What is the furosemide challenge test?

What is the Furosemide Stress Test?

The furosemide stress test (FST) is a functional biomarker that assesses renal tubular function by measuring urine output after a standardized dose of intravenous furosemide, used primarily to predict progression to severe acute kidney injury (stage 3 AKI) and the need for renal replacement therapy in critically ill patients with early-stage AKI. 1

Test Administration Protocol

• Administer 1 mg/kg IV furosemide in loop diuretic-naïve patients 1, 2, 3
• Administer 1.5 mg/kg IV furosemide in patients who received loop diuretics within the previous 7 days 3, 4
• Measure hourly urine output for 2-6 hours after administration 2, 3
• The test must only be performed after ensuring adequate intravascular volume status 1

Interpretation of Results

Response Thresholds

• Urine output >200 mL in the first 2 hours = FST-responsive (favorable prognosis) 1, 3, 4
• Urine output <200 mL in the first 2 hours = FST-nonresponsive (high risk for progression) 1, 3, 4
• The 2-hour urine output is the most predictive timepoint, with area under the curve (AUC) of 0.87-0.93 for predicting stage 3 AKI 1, 4

Predictive Performance

• For predicting progression to stage 3 AKI: AUC 0.87-0.93, sensitivity 73.9%, specificity 90.0% 1, 4
• For predicting need for renal replacement therapy: AUC 0.76-0.96 1, 2
• For predicting successful discontinuation of continuous RRT: AUC 0.84 1

Clinical Applications

Risk Stratification in Early AKI

• Use FST in critically ill patients with KDIGO stage 1 or 2 AKI to stratify risk of progression 1
• For patients with urine output <200 mL in first 2 hours, anticipate progression to stage 3 AKI and prepare for potential RRT 1
• Consider early nephrology consultation for RRT planning in patients with high-risk FST results 1

Predicting RRT Initiation

• FST-nonresponsive patients are 2.4 times more likely to require CRRT initiation than FST-responsive patients (95% CI 1.644-3.443) 3
• In one study, 89.2% of FST-nonresponsive patients required CRRT versus only 37.5% of FST-responsive patients 3

Guiding RRT Discontinuation

• Consider performing FST before attempting CRRT discontinuation in non-septic patients with AKI 1
• The test can predict successful CRRT cessation with AUC of 0.84 1

Critical Contraindications and Safety Warnings

Absolute Contraindications

• Do NOT perform FST in hemodynamically unstable patients—it may precipitate volume depletion, hypotension, and further renal hypoperfusion 1
• Do NOT use FST in patients with cirrhosis and new-onset AKI; furosemide should be withdrawn immediately in this population 1
• Do NOT perform FST in anuric patients 5

Important Caveats

• Furosemide is associated with worsening renal function and each nephrotoxic medication increases AKI odds by 53% 1
• High-dose furosemide produces acute, transient worsening of hemodynamics for 1-2 hours after administration, including increased systemic vascular resistance and reduced stroke volume 6
• The FST should NOT be used as the sole criterion to initiate RRT in euvolemic patients without an absolute indication 1

Integration with Clinical Decision-Making

KDIGO 2020 Recommendations

• A standardized FST may be considered in AKI to quantify the probability of progression; the result should be integrated with all other available clinical information when making RRT decisions 1
• Biomarkers alone (including FST) are not recommended for deciding to start RRT 1
• Initiation of RRT should be considered when metabolic and volume demands exceed the residual renal capacity of the patient 1

Post-Test Monitoring Requirements

• Monitor electrolytes every 12-24 hours during IV diuretic therapy 1
• Perform daily renal function assessment (serum creatinine, BUN) 1
• Reassess volume status after the test 1
• Intensify monitoring of renal function and fluid balance in FST-nonresponsive patients 1

Pediatric Considerations

• In critically ill children, use 1 mg/kg IV furosemide for FST 2
• Urine output >2 mL/kg within the first 2 hours is deemed furosemide responsive in pediatric patients 2
• FST demonstrated excellent predictive performance in children with AUC 0.93 for stage 3 AKI and 0.96 for need for kidney support therapy 2

Relationship to Renal Reserve Assessment

• The FST represents one method of tubular stress testing to assess renal reserve, particularly relevant for patients recovering from AKI who may have lost functional reserve despite normalized creatinine 1
• The Acute Disease Quality Initiative (ADQI) consensus framework recognizes FST as a tool to evaluate renal reserve in patients with acute kidney disease 1