Hyperuricosuria and Gouty Diathesis Workup

Updated: Apr 20, 2015
  • Author: Bijan Shekarriz, MD; Chief Editor: Bradley Fields Schwartz, DO, FACS  more...
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Workup

Laboratory Studies

Laboratory evaluation should include urinalysis and urine culture. In patients with known uric acid stones, urinary pH should be recorded using pH paper. Blood chemistry tests should include the following:

  • Calcium
  • Phosphorus
  • Electrolytes
  • Creatinine
  • Uric acid
  • Parathyroid hormone levels
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Imaging Studies

All patients with questionable urinary calculi should undergo a baseline radiologic evaluation, including a scout film (ie, kidneys, ureters, bladder [KUB]). The scout film is useful not only as a baseline for tracking radiopaque calculi, but also for establishing radiolucency of possible uric acid stones.

Pure uric acid stones are relatively radiolucent but produce a bright-white signal on noncontrast CT scans. The diagnosis of uric acid stones can be suggested by a persistent urinary pH of less than 5.5, uric acid crystals visible on microscopic urinalysis, hyperuricemia, a KUB study that fails to show a calcific stone, or a history of gout or previous uric acid calculi.

CT scan demonstrating right partial staghorn uric CT scan demonstrating right partial staghorn uric acid calculus. Uric acid stones appear dense on CT scan and radiolucent on kidneys, ureters, and bladder (KUB) imaging (not shown).
Follow-up CT scan of patient in the image above (i Follow-up CT scan of patient in the image above (ie, with partial staghorn uric acid calculus) 1 year later. This patient was treated with oral urinary alkalinization with sodium bicarbonate. Note only a small residual fragment is present (right image).

Intravenous urography (ie, intravenous pyelography [IVP]) or renal ultrasonography may be useful. Contrast is used for IVP, which can make a uric acid stone appear as a filling defect, especially in the renal pelvis. Ultrasonography is a very good tool for identifying and tracking larger uric acid calculi, especially in the renal pelvis, because their radiolucency does not affect sonograms. This modality is less useful for ureteral calculi.

Noncontrast CT scanning is the imaging modality of choice for the differential diagnoses of urinary calculi. With noncontrast CT scanning, uric acid calculi, despite being radiolucent on conventional radiographs, appear as bright-white images, as do other calculi. The average density readings of uric acid stones on CT scans are substantially less than calcium-containing stones but are still well above the threshold of optical visualization as anything but a bright-white spot. In these cases, performing KUB imaging at the same time as CT scanning is essential to help indicate that the stone is relatively radiolucent and therefore likely to be composed of uric acid. If the stone is located in the kidney, ultrasonography can be particularly useful for tracking the progress or dissolution of the stone.

Uric acid and non-uric acid renal stones can be differentiated using two consecutive spatially registered low- and high-energy scans acquired on a conventional CT scanning system. In a study of 33 patients who underwent clinically indicated dual-source dual-energy CT followed by two consecutive scans (at 80 kV and 140 kV) on a conventional CT scanner over the region identified on the initial scan, the overall sensitivity, specificity, and accuracy for identifying uric acid stones were 73.1%, 90.1%, and 89.1%, respectively. For stones 3 mm or larger, the sensitivity, specificity, and accuracy were 94.7%, 96.9%, and 96.8%, respectively. [5]

In rare circumstances (eg, intravenous contrast allergy, unavailability of CT scanning), retrograde pyelography may help to confirm the diagnosis.

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Other Tests

Perform a 24-hour urine collection for volume, pH, calcium, uric acid, oxalate, citrate, phosphorus, sodium, and creatinine analysis after the acute stone event has resolved. This test should help elucidate the metabolic abnormality associated with stone formation. The most common urinary abnormalities in patients with uric acid stones are persistently acidic urine (< 5.5), low volume, and hyperuricosuria.

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Staging

See the list below:

  • Staghorn calculi are stones in the renal pelvis that extend into at least 2 calyceal groups. A complete staghorn calculus fills the entire renal collecting system.
  • Proximal ureteral calculi are stones in the ureter distal to the ureteropelvic junction and anterior to the superior edge of the sacroiliac joint.
  • Midureteral calculi are stones in the ureter that overlie the sacroiliac joint as seen on KUB images.
  • Distal ureteral calculi are stones in the ureter below the inferior margin of the sacroiliac joint down to the ureterovesical junction.
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