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Kidney stones in adults: Evaluation of the patient with established stone disease

Kidney stones in adults: Evaluation of the patient with established stone disease
Authors:
Glenn M Preminger, MD
Gary C Curhan, MD, ScD
Section Editor:
Michael P O'Leary, MD, MPH
Deputy Editor:
Albert Q Lam, MD
Literature review current through: Dec 2022. | This topic last updated: Aug 23, 2021.

INTRODUCTION — Kidney stone disease (nephrolithiasis) is a common problem in primary care practice. Patients may present with the classic symptoms of renal colic and hematuria. Some patients may be asymptomatic or have atypical symptoms such as vague abdominal pain, while others will have more typical symptoms, such as acute abdominal or flank pain, nausea, urinary urgency or frequency, difficulty urinating, penile pain, or testicular pain.

This topic will review the evaluation of the patient with established stone disease (either newly diagnosed or recurrent stones) or asymptomatic stones. Other aspects of kidney stones in adults are discussed separately:

(See "Kidney stones in adults: Epidemiology and risk factors".)

(See "Kidney stones in adults: Diagnosis and acute management of suspected nephrolithiasis".)

(See "Kidney stones in adults: Prevention of recurrent kidney stones".)

(See "Kidney stones in adults: Surgical management of kidney and ureteral stones".)

GOAL OF EVALUATION — In patients with established kidney stone disease, the goal of a diagnostic evaluation is to identify, as efficiently and economically as possible, the particular physiologic differences present in a given patient so that effective therapy to prevent recurrent stones can be established and the prognosis can be better defined. Thus, the type and extent of evaluation depend in part upon the following:

The severity and type of stone disease

Whether it is a first or a recurrent stone

Presence or absence of systemic disease and/or risk factors for recurrent stone formation

Family history of nephrolithiasis

The patient's interest in stone prevention

APPROACH TO EVALUATION

Patients with established stone disease — All patients presenting with established kidney stone disease (either newly diagnosed or recurrent stones) should undergo a focused history, radiologic imaging, stone analysis (if available), and at least a limited laboratory evaluation. The approach for patients with asymptomatic stones is discussed elsewhere in this topic. (See 'Patients with asymptomatic stones' below.)

Focused history for stone risk factors — The purpose of the focused history is to identify stone risk factors, such as a family history of stone disease and certain dietary habits (table 1). These factors are described in detail elsewhere. (See "Kidney stones in adults: Epidemiology and risk factors".)

Summarized briefly, adverse dietary habits include:

Low fluid intake or a high fluid loss (eg, from sweating or gastrointestinal losses), which leads to a lower urine volume and, therefore, a higher concentration of lithogenic factors.

A very high animal protein diet, which can lead to higher excretion of calcium and uric acid and lower excretion of urine citrate (figure 1).

Higher sodium diet, which increases urinary calcium excretion [1].

Increased intake of higher oxalate-containing foods, particularly spinach. The magnitude of the contribution of dietary oxalate to urinary oxalate is controversial and likely varies considerably from person to person.

Lower calcium intake, which acts by increasing the absorption of dietary oxalate and subsequent higher excretion of oxalate due to decreased calcium oxalate complex formation within the intestinal lumen [2,3]. The effect of lower calcium intake on oxalate more than counterbalances the decrease in calcium absorption and excretion.

Excessive vitamin C and D supplementation, which may increase urinary oxalate or calcium, respectively.

Excessive sugar (sucrose and fructose) intake, which may increase calcium and/or oxalate excretion.

In addition, certain medications can occasionally crystallize in the urine and lead to stone formation. Examples include atazanavir, sulfadiazine, and triamterene. (See "Crystal-induced acute kidney injury" and "Triamterene nephrotoxicity".)

Radiologic testing — If not yet performed during the initial evaluation, radiographic examination, preferably with noncontrast, low-dose computed tomography (CT), should be obtained to search for residual stones within the urinary tract. Diagnostic tests for the detection of nephrolithiasis are discussed in detail elsewhere. (See "Kidney stones in adults: Diagnosis and acute management of suspected nephrolithiasis", section on 'Diagnostic imaging'.)

