What is the purpose of a Lasix (furosemide) challenge after an Acute Kidney Injury (AKI)?

The Purpose of a Furosemide Challenge After Acute Kidney Injury

The primary purpose of a furosemide challenge (furosemide stress test) after an acute kidney injury is to predict the risk of AKI progression, need for renal replacement therapy, and assess residual kidney function, helping clinicians make critical treatment decisions regarding patient management. 1

Mechanism and Rationale

The furosemide stress test (FST) works based on specific physiological principles:

• Furosemide is actively secreted by proximal tubules into the urine before reaching its site of action at the ascending limb of loop of Henle 2
• The urinary response to furosemide depends on:
  • Adequate renal blood flow
  • Functioning proximal tubular secretion
  • Intact tubular function at the loop of Henle

Clinical Applications of the Furosemide Challenge

1. Prediction of AKI Progression

   • Identifies patients at higher risk of AKI progression 3
   • Helps predict which patients will require renal replacement therapy 1
   • Poor response suggests more severe tubular damage

2. Assessment of Residual Renal Function

   • A good urinary response may be considered a "proxy" for having some residual renal function 2
   • Measured creatinine clearance strongly correlates with urinary furosemide excretion 4

3. Prediction of Recovery from RRT

   • Can help predict successful cessation of continuous renal replacement therapy in patients with established AKI 3

4. Risk Stratification

   • Helps in clinical decision-making regarding level of care and monitoring
   • Assists in determining appropriate follow-up after AKI 1

How to Perform the Furosemide Challenge

1. Dosing:

   • Typically 1.0-1.5 mg/kg IV furosemide for furosemide-naïve patients
   • 1.5-2.0 mg/kg IV for patients with prior furosemide exposure

2. Monitoring Response:

   • Measure urine output for 2-6 hours following administration
   • In children, cutoffs for predicting severe AKI are:
     • <4 mL/kg/hour at 2 hours
     • <3 mL/kg/hour at 6 hours 5

3. Interpretation:

   • Poor response (low urine output) indicates higher risk of AKI progression
   • Strong response suggests better prognosis and potential for recovery

Important Caveats and Considerations

• Not for Prevention or Treatment: Furosemide should not be used to prevent AKI (1B recommendation) or treat AKI except for volume overload management (2C recommendation) 1, 6

• Potential Harm: Furosemide may increase renal oxidative stress in AKI, with the greatest increase seen in patients with the most severe AKI 7

• Pharmacokinetic/Pharmacodynamic Changes: Both the pharmacokinetics and pharmacodynamics of furosemide are altered in AKI, particularly when creatinine clearance is below 40 mL/min/1.73m² 4

• Volume Status: Ensure patient is not hypovolemic before administration, as furosemide can worsen renal function through reduced renal blood flow and activation of the renin-angiotensin-aldosterone system 6

• Predictive Value: Creatinine clearance is the most reliable predictor of urinary output response to furosemide in AKI (AUC 0.75; 95% CI, 0.57-0.93) 4

When to Use the Furosemide Challenge

• When determining the severity and prognosis of AKI
• When deciding whether to initiate renal replacement therapy
• When assessing potential for recovery from established AKI
• When planning appropriate follow-up and monitoring after AKI

The furosemide stress test represents an important diagnostic and prognostic tool in the management of AKI, providing valuable information about kidney function and likely clinical course while helping guide treatment decisions that impact morbidity and mortality.