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However, when these substances fall below their normal proportions, stones can form from an aggregation of crystals. Kidney stones often result from a combination of factors, rather than a single, well-defined cause. Stones are more common in people whose diet is very high in animal protein or who do not consume enough water or calcium.

1 They can result from an underlying metabolic condition, such as distal renal tubular acidosis,20 Dent's disease,21 hyperparathyroidism,22 primary hyperoxaluria23 or medullary sponge kidney. In fact, studies show about 3% to 20% of people who form kidney stones have medullary sponge kidney. 1524 Kidney stones are also more common in people with Crohn's disease.

25 People with recurrent kidney stones are often screened for these disorders. This is typically done with a 24-hour urine collection that is chemically analyzed for deficiencies and excesses that promote stone formation. Diagnosis of kidney stones is made on the basis of information obtained from the history, physical examination, urinalysis, and radiographic studies.

26 Clinical diagnosis is usually made on the basis of the location and severity of the pain, which is typically colicky in nature (comes and goes in spasmodic waves). Pain in the back occurs when calculi produce an obstruction in the kidney. 27 Physical examination may reveal fever and tenderness at the costovertebral angle on the affected side.

Calcium-containing stones are relatively radiodense, and they can often be detected by a traditional radiograph of the abdomen that includes the kidneys, ureters, and bladder (KUB film). 28 Some 60% of all renal stones are radiopaque. 2930 In general, calcium phosphate stones have the greatest density, followed by calcium oxalate and magnesium ammonium phosphate stones.

Cystine calculi are only faintly radiodense, while uric acid stones are usually entirely radiolucent. Where available, a noncontrast helical CT scan with 5 millimeters (0.20 in) sections is the diagnostic modality of choice in the radiographic evaluation of suspected nephrolithiasis. 326293233 All stones are detectable on CT scans except very rare stones composed of certain drug residues in the urine,28 such as from indinavir.

Where a CT scan is unavailable, an intravenous pyelogram may be performed to help confirm the diagnosis of urolithiasis. This involves intravenous injection of a contrast agent followed by a KUB film. Uroliths present in the kidneys, ureters or bladder may be better defined by the use of this contrast agent.

Stones can also be detected by a retrograde pyelogram, where a similar contrast agent is injected directly into the distal ostium of the ureter (where the ureter terminates as it enters the bladder). Ultrasound imaging of the kidneys can sometimes be useful, as it gives details about the presence of hydronephrosis, suggesting the stone is blocking the outflow of urine. 28 Radiolucent stones, which do not appear on CT scans, may show up on ultrasound imaging studies.

Other advantages of renal ultrasonography include its low cost and absence of radiation exposure. Ultrasound imaging is useful for detecting stones in situations where X-rays or CT scans are discouraged, such as in children or pregnant women. 34 Despite these advantages, renal ultrasonography is not currently considered a substitute for noncontrast helical CT scan in the initial diagnostic evaluation of urolithiasis.

32 The main reason for this is that compared with CT, renal ultrasonography more often fails to detect small stones (especially ureteral stones), as well as other serious disorders that could be causing the symptoms. Collection of stones (by urinating through a StoneScreen kidney stone collection cup or a simple tea strainer) is useful. Chemical analysis of collected stones can establish their composition, which in turn can help to guide future preventive and therapeutic management.

Kidney stones are typically classified by their location and chemical composition. By far, the most common type of kidney stones worldwide contains calcium. For example, calcium-containing stones represent about 80% of all cases in the United States; these typically contain calcium oxalate either alone or in combination with calcium phosphate in the form of apatite or brushite.

1519 Factors that promote the precipitation of oxalate crystals in the urine, such as primary hyperoxaluria, are associated with the development of calcium oxalate stones. 23 The formation of calcium phosphate stones is associated with conditions such as hyperparathyroidism22 and renal tubular acidosis. About 10–15% of urinary calculi are composed of struvite (ammonium magnesium phosphate, NH4MgPO4·6H2O).

37 Struvite stones (also known as "infection stones", urease or triple-phosphate stones), form most often in the presence of infection by urea-splitting bacteria. Using the enzyme urease, these organisms metabolize urea into ammonia and carbon dioxide. This alkalinizes the urine, resulting in favorable conditions for the formation of struvite stones.

Proteus mirabilis, Proteus vulgaris, and Morganella morganii are the most common organisms isolated; less common organisms include Ureaplasma urealyticum, and some species of Providencia, Klebsiella, Serratia, and Enterobacter. These infection stones are commonly observed in people who have factors that predispose them to urinary tract infections, such as those with spinal cord injury and other forms of neurogenic bladder, ileal conduit urinary diversion, vesicoureteral reflux, and obstructive uropathies. They are also commonly seen in people with underlying metabolic disorders, such as idiopathic hypercalciuria, hyperparathyroidism, and gout.

Infection stones can grow rapidly, forming large calyceal staghorn (antler-shaped) calculi requiring invasive surgery such as percutaneous nephrolithotomy for definitive treatment. About 5–10% of all stones are formed from uric acid. 20 People with certain metabolic abnormalities, including obesity,6 may produce uric acid stones.

They also may form in association with conditions that cause hyperuricosuria (an excessive amount of uric acid in the urine) with or without hyperuricemia (an excessive amount of uric acid in the serum). They may also form in association with disorders of acid/base metabolism where the urine is excessively acidic (low pH), resulting in precipitation of uric acid crystals.