What is the purpose and protocol of the Furosemide (Lasix) stress test in patients with suspected acute kidney injury or chronic kidney disease?

Furosemide Stress Test in Acute Kidney Injury

Purpose and Clinical Utility

The furosemide stress test (FST) is a functional biomarker used to predict progression to severe AKI (stage 3) and the need for renal replacement therapy in patients with early-stage (stage 1 or 2) AKI. 1

The FST serves three primary clinical applications:

• Predicting progression to stage 3 AKI in patients with early-stage disease, with area under the curve (AUC) values of 0.87-0.93 1, 2, 3
• Identifying patients who will require RRT, with AUC values of 0.76-0.96 for predicting dialysis need 1, 4
• Predicting successful discontinuation of continuous RRT in patients with established AKI (AUC = 0.84) 1

The FST represents a 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

Protocol and Administration

Patient Selection Criteria

The FST should only be performed in hemodynamically stable patients with stage 1 or 2 AKI after ensuring adequate intravascular volume. 1, 2

Critical exclusion criteria include:

• Hemodynamically unstable patients - the test may precipitate volume depletion, hypotension, and further renal hypoperfusion 1, 5
• Patients with cirrhosis and new-onset AKI - furosemide should be withdrawn immediately in this population 1, 5
• Patients with severe chronic kidney disease 2

Dosing Protocol

The standardized FST dosing is:

• 1.0 mg/kg IV furosemide for loop diuretic-naïve patients 2, 3
• 1.5 mg/kg IV furosemide for patients previously exposed to loop diuretics 2, 3

Urine Output Measurement and Interpretation

Measure hourly urine output for 2-6 hours after furosemide administration, with the first 2 hours being most predictive. 2, 3

The critical threshold for risk stratification:

• 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% 1, 2
• Urine output >2 mL/kg within the first 2 hours (in pediatric populations) indicates furosemide responsiveness 4

Monitoring Requirements

During the Test

• Hourly urine output monitoring for at least 2 hours, up to 6 hours 1, 5
• Volume status reassessment after the test 1

Post-Test Monitoring

• Electrolyte monitoring every 12-24 hours during IV diuretic therapy 1, 5
• Daily renal function assessment (serum creatinine, BUN) 1, 5

Clinical Decision-Making Based on Results

High-Risk Results (Urine Output <200 mL in 2 Hours)

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

Specific actions include:

• Intensify monitoring of renal function and fluid balance 1
• Early nephrology consultation for RRT planning 1
• Avoid additional nephrotoxic medications - each nephrotoxin increases AKI odds by 53% 1, 6, 5

Low-Risk Results (Adequate Urine Response)

Patients with adequate urine output (>200 mL in 2 hours) have significantly lower risk of progression, though continued monitoring remains essential 2, 3

Critical Contraindications and Warnings

Absolute Contraindications

• Cirrhosis with new-onset AKI - furosemide must be withdrawn immediately per International Club of Ascites guidelines 1, 5
• Hemodynamic instability - risk of precipitating volume depletion and hypotension 1, 5

Relative Contraindications and Cautions

• Severe chronic kidney disease - limited data in this population 2
• Concurrent nephrotoxic medications - each additional nephrotoxin increases AKI risk by 53% 6, 5

Important Clinical Caveats

The FST should not be used to convert oliguric to non-oliguric AKI, as this practice lacks evidence of benefit. 5

The test is purely diagnostic for risk stratification, not therapeutic 5. The KDIGO guidelines state that diuretics should not be used to prevent AKI (Grade 1B) and should not be used to treat AKI except in the management of volume overload (Grade 2C) 6, 5

Combining FST with elevated biomarker levels (such as TIMP-2 × IGFBP7) improves risk stratification, with AUC improving to 0.90-0.91 for predicting progression and RRT need 3

The FST outperformed biochemical biomarkers (NGAL, PENK) for prediction of progressive AKI, need for RRT, and inpatient mortality 4, 3