross hematuria also occurs.

Pain (renal colic) is of variable intensity but is typically excruciating and intermittent, often occurs cyclically, and lasts 20 to 60 min. Nausea and vomiting are common. Pain in the flank or kidney area that radiates across the abdomen suggests upper ureteral or renal pelvic obstruction. Pain that radiates along the course of the ureter into the genital region suggests lower ureteral obstruction. Suprapubic pain along with urinary urgency and frequency suggests a distal ureteral, ureterovesical, or bladder calculus (see Obstructive Uropathy : Symptoms and Signs).

On examination, patients may be in obvious extreme discomfort, often ashen and diaphoretic. Patients with renal colic may be unable to lie still and may pace, writhe, or constantly shift position. The abdomen may be somewhat tender on the affected side as palpation increases pressure in the already-distended kidney (costovertebral angle tenderness), but peritoneal signs (guarding, rebound, rigidity) are lacking.

For some patients, the first symptom is hematuria or either gravel or a calculus in the urine. Other patients may have symptoms of a UTI, such as fever, dysuria, or cloudy or foul-smelling urine.


  • Clinical differential diagnosis

  • Urinalysis

  • Imaging

  • Determination of calculus composition

The symptoms and signs may suggest other diagnoses, such as

  • Peritonitis (eg, due to appendicitis, ectopic pregnancy, or pelvic inflammatory disease): Pain is usually constant, and patients lie still because movement worsens pain; patients often also have rebound tenderness or rigidity.

  • Cholecystitis: May cause colicky pain, usually in the epigastrium or right upper quadrant, often with Murphy sign.

  • Bowel obstruction: May cause colicky abdominal pain and vomiting, but the pain is usually bilateral and not located primarily in the flank or along the ureter.

  • Pancreatitis: May cause upper abdominal pain and vomiting, but the pain is usually constant, may be bilateral, and is usually not along the flank or ureter.

With most of these disorders, urinary symptoms are uncommon and other symptoms may suggest which organ system is actually involved (eg, vaginal discharge or bleeding in pelvic disorders among females). Dissecting aortic aneurysm must be considered, particularly in the elderly, because, if a renal artery is affected, it can cause hematuria, pain that radiates along a ureteral distribution, or both. Other considerations in the general evaluation of acute abdominal pain are discussed elsewhere (see Acute Abdominal Pain : Evaluation).

Pearls & Pitfalls

  • Giving fluid (oral or IV) does not speed the passage of urinary calculi.

Patients suspected of having a calculus causing colic require urinalysis and usually an imaging study. If a calculus is confirmed, evaluation of the underlying disorder, including calculus composition testing, is required.


Macroscopic or microscopic hematuria is common, but urine may be normal despite multiple calculi. Pyuria with or without bacteria may be present. Pyuria suggests infection, particularly if combined with suggestive clinical findings, such as foul-smelling urine or a fever. A calculus and various crystalline substances may be present in the sediment. If so, further testing is usually necessary because the composition of the calculus and crystals cannot be determined conclusively by microscopy. The only exception is when typical hexagonal crystals of cystine are found in a concentrated, acidified specimen, confirming cystinuria.

Imaging tests

Noncontrast helical CT is the initial imaging study. This study can detect the location of a calculus as well as the degree of obstruction. Moreover, helical CT may also reveal another cause of the pain (eg, aortic aneurysm). For patients who have recurrent calculi, cumulative radiation exposure from multiple CT scans is a concern. However, the routine use of low-dose renal CT can meaningfully reduce cumulative radiation dose with little loss of sensitivity (1). For patients with typical symptoms, ultrasonography or plain abdominal x-rays can usually confirm presence of a calculus with minimal or no radiation exposure. MRI may not identify calculi.

