How do you interpret a renal Lasix (furosemide) scan?

Interpreting a Renal Lasix Scan

A renal Lasix (furosemide) scan is primarily used to differentiate between obstructive and non-obstructive hydronephrosis by evaluating renal function and urinary drainage patterns. The test combines nuclear medicine imaging with diuretic administration to assess kidney function and urinary tract patency.

Basic Principles and Technique

A renal Lasix scan involves two main components:

1. Initial Radionuclide Scan:

   • Technetium-99m MAG3 (mercaptoacetyltriglycine) is the preferred radiopharmaceutical for suspected obstruction or impaired renal function 1
   • MAG3 is primarily excreted through active renal tubular transport with an extraction fraction of 40-50%, providing better visualization than DTPA 1
   • The scan provides information on split renal function and baseline drainage patterns

2. Diuretic Challenge:

   • Furosemide (Lasix) is administered intravenously during the study
   • The diuretic increases urine production to help differentiate between true obstruction and urinary stasis
   • Typically administered after initial imaging shows retention of the radiopharmaceutical

Interpretation Parameters

1. Split Renal Function

• Quantifies the relative contribution of each kidney to total renal function
• Normal: approximately 50/50 split (45-55% range)
• Differential function <40% suggests significant renal impairment on the affected side 1

2. Time-Activity Curves and T1/2

• T1/2 (washout half-time): Time required for activity in the collecting system to decrease by 50% after furosemide administration
• Interpretation:
  • T1/2 <10 minutes: Normal drainage (non-obstructive)
  • T1/2 10-20 minutes: Indeterminate/equivocal
  • T1/2 >20 minutes: Suggestive of obstruction 1

3. Drainage Patterns

• Normal: Prompt clearance of tracer after furosemide
• Obstructive: Persistent retention of tracer despite furosemide challenge
• Dilated non-obstructive: Initial retention with eventual clearance after furosemide

Clinical Applications

1. Evaluation of Hydronephrosis:

   • Differentiates between obstructive and non-obstructive causes
   • Particularly useful for ureteropelvic junction obstruction (UPJO) assessment 1

2. Monitoring Known Obstruction:

   • Serial scans can track changes in differential renal function over time
   • A decrease of >5% in differential function on consecutive scans suggests deterioration requiring intervention 1

3. Surgical Decision-Making:

   • Surgical intervention is typically considered when:
     • T1/2 >20 minutes (obstruction)
     • Differential renal function <40%
     • Deteriorating function (>5% decrease between scans)
     • Worsening drainage on serial imaging 1

Potential Pitfalls and Considerations

1. Dehydration: May mask obstruction by reducing urine production

   • Ensure adequate hydration before the study

2. Impaired Renal Function:

   • May affect tracer clearance and interpretation
   • MAG3 is preferred over DTPA in patients with impaired renal function 1

3. Bladder Fullness:

   • A distended bladder can cause artifactual hydronephrosis and affect interpretation
   • Consider post-void images if needed 1

4. Patient Positioning:

   • Gravity-dependent drainage can affect results
   • Upright positioning may help differentiate in equivocal cases

5. Timing of Furosemide Administration:

   • F+0 protocol: Furosemide given at start of study
   • F+20 protocol: Furosemide given 20 minutes after tracer injection
   • Different protocols may affect interpretation

Comparison with Other Imaging Modalities

• Ultrasound: Provides anatomical information but limited functional assessment

  • May show hydronephrosis but cannot reliably determine if it's obstructive 1

• CT/MRI: Better anatomical detail but limited functional information

  • MR urography can assess degree of obstruction but has limitations in estimating split renal function in severely hydronephrotic kidneys 1

• Lasix Scan Advantages: Provides both functional and drainage information in a single study with relatively low radiation exposure

Documentation and Reporting

Key elements that should be included in the report:

• Split renal function percentages
• T1/2 values for each kidney
• Visual assessment of drainage patterns
• Comparison with prior studies if available
• Clinical interpretation regarding presence/absence of obstruction 1

By systematically evaluating these parameters, a renal Lasix scan provides valuable information about kidney function and urinary tract patency that guides clinical management decisions, particularly regarding the need for surgical intervention in cases of suspected obstruction.