How is a DTPA (Diethylene Triamine Pentaacetate) renogram interpreted?

Interpreting a DTPA Renogram

A DTPA renogram is interpreted by evaluating sequential images obtained at 2-3 minute intervals for relative uptake, kidney size and shape, and transit abnormalities from cortex to pelvis, with time-activity curves generated over each kidney to demonstrate semi-quantitative uptake and excretion patterns over 20-30 minutes. 1

Core Interpretation Parameters

Visual Image Analysis

• Assess relative uptake between kidneys at early time points (first 2-3 minutes), looking for asymmetry that may indicate differential function 1
• Evaluate kidney size, shape, and position on the sequential images to detect anatomical abnormalities or parenchymal disease 1
• Identify transit abnormalities from renal cortex to renal pelvis and bladder, which helps detect parenchymal disease and delays in transport from the renal pelvis 1

Time-Activity Curve Analysis

• Generate time-activity curves over each kidney to demonstrate relative uptake and excretion in a semi-quantitative manner 1
• Calculate differential renal function from the uptake phase, with normal kidneys showing approximately 50% uptake each; uptake <40% on one side suggests significant functional impairment 2, 3
• Measure time to peak uptake, with delays >5 minutes longer than the contralateral kidney suggesting renal artery stenosis or obstruction 3

Specific Diagnostic Patterns

Obstruction Assessment

• T1/2 drainage time >20 minutes after furosemide administration indicates obstruction 2, 4
• Persistent nephrogram without excretion characterizes an obstructive pattern 4
• Renal output efficiency (OE40) <80% has 92% sensitivity and 99% specificity for diagnosing obstruction 5
• Normalized residual activity (NORAPM) ≥0.23 on post-void imaging has 97% sensitivity and 99% specificity for obstruction, superior to conventional interpretation 5

Renovascular Disease (with ACE Inhibitor)

• Compare baseline and post-captopril studies for changes in uptake, time to peak, or retention patterns 1, 3
• Captopril-induced asymmetry (uptake <40%, delayed time to peak >5 minutes, or retention at 15 minutes >20% greater than contralateral side) has 94% sensitivity and 95% specificity for renal artery stenosis 3
• A normal post-captopril renogram makes hemodynamically significant renal artery stenosis unlikely 1

Technical Considerations

Radiopharmaceutical Properties

• DTPA is excreted by glomerular filtration, making it useful for estimating single-kidney GFR 1, 6
• DTPA has limitations in patients with reduced renal function, where it may produce equivocal or false-positive results compared to MAG3 1, 4
• MAG3 is preferred over DTPA for obstruction evaluation due to more efficient tubular extraction and easier washout assessment 1, 4

Timing and Protocol

• Acquire images for at least 20-30 minutes to adequately assess excretion patterns 1
• Administer furosemide at 20 minutes (F+20 protocol) to differentiate true obstruction from dilated non-obstructed systems 4
• Obtain post-void images to calculate normalized residual activity, which significantly improves diagnostic accuracy 5

Common Pitfalls and Limitations

False Positives/Negatives

• Dehydration can mimic obstruction by causing delayed excretion without true mechanical blockage 6
• Bilateral disease is difficult to assess because asymmetry criteria cannot identify which kidney is abnormal when both are affected 3
• Poor renal function (GFR <15-20 mL/min) limits interpretation due to inadequate tracer uptake and prolonged background activity 1, 6

Clinical Context Requirements

• Results must be interpreted with clinical findings and other imaging studies, as renograms are sensitive but often lack specificity for exact diagnoses 6
• Serial scans showing >5% functional decline indicate progressive injury requiring intervention, even if a single scan appears borderline 2, 4
• In newborns, delay examination until at least 2 months of age due to immature glomerular filtration rates 4