Stone analysis — Analysis of the stone is an essential part of the evaluation [4-6]. Patients should be encouraged to retrieve stones they pass spontaneously for analysis, although novel CT imaging techniques may permit noninvasive discrimination among the main subtypes of urinary calculi. Similarly, stones that are surgically removed should also be submitted for analysis. (See "Kidney stones in adults: Diagnosis and acute management of suspected nephrolithiasis", section on 'Determination of stone composition'.)

Number of stones to analyze — At least one stone should be analyzed in every patient. Given sampling issues, analyzing two stones provides useful information about other possible and/or relevant components. If the two stone composition reports are sufficiently similar, then future stones do not need to be analyzed unless there has been a clinically relevant change. However, some patients produce more than one stone type. In a study of patients with bilateral stones, for example, the major stone component was discordant between the two kidneys in 25 percent of individuals [7]. In this setting, it would be reasonable to analyze more than one stone that is passed from each side. Also, if a new stone forms after many years, it should be sent for analysis if passed or retrieved.

Occasionally, treatment to prevent one stone type may inadvertently lead to the formation of a different stone type (though this is uncommon). As an example, over-alkalinization of the urine of a patient with uric acid stones could increase the risk of calcium phosphate crystal formation. Thus, analysis of a newly formed stone after the initiation of treatment is indicated.

Individual crystalline components — The most common crystalline materials found in kidney stones are calcium oxalate, calcium phosphate, uric acid, and struvite (magnesium ammonium phosphate). It is not uncommon for a stone to contain more than one crystalline component.

Calcium oxalate – Calcium oxalate is the most common component found in kidney stones (approximately 70 to 80 percent). Calcium oxalate can be found in monohydrate and dihydrate forms (picture 1A). Calcium oxalate can also be present in combination with uric acid or calcium phosphate. Because calcium oxalate stones typically grow on a Randall's plaque (composed of calcium phosphate) on the papillary tip [8,9], a laboratory that examines the composition of the nidus may report a stone with an eccentric calcium phosphate nidus (usually 5 percent) and a calcium oxalate body (95 percent).

Calcium phosphate – Calcium phosphate is found in approximately 15 percent of kidney stones and can be present in combination with calcium oxalate or struvite. Because of differences in solubility due to urine pH, calcium phosphate is not found mixed with uric acid. The two forms of calcium phosphate include apatite (sometimes reported as carbonate apatite), which is the crystal type found in bone, or calcium hydrogen phosphate (brushite); the frequency of apatite is much greater than brushite. Calcium phosphate crystals in the urine sediment are typically dark and amorphous.

Uric acid – Uric acid is the most common crystal form that contains urate (picture 2A). Rare crystals that contain urate include sodium urate (which is present in the joint fluid of patients with gouty arthritis) and ammonium urate. Uric acid is present in approximately 8 percent of analyzed stones, sometimes in combination with calcium oxalate. (See "Kidney stones in adults: Uric acid nephrolithiasis".)

Struvite – Struvite is the crystal name for stones that form only in the presence of urease-producing bacteria (eg, Proteus mirabilis, Klebsiella pneumoniae, Corynebacterium species, Ureaplasma urealyticum) in the upper urinary tract (picture 3). Other names for this crystal type include "triple phosphate" (because the phosphate is in the triple-negative form) and magnesium ammonium phosphate carbonate apatite. Struvite is found in approximately 1 percent of analyzed stones and is much more common in females than in males (due to the higher risk of urinary tract infections in females). If a preexisting calcium-containing kidney stone is subsequently infected with a urease-producing bacterium, the stone analysis may report that the composition of the stone includes calcium oxalate or calcium phosphate in addition to struvite. (See "Kidney stones in adults: Struvite (infection) stones".)

