What is the furosemide (Lasix) stress test for assessing renal function in patients with suspected acute kidney injury or chronic kidney disease?

What is the Furosemide Stress Test?

The furosemide stress test (FST) is a functional biomarker that assesses tubular secretory capacity by administering intravenous furosemide and measuring subsequent urine output to predict progression to severe AKI (stage 3) and the need for renal replacement therapy in patients with early-stage acute kidney injury. 1

Test Protocol and Administration

The FST involves administering a standardized dose of intravenous furosemide based on prior diuretic exposure 1, 2:

• 1.0 mg/kg IV for loop diuretic-naïve patients 2
• 1.5 mg/kg IV for patients previously exposed to loop diuretics 2

Urine output is then measured hourly for 6 hours, with the first 2 hours being the most predictive time window. 2, 3

Clinical Interpretation and Predictive Value

The test demonstrates excellent predictive accuracy for adverse outcomes 1, 2:

• Urine output <200 mL in the first 2 hours predicts progression to stage 3 AKI with sensitivity of 73.9% and specificity of 90.0% (AUC 0.87-0.93) 1, 2
• Predicts need for renal replacement therapy with AUC values of 0.76-0.96 1
• Can predict successful discontinuation of continuous RRT in established AKI patients (AUC 0.84) 1

In pediatric populations, FST shows even stronger predictive performance with AUC of 0.93 for stage 3 AKI and 0.96 for kidney support therapy need 3.

Appropriate Clinical Scenarios

The FST should be considered for risk stratification in specific patient populations 1, 4:

• Critically ill patients with stage 1 or 2 AKI who are hemodynamically stable 1, 4
• Patients requiring assessment before attempting CRRT discontinuation (particularly non-septic patients) 1
• Risk stratification to guide early nephrology consultation and RRT planning 1

The test represents tubular secretory capacity since furosemide is actively secreted through kidney tubules 5, 6.

Critical Contraindications and Safety Warnings

The FST must NOT be performed in several high-risk situations 1, 4:

• Hemodynamically unstable patients - may precipitate volume depletion, hypotension, and further renal hypoperfusion 1, 4
• Patients with cirrhosis and new-onset AKI - furosemide should be withdrawn immediately in this population 1, 4
• Before ensuring adequate intravascular volume - volume status must be optimized first 1

Each nephrotoxic medication increases AKI odds by 53%, and furosemide itself is associated with worsening renal function, making careful patient selection essential 1, 4.

Required Monitoring During and After Testing

Rigorous monitoring protocols are mandatory when performing the FST 1, 4:

• Hourly urine output monitoring during the test period 4
• Electrolyte monitoring every 12-24 hours during IV diuretic therapy 1, 4
• Daily renal function assessment (serum creatinine, BUN) 1, 4
• Volume status reassessment after test completion 1

Integration into Clinical Decision-Making

For patients with urine output <200 mL in the first 2 hours, clinicians should anticipate progression to stage 3 AKI and prepare for potential RRT 1. This includes:

• Intensifying monitoring of renal function and fluid balance 1
• Early nephrology consultation for RRT planning 1
• Avoiding additional nephrotoxic exposures 4

The FST serves as a functional assessment of tubular integrity and represents an economical, easily administered tool to assess renal tubular function in the acute setting 7, 5.

Relationship to Renal Reserve Assessment

The FST represents one method of tubular stress testing to assess renal reserve, which is particularly relevant for patients recovering from AKI who may have lost functional reserve despite normalized creatinine 8. The ADQI consensus framework recognizes both glomerular and tubular stress testing as tools to evaluate renal reserve in patients with acute kidney disease 8.

Common Pitfalls to Avoid

Do not use the FST to convert oliguric to non-oliguric AKI - this practice lacks evidence of benefit and may cause harm 4. The test is purely diagnostic for risk stratification, not therapeutic 1, 5. Additionally, avoid performing the test in volume-depleted patients or combining it with other nephrotoxic medications 1, 4.