Although most urinary calculi are demonstrable on plain x-ray, neither their presence nor their absence obviates the need for more definitive imaging, so this study can be avoided except in some patients with suspected recurrent calculi. Both renal ultrasonography and excretory urography (previously called intravenous urography) can identify calculi and hydronephrosis. However, ultrasonography is less sensitive for small or ureteral calculi in patients without hydronephrosis, and excretory urography is time consuming and exposes the patient to the risk of IV contrast agents. These studies are generally used when helical CT is unavailable.

Identifying the cause

The calculus is obtained by straining the urine (or, if necessary, during operative removal) and sent to the laboratory for stone analysis. Some calculi are brought in by patients. Urine specimens that show microscopic crystals are sent for crystallography.

In patients with a single calcium calculus and no additional risk factors for calculi, evaluation to exclude hyperparathyroidism is sufficient. Evaluation entails urinalysis and determination of plasma calcium concentration on 2 separate occasions. Predisposing factors, such as recurrent calculi, a diet high in animal protein, or use of vitamin C or D supplements, should be sought.

Patients with a strong family history of calculi, conditions that might predispose to calculi formation (eg, sarcoidosis, bone metastases, multiple myeloma), or conditions that would make it difficult to treat calculi (eg, solitary kidney, urinary tract anomalies) require evaluation for all possible causative disorders and risk factors. This evaluation should include serum electrolytes, uric acid, and calcium on 2 separate occasions. Follow-up determination of parathyroid hormone levels is done if necessary. Urine tests should include routine urinalysis and 2 separate 24-h urine collections to determine urine volume, pH, and excretion of calcium, uric acid, citrate, oxalate, sodium, and creatinine (2).


  • Analgesia

  • Facilitate calculus passage, eg, with alpha-receptor blockers such as tamsulosin (described as medical expulsive therapy)

  • For persistent or infection-causing calculi, complete removal using primarily endoscopic techniques


Renal colic may be relieved with opioids, such as morphine and, for a rapid onset, fentanyl. Ketorolac 30 mg IV is rapidly effective and nonsedating. Vomiting usually resolves as pain decreases, but persistent vomiting can be treated with an antiemetic (eg, ondansetron 10 mg IV).

Medical expulsive therapy

Although increasing fluids (either oral or IV) has traditionally been recommended, increased fluid administration has not been proven to speed the passage of calculi. Patients with calculi < 1 cm in diameter who have no infection or obstruction, whose pain is controlled with analgesics, and who can tolerate liquids can be treated at home with analgesics and alpha-receptor blockers (eg, tamsulosin 0.4 mg po once/day) to facilitate calculus passage. Calculi that have not passed within 6 to 8 wk typically require removal. In patients with infection and obstruction, initial treatment is relief of obstruction with a ureteral stent and treatment of the infection followed by removal of calculi as soon as possible.

Calculus removal

The technique used for removal depends on the location and size of the calculus. Techniques include shock wave lithotripsy and, to ensure complete removal or for larger calculi, endoscopic techniques. Endoscopic techniques may involve rigid or flexible ureteroscopes (endoscopes) and may involve direct-vision removal (basketing), fragmentation with some sort of lithotripsy device (eg, pneumatic, ultrasonic, laser), or both.

For symptomatic calculi < 1 cm in diameter in the renal collecting system or proximal ureter, shock wave lithotripsy is a reasonable first option for therapy.

For larger calculi or if shock wave lithotripsy is unsuccessful, ureteroscopy (done in a retrograde fashion) with holmium laser lithotripsy is usually used. Sometimes removal is possible using an endoscope inserted anterograde through the kidney. For renal stones > 2 cm, percutaneous nephrolithotomy, with insertion of a nephroscope directly into the kidney, is the treatment of choice.

For midureteral calculi , ureteroscopy with holmium laser lithotripsy is usually the treatment of choice. Shock wave lithotripsy is an alternative.

For distal ureteral calculi , endoscopic techniques (ureteroscopy), such as direct removal and use of intracorporeal lithotripsy (eg, pneumatic, electrohydraulic, laser), are considered by many to be the procedures of choice. Shock wave lithotripsy can also be used.