Other crystal types – Rare crystal types include [10]:

Cystine (picture 4). (See "Cystinuria and cystine stones".)

2,8-dihydroxyadenine (DHA) - DHA crystals form as a result of adenine phosphoribosyltransferase deficiency, a rare autosomal recessive disorder; DHA crystals may be incorrectly reported as uric acid by some laboratories [11].

Triamterene. (See "Triamterene nephrotoxicity", section on 'Triamterene stones'.)

Acyclovir. (See "Crystal-induced acute kidney injury", section on 'Acyclovir'.)

Indinavir. (See "Crystal-induced acute kidney injury", section on 'Indinavir'.)

Clinical relevance — Knowing the composition of the stone assists with clinical decision-making for the treatment of existing stones and prevention of new stone formation. Examples of how this knowledge can impact clinical decision making include:

Calcium oxalate monohydrate and brushite are hard stones and may not be fragmented as easily with shock wave lithotripsy.

Uric acid stones form in acid urine, and alkalinization of the urine can both dissolve existing uric acid stones and prevent new stones from forming. (See "Kidney stones in adults: Uric acid nephrolithiasis", section on 'Urinary alkalinization'.)

Calcium phosphate stones form in alkaline urine, and therefore, increasing the urine pH may increase the likelihood of calcium phosphate precipitation. While reducing the urine pH would be helpful, this is often not clinically possible for most patients who form calcium phosphate stones (for unclear reasons).

The presence of certain stone types may indicate the existence of an underlying predisposing condition. As examples, calcium phosphate stones are more frequent in individuals with primary hyperparathyroidism and distal renal tubular acidosis, and struvite stones form in the presence of an upper urinary tract infection. Uric acid stones may be more common in individuals with diabetes mellitus (due to impaired ammoniagenesis), metabolic syndrome [12], or gout. (See "Kidney stones in adults: Epidemiology and risk factors", section on 'Medical conditions'.)

When designing a preventive regimen for a patient, the individual components of the stone need to be considered:

For pure stones, the focus is on modifying the urine composition to prevent the precipitation of that specific crystal type, even if the 24-hour urine composition suggests a high risk for precipitation of another crystal type. (See '24-hour urine collections' below.)

For mixed stones, the treatment recommendations depend upon the specific components and the relative amounts present. As an example, for a stone that is reported to contain 95 percent calcium oxalate and 5 percent calcium phosphate, the focus should be on reducing the supersaturation of calcium oxalate. However, these two crystal types share risk factors (lower urine volume, higher urine calcium, and lower urine citrate), so modifying these components should reduce the risk of both crystal types. (See "Kidney stones in adults: Prevention of recurrent kidney stones".)

Given the variability in reporting of mixed stones by commercial laboratories, it is important to keep in mind the clinical setting and other available information and to question the reliability of the stone composition report if it seems inconsistent with the patient's history. A report indicating the presence of struvite, for example, is probably inaccurate in a patient with no documented infection with a urease-producing bacterium. It is also important to examine the urine sediment for crystals as this might help identify the crystal type. (See 'Urinalysis' below.)

Laboratory testing

Approaches to laboratory testing — Three options have been proposed for laboratory evaluation after a first stone: a limited evaluation, a complete metabolic evaluation, or a targeted approach. Although there is disagreement whether a complete metabolic evaluation should be performed after the first kidney stone, there is general agreement that a complete metabolic evaluation is indicated in all patients with multiple stones at first presentation (including those who have passed a single stone but have other asymptomatic stones found in the kidney by imaging), patients with a strong family history of stones, and individuals with active stone disease (defined as recurrent stone formation, enlargement of existing stones, or the recurrent passage of gravel). The decision about which option to pursue should be shared by the clinician and patient.

Limited evaluation – A limited laboratory evaluation includes a urinalysis and routine blood chemistries. Some clinicians prefer this approach after a first stone because of the variable rate of stone recurrence and data suggesting that a comprehensive medical evaluation is not cost-effective for patients who have only formed one stone [13,14]. This approach is based upon the availability of non-operative therapy for most symptomatic stones and avoids unnecessary therapy in those who would not have a recurrence. (See 'Urinalysis' below and 'Blood tests' below.)

