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A Rising Renogram Curve: Is It Always Obstruction?

Renal & GU · 3 min read

No. The curve measures drainage, and drainage fails for more reasons than blockage.

A curve that keeps climbing after furosemide looks like obstruction. It only shows that tracer is leaving the kidney slowly — and weak function, a large open pelvis, lying flat or a full bladder do the same.

What the curve measures

  • Kidney counts = tracer in minus tracer out. Rising means more is arriving than leaving.
  • “Out” depends on urine flow, pelvic size, gravity, bladder pressure — and outlet resistance. Only resistance is obstruction.
  • Obstruction is harm over time: split function falling by more than 10%, or dilatation worsening (Koff 1994).
Schematic F0 renogram curves. The normal kidney peaks and washes out. At 20 minutes the dilated but unobstructed kidney and the obstructed kidney both look high; after voiding upright and on a delayed image about 3 hours later the dilated kidney drains while the obstructed kidney stays high.01020minutes (F0: tracer + furosemide at 0)Post-voiduprightDelayed~3 hkidney countsNormalDilated, not obstructedObstructed
Figure 1 (schematic). At 20 minutes the dilated and obstructed kidneys look alike; only the late images separate them.
Remember

“The renogram measures drainage, not obstruction.”

Four Ps before blockage

PWhy the curve risesHow to tell
Poor flowToo little urine to wash tracer out — weak or immature kidney, dehydrationCheck split function and hydration; washout is unreliable when function is poor
PelvisA large, open reservoir empties slowlyResidual activity keeps falling on post-void and delayed images
Posture & bladderLying flat wastes gravity; a full bladder or reflux slows emptyingVoid, stand, re-image; catheterise if reflux
ParenchymaTracer held in the cortex — dehydration, acute tubular injury, ACE inhibitor with renal artery stenosis, or acute obstructionCortex or pelvis on the images? Context and ultrasound decide

Rising at the end of an F0 study

  • In F0, furosemide goes in with the tracer, so almost the whole 20 minutes is under diuresis.
  • Poor drainage at 20 minutes is common in dilated, unobstructed kidneys. It then improves sharply after voiding upright (Nogarède 2010).
  • Don’t stop at 20 minutes — get the post-void upright image and measure NORA or output efficiency. Final drainage is similar with F0 or F+20 (Donoso 2003).
  • Still poor? Then add a delayed image at 1–3 hours.
  • Don’t quote a T½ for a curve that never peaked.

What the evidence shows

  • Poor drainage without obstruction is common. In 24 children with stable function and dilatation for 2+ years, drainage still looked impaired in 68% by T½ and 44% after voiding upright (Amarante 2003).
  • Few need surgery. Of 104 neonates followed whatever their curve, 7% needed pyeloplasty; with split function ≤40%, washout could not predict deterioration (Koff 1994).
  • Late images help rule obstruction out. With T½ over 10 minutes, a gravity-assisted image separated obstruction with 88–100% sensitivity (Wong 2000). A 3-hour image raised specificity from 82% to 96%, with 100% negative predictive value: ≥15% drained argues against obstruction; under 10% is not proof — only about a third of those were obstructed (Simal 2018).
Pitfalls and reporting
  • T½ is a guide, not a verdict — cut-offs shift with protocol and technique.
  • Report “impaired drainage — obstruction not proven” and follow split function.
Take home
  • A rising curve means slow drainage, not necessarily obstruction.
  • Check the four Ps; always image post-void upright, and add a delayed image if still equivocal.
Sources
  1. Taylor AT, Brandon DC, de Palma D, et al. SNMMI procedure standard/EANM practice guideline for diuretic renal scintigraphy in adults with suspected upper urinary tract obstruction 1.0. Semin Nucl Med. 2018;48(4):377-90.
  2. Gordon I, Piepsz A, Sixt R. Guidelines for standard and diuretic renogram in children. Eur J Nucl Med Mol Imaging. 2011;38(6):1175-88.
  3. Eskild-Jensen A, Gordon I, Piepsz A, Frøkiaer J. Interpretation of the renogram: problems and pitfalls in hydronephrosis in children. BJU Int. 2004;94(6):887-92.
  4. Nogarède C, Tondeur M, Piepsz A. Normalized residual activity and output efficiency in case of early furosemide injection in children. Nucl Med Commun. 2010;31(5):355-8.
  5. Donoso G, Kuyvenhoven JD, Ham H, Piepsz A. 99mTc-MAG3 diuretic renography in children: a comparison between F0 and F+20. Nucl Med Commun. 2003;24(11):1189-93.
  6. Amarante J, Anderson PJ, Gordon I. Impaired drainage on diuretic renography using half-time or pelvic excretion efficiency is not a sign of obstruction in children with a prenatal diagnosis of unilateral renal pelvic dilatation. J Urol. 2003;169(5):1828-31.
  7. Koff SA, Campbell KD. The nonoperative management of unilateral neonatal hydronephrosis: natural history of poorly functioning kidneys. J Urol. 1994;152(2 Pt 2):593-5.
  8. Wong DC, Rossleigh MA, Farnsworth RH. Diuretic renography with the addition of quantitative gravity-assisted drainage in infants and children. J Nucl Med. 2000;41(6):1030-6.
  9. Simal CJR. 99mTc-DTPA diuretic renography with 3 hours late output fraction in the evaluation of hydronephrosis in children. Int Braz J Urol. 2018;44(3):577-84.
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