What is the role of flat plate radiography in the diagnosis of nephrolithiasis?

Updated: Jun 21, 2018
  • Author: Chirag N Dave, MD; Chief Editor: Bradley Fields Schwartz, DO, FACS  more...
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Answer

Plain abdominal radiography (also referred to as flat plate or KUB radiography) is useful for assessing total stone burden, as well as the size, shape, composition, and location of urinary calculi in some patients. Calcium-containing stones (approximately 85% of all upper urinary tract calculi) are radiopaque, but pure uric acid, indinavir-induced, and cystine calculi are relatively radiolucent on plain radiography.

When used with other imaging studies, such as a renal ultrasonography or, particularly, CT scanning, the plain film helps provide a better understanding of the characteristics of urinary stones revealed with these other imaging studies. This may also be helpful in planning surgical therapy.

The flat plate radiograph uses the same orientation and anatomical presentation that is observed on fluoroscopy images and retrograde pyelograms or during endoscopic ureteral surgery, such as ureteroscopy or intracorporeal lithotripsy. Not all urinary calculi may be visible on the KUB radiograph, whether because of their small size, stone radiolucency, or overlying gas, stool, or bone. The stones that are observed can be correlated with opacities found on other studies for identification and tracking progress.

If a stone is not visible on a flat plate radiograph, it could be a radiolucent uric acid stone that can be dissolved with alkalinizing medication. Such a stone is more likely if the urine pH indicates very acidic urine. In practice, any patient with symptoms of acute renal colic who demonstrates a urine pH lower than 6.0 should be considered at risk for a possible uric acid stone. If a stone of adequate size is visible on a CT scan but not visible on KUB, then uric stones should be considered.

The flat plate radiograph is inexpensive, quick, and usually helpful even if no specific stone is observed. It is extremely useful in following the progress of previously documented radiopaque calculi and checking the position of any indwelling double-J stents. The KUB radiograph can suggest the fluoroscopic appearance of a stone, which determines whether it can be targeted with extracorporeal shockwave lithotripsy (ESWL).


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