Complete metabolic evaluation – A complete metabolic evaluation consists of a urinalysis, routine blood chemistries, and at least two 24-hour urine collections for analysis of urine composition. Some clinicians recommend this approach after the first stone because of the potentially high rate of recurrence and potential morbidity from recurrent stones. Limited data suggest that single-stone formers have similar metabolic abnormalities as patients with recurrent nephrolithiasis [15,16]. In addition, there are several studies suggesting that the likelihood of stone formation can be predicted reasonably well from the 24-hour urine values [17-19]. This approach should be followed only in individuals willing to make changes to their diet or fluid intake or to take medical therapy if warranted by the work-up. (See 'Complete metabolic evaluation' below.)

Targeted approach – A third approach is to base the extent of the laboratory evaluation upon an estimation of the risk for new stone formation [18]. A complete metabolic evaluation would be performed in patients at moderate to high risk for recurrent disease. Patients at high risk for recurrent disease include:

Patients who have formed more than one kidney stone

Patients with a family history of stones

Patients with chronic diarrheal states and/or malabsorption, pathologic skeletal fractures, osteoporosis, urinary tract infection, diabetes, and/or gout

Patients taking medication that may put them at higher risk (eg, topiramate, acetazolamide)

Patients with stones composed of cystine, uric acid, or calcium phosphate

Patients with dietary habits associated with higher risk of stone formation

Complete metabolic evaluation — The complete metabolic evaluation for nephrolithiasis consists of both blood and urine testing, including at least two 24-hour urine collections.

Urinalysis — A urinalysis should be performed on a voided urinary specimen. The urinalysis should include pH determination since a pH greater than 7.5 raises the possibility of a stone due to urease-producing bacteria, whereas a pH less than 5.5 favors uric acid lithiasis. (See "Urinalysis in the diagnosis of kidney disease".)

The urine sediment should also be examined for crystalluria since particular crystal types may provide a clue as to the composition of stones (see "Urinalysis in the diagnosis of kidney disease"):

Uric acid crystals – Uric acid crystals are observed in acid urine (usually pH <5.5), a milieu that favors the conversion of the relatively soluble urate salt into the insoluble uric acid (picture 2A-B). (See "Uric acid kidney diseases".)

Calcium phosphate or calcium oxalate crystals – The formation of calcium oxalate crystals is not dependent upon the urine pH, while calcium phosphate crystals only form in a relatively alkaline urine (usually pH >6.8) (picture 1A-B). (See "Kidney stones in adults: Epidemiology and risk factors".)

Cystine crystals – Cystine crystals, with their characteristic hexagonal shape, are diagnostic of cystinuria (picture 5). (See "Cystinuria and cystine stones".)

Magnesium ammonium phosphate crystals – Magnesium ammonium phosphate (struvite) and calcium carbonate apatite are the constituents of struvite stones (picture 3) (see "Kidney stones in adults: Struvite (infection) stones"). Normal urine is undersaturated with magnesium ammonium phosphate, and struvite stone formation occurs only when ammonia production is increased and the urine pH is elevated. Both of these requirements are only met when an upper urinary tract infection occurs with a urease-producing bacterium, such as Proteus or Klebsiella.

Blood tests — A routine chemistry profile should be obtained, including measurement of serum electrolytes, serum creatinine, and serum calcium. The results may help identify certain disorders, such as primary hyperparathyroidism, hyperuricemia, and distal renal tubular acidosis, that are associated with nephrolithiasis. (See "Primary hyperparathyroidism: Diagnosis, differential diagnosis, and evaluation" and "Kidney stones in adults: Uric acid nephrolithiasis" and "Nephrolithiasis in renal tubular acidosis".)

