Laboratory Studies
Any child with a stone should have a 24-hour urine sample collected for analysis of calcium, magnesium, uric acid, citrate, sodium, and urine volume. [2] A spot urine pH level should be obtained.
Serum uric acid, 24-hour acid excretion, urine uric acid, creatinine, and serum creatinine can be used to assess uric acid production and excretion.
Blood should be obtained for measurement of blood urea nitrogen (BUN), creatinine, calcium, phosphorus, bicarbonate, uric acid, and parathyroid hormone levels.
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These test results are used to estimate the fractional excretion of uric acid.
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Reference range values for uric acid excretion by children and infants are shown in the table above.
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Results are used to determine if the elevated urinary uric acid concentration is caused by uric acid overproduction, decreased net renal tubular uric acid reabsorption, or increased net renal tubular uric acid secretion.
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Overproduction is indicated by high uric acid excretion for 24 hours with high or reference range serum uric acid levels and reference range or increased fractional excretion of uric acid.
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Decreased net renal tubular reabsorption or increased net tubular secretion is indicated by reference range or low uric acid excretion with low or low-normal serum uric acid and high fractional excretion of uric acid.
Once the problem is recognized as overproduction or increased tubular secretion or decreased net tubular reabsorption, specific testing for the primary cause can be undertaken.
Other tests
Any stones collected should be sent for crystallographic analysis.
Imaging Studies
For children, renal ultrasonography and abdominal flat plate radiography are as effective as intravenous pyelography (IVP) for identifying stones and do not expose the child to the risk of contrast agents. In addition, the radiation exposure is less. Renal ultrasonography is nonpainful, noninvasive, and creates no radiation exposure. [7] Ultrasonography can reveal hydronephrosis and, frequently, acoustic shadowing produced by a renal stone. Although the stone position can be identified, its composition cannot be determined using ultrasonography.
Noncontrast CT scanning (spiral CT scanning) is the most sensitive and specific study to search for uric acid stones. [7, 11] . A combination of stone size, attenuation intensity, and attenuation pattern from conventional computerized tomography can distinguish uric acid stones from calcium oxalate stones with high sensitivity and specificity. [12] A meta-analysis by McGrath et al found that the use of dual-energy CT to detect uric acid–dominant stones had a sensitivity of 88% and a specificity of 98%; for uric acid–containing stones, dual-energy CT had a mean sensitivity of 82% and a specificity of 97%. [13]
Retrograde pyelography may be necessary to delineate upper tract anatomy and localize small or radiolucent calculi.
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Uric acid stones.
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Printer friendly version of the table.
Tables
|
Neonates* |
Children |
Adults |
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29-33 wk |
34-37 wk |
38-40 wk |
3-4 y |
5-9 y |
10-14 y |
40-44 y |
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Male |
Female |
Male |
Female |
Male |
Female |
Male |
Female |
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Serum uric acid (mg/dL) |
7.71±2.65 |
6.04±2.19 |
5.19±1.57 |
3.45±1.01 |
3.44±0.8 |
3.63±1.04 |
3.71±0.92 |
4.28±1.19 |
4.09±1.2 |
5.134±1.25 |
4.25±1.1 |
Uric acid excretion (mg/dL GFR†) |
4.8±2.23 |
2.81±0.93 |
1.69±0.84 |
0.34±0.11 |
0.403±0.095 |
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Uric acid excretion (mg/kg/d) |
N/A |
N/A |
19.6 |
13.5±3.75 (3 y) |
11.5±3.75 (7 y) |
9±3.75 (12 y) |
10 |
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Fractional excretion of uric acid (%) |
61.24±12.21 |
44.52±15.23 |
38.19±13.61 |
12±3.75 (3 y) |
10±3 (7 y) |
7.6±3.75 (12 y) |
7±1.6 |
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*Gestational ages † Glomerular filtration rate |
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