Factors Contributing to a False Positive Result on a Renal Function Scan

The following factors may produce a false positive (i.e., suggestion of obstruction) on renal function scans:

• Poor renal diuretic response: Due to poor hydration or diminished renal function.
• Non-compliant renal pelvis: Produces increasing resistance to increasing urine flow.
• Over-compliant renal pelvis: The urine excreted during the diuretic phase may not be able to completely fill an overly compliant renal pelvis, leading to continuously increasing counts post diuretic.
• Massive hydronephrosis: Same concept as above, but instead of an overly compliant renal pelvis, we simply have too much space to fill because of massive hydronephrosis, leading to continuously increasing counts post diuretic. This is called the reservoir effect.
• Increased bladder pressure: Either due to over-distension or noncompliance, which can impair drainage from the ureters.

References

• Achong DM, Tenorio LE. Abnormal MAG3 renal scintigraphy resulting from dehydration. Clin Nucl Med. 2003 Aug;28(8):683-4.
• Mettler FA and Guiberteau MJ. Chapter 10: Genitourinary System and Adrenal Glands. In Essentials of Nuclear Medicine Imaging. Fifth Edition. Saunders, Philadelphia. 2006.