The serum calcium concentration should be measured looking for hypercalcemia; if high-normal (which we define as above the midpoint of the normal range), the serum calcium should be repeated. A measurement of intact parathyroid hormone is warranted in patients with serum calcium values in the high-normal range or if the urine calcium is high since primary hyperparathyroidism is often associated with only intermittent and mild elevations in the serum calcium concentration [20-22]. In one series of 48 patients with nephrolithiasis and primary hyperparathyroidism, 30 (63 percent) had serum calcium concentrations between 10.2 and 11 mg/dL (2.55 and 2.75 mmol/L) [20]. (See "Primary hyperparathyroidism: Diagnosis, differential diagnosis, and evaluation".)

Although usually in the middle of the reference range, the presence of a lower serum bicarbonate concentration raises the possibility of distal renal tubular acidosis or chronic diarrhea.

24-hour urine collections — An important component of the evaluation of patients at moderate to high risk for recurrent stone disease is the assessment of urine composition:

Number of collections – At least two 24-hour urine collections should be obtained in the outpatient setting, with the patient following their usual diet, fluid intake, and physical activity. The differences among the collections are often substantial, so an important contributor may be missed in many patients if only one sample is collected (figure 2) [23,24]. The validity of this approach was illustrated in the following studies:

A study of 75 recurrent idiopathic calcium kidney stone formers examined the relative diagnostic utility of one, two, and three urine collections [23]. When compared with one or any combination of two urine collections, three urine collections were significantly associated with the highest yield of identifying a urinary abnormality (figure 2).

Similar findings were noted in another report of over 1000 stone formers in whom three 24-hour urine collections were obtained [24]. Differences in urinary biochemical risk factors among the three collections were substantial enough that an important metabolic abnormality would have been missed in many patients if only one sample had been collected.

Thus, we recommend that a minimum of two collections be performed as part of the initial evaluation.

Timing of collections – The urine collections should be obtained while the patient is on his or her usual diet. Values should not be measured immediately after the acute stone episode; it is common practice to wait at least one to two months after a stone event to obtain the collections [23]. One should also wait at least one to two months after the patient has completely recovered from any interventions, such as shock wave lithotripsy, ureteroscopy, or percutaneous stone removal. Ideally, the patient should be free of pain, infection, and obstruction and following their "usual routine" when performing their urine collections.

Tests to include – The urine volume and excretion of calcium, uric acid, citrate, oxalate, creatinine (to assess the completeness of the collection), pH, sodium, and magnesium should all be measured. Ideally, the supersaturation of lithogenic substances should be calculated. The results of the urine collections and stone analysis (if available) dictate subsequent evaluation and management. (See "Kidney stones in adults: Prevention of recurrent kidney stones" and "Kidney stones in adults: Struvite (infection) stones" and "Kidney stones in adults: Uric acid nephrolithiasis" and "Cystinuria and cystine stones".)

A variety of definitions for "normal" are used by different laboratories for each of the urinary parameters. Below are some more common definitions:

Calcium − Less than 200 mg (5.0 mmol) per day in females or less than 250 mg (6.25 mmol) per day in males

Uric acid − Less than 750 mg (4.5 mmol) per day in females or less than 800 mg (4.8 mmol) per day in males

Oxalate − Less than 40 mg (0.44 mmol) per day in both females and males

Citrate − Greater than or equal to 450 mg per day in both females and males

However, these values are arbitrary and additional data suggest a more linear relation for each of these factors. As an example, the risk continues to decrease as urinary calcium falls below 250 mg/day (6.25 mmol/day) in both males and females [25]. In addition, the concentration of lithogenic factors and the urinary supersaturation, as calculated in an experienced laboratory, are more important than the absolute amounts with respect to stone formation [25]. The definition of "normal" for calculated supersaturation is also arbitrary; the risk continues to decline even when the supersaturation is lower than the threshold [26].

Measurement of sodium excretion is also important. Higher sodium intake can contribute to increased calcium excretion and will affect the response to a thiazide diuretic when prescribed to reduce urine calcium. (See "Kidney stones in adults: Epidemiology and risk factors".)