Calculus dissolution

Uric acid calculi in the upper or lower urinary tract occasionally may be dissolved by prolonged alkalinization of the urine with potassium citrate 20 mEq po bid to tid, but chemical dissolution of calcium calculi is not possible and of cystine calculi is difficult.


In a patient who has passed a first calcium calculus, the likelihood of forming a 2nd calculus is about 15% at 1 yr, 40% at 5 yr, and 80% at 10 yr. Drinking large amounts of fluids—8 to 10 ten-ounce (300-milliliter) glasses a day—is recommended for prevention of all stones. Recovery and analysis of the calculus, measurement of calculus-forming substances in the urine, and the clinical history are needed to plan other prophylactic measures.

In < 3% of patients, no metabolic abnormality is found. These patients seemingly cannot tolerate normal amounts of calculus-forming salts in their urine without crystallization. Thiazide diuretics, potassium citrate, and increased fluid intake may reduce their calculus production rate.

For hypercalciuria, patients may receive thiazide diuretics (eg, chlorthalidone 25 mg po once/day or indapamide 1.25 mg po once/day) to lower urine calcium excretion and thus prevent urinary supersaturation with calcium oxalate. Patients are encouraged to increase their fluid intake to 3 L/day. A diet that is low in sodium and high in potassium is recommended. Even with a high potassium intake, supplementation with potassium citrate is recommended to prevent hypokalemia. Restriction of dietary animal protein is also recommended.

For patients with hypocitruria, potassium citrate (20 mEq po bid) enhances citrate excretion. A normal calcium intake (eg, 1000 mg or about 2 to 3 dairy servings per day) is recommended, and calcium restriction is avoided. Oral orthophosphate has not been thoroughly studied.

Hyperoxaluria prevention varies. Patients with small-bowel disease can be treated with a combination of high fluid intake, calcium loading (usually in the form of calcium citrate 400 mg po bid with meals), cholestyramine, and a low-oxalate, low-fat diet. Hyperoxaluria may respond to pyridoxine 100 to 200 mg po once/day, possibly by increasing transaminase activity, because this activity is responsible for the conversion of glyoxylate, the immediate oxalate precursor, to glycine.

In hyperuricosuria, intake of animal protein should be reduced. If the diet cannot be changed, allopurinol 300 mg each morning lowers uric acid production. For uric acid calculi, the urine pH must be increased to between 6 and 6.5 by giving an oral alkalinizing drug that contains potassium (eg, potassium citrate 20 mEq bid) along with increased fluid intake.

Infection with urea-splitting bacteria requires culture-specific antibiotics and complete removal of all calculi. If eradication of infection is impossible, long-term suppressive therapy (eg, with nitrofurantoin) may be necessary. In addition, acetohydroxamic acid can be used to reduce the recurrence of struvite calculi.

To prevent recurrent cystine calculi, urinary cystine levels must be reduced to < 250 mg cystine/L of urine. Any combination of increasing urine volume along with reducing cystine excretion (eg, with alpha-mercaptopropionylglycine or penicillamine) should reduce the urinary cystine concentration.

Diagnosis and treatment references

Key Points

  • 85% of urinary calculi are calcium, mainly calcium oxalate (see Table: Composition of Urinary Calculi); 10% are uric acid; 2% are cystine; and most of the remainder are magnesium ammonium phosphate (struvite).

  • Larger calculi are more likely to obstruct; however, obstruction can occur even with small ureteral calculi (ie, 2 to 5 mm).

  • Symptoms include hematuria, symptoms of infection, and renal colic.

  • Test usually with urinalysis, imaging, and if the calculus can later be retrieved, determination of calculus composition.

  • Give analgesics and drugs to facilitate calculus passage (eg, alpha-receptor blockers) acutely and remove calculi that cause infection or persist endoscopically.

  • Decrease the risk of subsequent calculus formation by treating with measures such as thiazide diuretics, potassium citrate, increases in fluid intake, and decreases in dietary animal protein, depending on calculus composition.

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