Mechanism 13 of 16

Renal filtration versus secretion

The kidney offers two ways to clear a molecule. Filtration is passive and limited to what the glomerulus lets through; tubular secretion is active and clears about twice as much. Which tracer you pick decides which function you measure.

The practical consequence is image quality in the failing kidney. DTPA is filtered and nothing more, so as GFR falls the images fade. MAG3 is secreted by the organic anion transporters with an extraction fraction of 40–50% per pass — roughly twice DTPA’s — so it still produces a diagnostic renogram at a GFR of 15. That is why MAG3 is the default in obstruction, transplant assessment and paediatrics.

Two dodges are worth remembering, because they explain most false positives. Dehydration and a full bladder both slow drainage and mimic obstruction — hydrate, and catheterise when the bladder is the question. And a dilated but unobstructed system can fail to wash out after furosemide simply because the collecting system is so capacious that it acts as a reservoir.

filtered / secretedbound in the tubular celltransporter

Three tracers, three exits. DTPA is filtered at the glomerulus and then simply flows away — its clearance is GFR. MAG3 never needs to be filtered: it is pulled from the peritubular capillary across the tubular cell by OAT1/OAT3 and dumped into the lumen, which is why it still works when the glomerulus barely does. DMSA binds sulfhydryl groups in the tubular cell and stays, giving cortical anatomy instead of function.

The agents

Tc-99m DTPApentetate

SPECT / planar

Tc-99m · t½ 6.01 h · extraction ~20% per pass

Handle: Glomerular filtration only
Trapping: Freely filtered, neither secreted nor reabsorbed — so its clearance is a direct measure of GFR.
Use: GFR quantification, renogram, obstruction, brain-death flow study, aerosol ventilation imaging.
Pitfall: Image quality degrades badly with impaired function — use MAG3 instead below a GFR of about 30. Some protein binding reduces GFR accuracy. Poor labelling gives free pertechnetate with gastric and thyroid activity.

Tc-99m MAG3mertiatide

SPECT / planar

Tc-99m · t½ 6.01 h · extraction 40–50% per pass

Handle: Organic anion transporters OAT1/OAT3 in the proximal tubule
Trapping: About 90% cleared by active tubular secretion, with roughly twice the per-pass extraction of DTPA. Measures effective renal plasma flow, and gives diagnostic images even in poor function.
Use: Renogram, obstruction with furosemide (washout t½ under 10 min normal, over 20 min obstructed), renovascular hypertension with captopril, transplant assessment, urine leak and reflux.
Pitfall: Highly protein-bound, so it is emphatically not a filtration marker — do not quote a GFR from it. Dehydration and a full bladder both mimic obstruction: hydrate, and catheterise when needed. A markedly dilated system may not respond to furosemide even when unobstructed.

Tc-99m DMSAsuccimer

SPECT / planar

Tc-99m · t½ 6.01 h · image at 2–3 h · 40–65% cortical retention at 2 h

Handle: Sulfhydryl groups on proximal tubular cells
Trapping: Binds and is retained in the renal cortex — the only agent that gives cortical morphology rather than flow or drainage.
Use: Acute pyelonephritis and cortical scarring in children, differential cortical function, ectopic and horseshoe kidney, column of Bertin versus a mass.
Pitfall: Not a functional drainage study — it answers a different question from MAG3 and cannot substitute for it. Delayed imaging is essential; early images are uninterpretable for scarring.

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