In many laboratories, two or three separate collections are required to obtain all of this information: uric acid is measured in a plain or alkaline solution, calcium and oxalate are measured in hydrochloric or nitric acid, and citrate is measured in an acidified solution. However, some specialized laboratories provide a kit that permits all of the above values to be measured and urinary supersaturation to be calculated from a single urine collection [27-29].

Monitoring for new stones — Radiologic monitoring, usually with ultrasonography, abdominal radiography, low-dose, noncontrast CT, or digital tomosynthesis, is warranted for the detection of new stones. Monitoring should be performed initially at one year and, if negative, every two to four years thereafter depending upon the severity of the stone disease and the 24-hour urine values. (See "Kidney stones in adults: Diagnosis and acute management of suspected nephrolithiasis".)

Several factors should be considered when choosing which modality to use for radiologic monitoring:

If stones were previously visible by ultrasonography, then using this modality will minimize cumulative radiation exposure; this is particularly important in patients of childbearing age.

To allow comparisons of stone burden over time, a modality should be selected that successfully detected the number and size of previous stones. As an example, ultrasound should not be used for radiologic monitoring if prior stones were only quantifiable with noncontrast CT, abdominal radiographs, or digital tomograms.

Noncontrast CT remains the most sensitive test for detecting stones, particularly small stones, but is an expensive option for routine monitoring of stone burden [30]. Digital tomosynthesis, a high-resolution radiograph-based imaging technique that is used routinely for breast cancer screening, may be an effective and lower cost alternative to noncontrast CT. One retrospective study comparing digital tomosynthesis with noncontrast CT for the follow-up of nephrolithiasis found similar stone detection rates between the two imaging modalities [31]. Digital tomosynthesis has also been shown to have lower radiation exposure compared with noncontrast CT [32,33]. (See "Kidney stones in adults: Diagnosis and acute management of suspected nephrolithiasis", section on 'Digital tomosynthesis'.)

Patients with asymptomatic stones — Some patients are found to have an asymptomatic kidney stone or stones by imaging performed for a different indication [34]. Approximately one-third of such patients will develop symptoms related to their kidney stones within three years, and as many as one-half of these symptomatic patients may require surgical treatment for their stones [35,36]. Thus, asymptomatic patients may benefit from a metabolic evaluation and appropriate medical therapy to prevent growth of any existing stones and to prevent new stone formation. We perform a complete metabolic evaluation in all asymptomatic patients who have multiple stones since we consider such patients to be recurrent stone formers. In addition, we perform a complete metabolic evaluation in select asymptomatic patients with a single stone, based upon their occupation (airline pilots, frequent business travelers), complexity (neurologic disease, anatomic abnormalities of the urinary tract, such as urinary diversion or solitary kidney), or the need for surgical stone removal. (See 'Complete metabolic evaluation' above.)

In asymptomatic patients with a single stone who do not warrant a complete metabolic evaluation, active surveillance with repeat imaging within one to two years (see 'Monitoring for new stones' above) is a reasonable approach. If repeat imaging shows no evidence of stone growth, imaging can be repeated every two years; if there is no stone growth after four to five years, active surveillance may be discontinued. If repeat imaging shows an increase in stone size or new stones, we perform a complete metabolic evaluation.

Several studies have examined the natural history of asymptomatic kidney stones. As examples:

A cohort of 110 patients with 160 asymptomatic kidney stones was followed with active surveillance (using kidney ultrasound performed every 6 to 12 months) [35]. During a mean follow-up of 3.4 years, 28 percent of stones produced symptoms and 17 percent required surgery for these symptoms; an additional 3 percent caused silent obstruction that required intervention. Lower pole stones were less likely to cause symptoms or pass spontaneously.

Another study monitored 107 such patients for a mean of 32 months [36]. The likelihood of developing symptoms was approximately 32 percent at 2.5 years and 49 percent at 5 years; the risk was lowest in patients who had no history of previous stones. Roughly one-half of symptomatic patients required a procedure (such as shock wave lithotripsy) for removal of the stone, while the remaining symptomatic patients passed the stone spontaneously.

