What is the role of radionuclide studies in the workup of azotemia?

Updated: Sep 19, 2018
  • Author: Moro O Salifu, MD, MPH, FACP; Chief Editor: Vecihi Batuman, MD, FASN  more...
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Answer

Technetium-99m dimercaptosuccinic acid (99m Tc DMSA) is heavily distributed within the renal parenchyma at first pass and so is best for detecting renal parenchymal scarring.

Technetium diethylenetriamine pentaacetic acid (99m Tc DTPA) is heavily filtered at first pass and therefore is best for qualitative assessment of renal function (filtration and excretion). Because it is heavily filtered, it is most sensitive in detecting urine leaks after renal transplant. For the same reason, it is also used concomitantly with a furosemide washout scan (see below) for assessing functional obstruction of the collecting system.

Mercaptoacetyltriglycine (MAG3) is evenly distributed at first pass in the kidney and so is best for qualitative assessment of perfusion, filtration, and excretion. It is the preferred test for assessing the 3 aspects of function after renal transplantation. It can be used to detect urine leaks or functional obstruction with furosemide, though99m Tc DTPA scanning remains the test of choice for these conditions. Voiding cystourethrography can be performed with a radionuclide study to detect vesicoureteral reflux.

In a furosemide washout scan, the renal scan usually is performed first. Then, if needed, the furosemide washout is done after the radionuclide has accumulated in the collecting system. Furosemide is used as a part of the renogram to separate nonobstructive hydronephrosis from obstructive hydronephrosis. If there is no obstruction, furosemide-induced flow containing little or no radionuclide will fill the collecting system, washing out radionuclide-containing urine. If obstruction is present, the radionuclide is not washed out as quickly.

The half-life or clearance of the radioisotope is plotted on a curve. A half-life shorter than 10 minutes is considered normal, one longer than 20 minutes is considered obstruction, and one of between 10-20 minutes is subject to further interpretation.

Conditions that can make it difficult to interpret the furosemide washout curve include a megaureter or pelvis that accepts a large bolus of urine and poor renal function. In patients with a megaureter, it can be difficult to determine when the renal pelvis is full, and in patients with renal disease, the onset of furosemide action may be delayed. To overcome the problem of poor renal function or relative hypovolemia if a patient has been fasting, the patient should be well hydrated with intravenous (IV) fluids before the study.

The test also is operator dependent, in that the furosemide should be administered at a time when the renal pelvis is believed to be full. A full bladder also delays washout of isotope. Therefore, the patient’s bladder must be catheterized before the study can be performed.


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