When is a Lasix (furosemide) scan indicated?

Indications for Lasix (Furosemide) Scan

A Lasix scan is primarily indicated for evaluating suspected urinary tract obstruction, particularly in cases of hydronephrosis, to differentiate between true mechanical obstruction and non-obstructive dilation of the collecting system.

What is a Lasix Scan?

A Lasix scan (also called diuretic renography) is a nuclear medicine procedure that combines conventional renal scintigraphy with the administration of furosemide (Lasix) to evaluate kidney function and urinary drainage.

Primary Indications

Suspected Urinary Tract Obstruction

• Evaluation of hydronephrosis detected on ultrasound or other imaging
• Differentiation between obstructive and non-obstructive causes of urinary tract dilation
• Assessment of the functional significance of anatomical abnormalities

Specific Clinical Scenarios

1. Antenatal Hydronephrosis

   • When ultrasound shows persistent hydronephrosis in children with antenatal hydronephrosis
   • To determine if surgical intervention is necessary 1

2. Ureteropelvic Junction (UPJ) Obstruction

   • To confirm the diagnosis and assess the functional impact
   • To guide decision-making regarding surgical intervention

3. Nephrolithiasis with Hydronephrosis

   • To evaluate the degree of obstruction caused by kidney stones
   • To assess renal function in the affected kidney 2

4. Post-Surgical Evaluation

   • To assess the success of surgical procedures to relieve obstruction
   • To monitor for recurrent obstruction

5. Renal Transplant Assessment

   • To evaluate for possible obstruction in transplanted kidneys
   • To differentiate between rejection and obstruction as causes of declining renal function

Technical Aspects of the Procedure

Methodology

1. A radiopharmaceutical (typically 99mTc-MAG3 or 99mTc-DTPA) is administered intravenously
2. Initial images are obtained to assess renal perfusion and function
3. Furosemide is administered to increase urine flow
4. Additional images are obtained to evaluate drainage response

Interpretation Parameters

• Time-activity curves are generated for each kidney
• Half-time of tracer clearance after furosemide (T½)
• Differential renal function
• Visual assessment of tracer retention

Diagnostic Criteria

Obstructive Pattern

• Prolonged T½ (>20 minutes)
• Continued accumulation or minimal washout of tracer after furosemide
• Absence of excretory segment on time-activity curve 2

Non-obstructive Pattern

• Rapid washout of tracer after furosemide (T½ <10 minutes)
• Clear visualization of the entire urinary tract
• Presence of excretory segment on time-activity curve

Equivocal Pattern

• Intermediate washout (T½ 10-20 minutes)
• May require clinical correlation or additional studies

Limitations and Considerations

Patient Factors

• Adequate hydration is essential for accurate results
• Renal function must be sufficient to respond to furosemide
• Results may be affected by severe dehydration or overhydration

Technical Factors

• Interpretation can be challenging and somewhat subjective
• Deep learning models are being developed to improve diagnostic accuracy 1
• False positives can occur with non-obstructive causes of delayed drainage

Potential Complications

• Transient worsening of renal function with furosemide administration in patients with compromised renal function 3
• Risk of acute kidney injury in patients with heart failure 3
• Fluid and electrolyte disturbances 4

Alternative Diagnostic Approaches

• CT urography
• MR urography
• Whitaker test (pressure-perfusion study)
• Ultrasound with Doppler

Conclusion

A Lasix scan is a valuable diagnostic tool for evaluating suspected urinary tract obstruction. It provides both anatomical and functional information about the kidneys and urinary tract, helping clinicians differentiate between true mechanical obstruction requiring intervention and non-obstructive dilation that can be managed conservatively.