In addition to these findings, a number of studies have determined that patients with residual stones following shock wave lithotripsy or percutaneous stone removal are at increased risk for symptomatic stone episodes. However, these investigations also suggest that appropriate medical stone management can significantly reduce recurrent stone formation or growth of existing stones [37-39].

SOCIETY GUIDELINE LINKS — Links to society and government-sponsored guidelines from selected countries and regions around the world are provided separately. (See "Society guideline links: Kidney stones".)

SUMMARY AND RECOMMENDATIONS

Goal of evaluation – In patients with established kidney stone disease, the goal of a diagnostic evaluation is to identify, as efficiently and economically as possible, the particular physiologic differences present in a given patient so that effective therapy to prevent recurrent stones can be established and the prognosis can be better defined. (See 'Goal of evaluation' above.)

Approach to evaluation – All patients presenting with established kidney stone disease should undergo a focused history, radiologic imaging, stone analysis (if available), and at least a limited laboratory evaluation:

Focused history – The purpose of the focused history is to identify stone risk factors, such as a family history of stone disease and certain dietary habits (table 1). (See 'Focused history for stone risk factors' above.)

Radiologic testing – If not yet performed during the initial evaluation, radiographic examination, preferably with noncontrast, low-dose computed tomography (CT), should be obtained to search for residual stones within the urinary tract. Diagnostic tests for the detection of nephrolithiasis are discussed in detail elsewhere. (See "Kidney stones in adults: Diagnosis and acute management of suspected nephrolithiasis", section on 'Diagnostic imaging'.)

Stone analysis – Analysis of the stone is an essential part of the evaluation. Patients should be encouraged to retrieve stones they pass spontaneously for analysis, although novel CT imaging techniques may permit noninvasive discrimination among the main subtypes of urinary calculi. Similarly, stones that are surgically removed should also be submitted for analysis. The most common crystalline materials found in kidney stones are calcium oxalate, calcium phosphate, uric acid, and struvite (magnesium ammonium phosphate). It is not uncommon for a stone to contain more than one crystalline component. (See 'Stone analysis' above.)

Laboratory testing – Three options have been proposed for laboratory evaluation after a first stone: a limited evaluation, a complete metabolic evaluation, or a targeted approach. Although there is disagreement whether a complete metabolic evaluation should be performed after the first kidney stone, a complete metabolic evaluation is indicated in all patients with multiple stones at first presentation, patients with a strong family history of stones, and individuals with active stone disease (defined as recurrent stone formation, enlargement of existing stones, or the recurrent passage of gravel). (See 'Approaches to laboratory testing' above.)

Complete metabolic evaluation – The complete metabolic evaluation for nephrolithiasis consists of both blood and urine testing, including at least two 24-hour urine collections. In each 24-hour urine collection, the urine volume, pH, and excretion of calcium, uric acid, citrate, oxalate, sodium, potassium, magnesium, and creatinine (to assess the completeness of the collection) should be measured. Also, urinary supersaturation should be calculated. Urine collections should not be performed if there is evidence of kidney/ureteral obstruction from existing stones or urinary tract infection. (See 'Complete metabolic evaluation' above.)

Monitoring for new stones – Radiologic monitoring, usually with ultrasonography; abdominal radiography; noncontrast, low-dose CT; or digital tomosynthesis, is warranted for the detection of new stones. Monitoring should be performed initially at one year and, if negative, every two to four years thereafter depending upon the severity of the stone disease and the 24-hour urine values. (See 'Monitoring for new stones' above.)

Asymptomatic stones – Some patients are found to have an asymptomatic kidney stone or stones by imaging performed for a different indication. Such patients may also benefit from a metabolic evaluation and appropriate medical therapy to prevent growth of any existing stones and to prevent new stone formation. (See 'Patients with asymptomatic stones' above.)

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