Pre-Furosemide Laboratory and Clinical Assessment
Before initiating furosemide, you must check serum electrolytes (particularly potassium and sodium), renal function (creatinine and BUN), volume status, and blood pressure to prevent life-threatening complications and ensure safe diuretic therapy. 1
Essential Laboratory Tests
Electrolytes
- Serum potassium: Critical to measure before starting therapy, as furosemide causes significant potassium wasting 1. Correct severe hypokalemia (<3 mmol/L) before initiation 2
- Serum sodium: Assess for hyponatremia, particularly in cirrhotic patients where severe hyponatremia (<120 mmol/L) contraindicates diuretic use 2
- Serum chloride: Important baseline as furosemide can cause hypochloremic alkalosis 1
- Serum magnesium and calcium: Should be measured as furosemide depletes both electrolytes 1
Renal Function
- Serum creatinine and BUN: Essential to establish baseline kidney function 1. The FDA label emphasizes determining these "frequently during the first few months of furosemide therapy" 1
- Estimated GFR: Particularly important as renal impairment requires caution when starting diuretics 2. In cirrhotic patients with significant renal dysfunction (creatinine >221 μmol/L or eGFR <30 mL/min/1.73 m²), seek specialist advice before initiating therapy 2
Additional Laboratory Monitoring
- Blood glucose: Check in diabetic patients or those suspected of latent diabetes, as furosemide may increase glucose levels 1
- Uric acid: Baseline measurement recommended as asymptomatic hyperuricemia commonly occurs 1
Critical Clinical Assessment
Volume Status Evaluation
- Assess for hypovolemia: Look for prolonged capillary refill time, tachycardia, hypotension, and oliguria 2. Furosemide should be used with extreme caution or avoided in hypovolemic patients 2
- Blood pressure: Measure carefully; symptomatic or severe hypotension (systolic <90 mmHg) requires caution 2. The FDA label warns that excessive diuresis may cause circulatory collapse 1
- Intravascular vs. extravascular fluid: In nephrotic syndrome, diuretics should only be used when there is evidence of intravascular fluid overload (good peripheral perfusion and high blood pressure), not just edema 2
Specific Clinical Scenarios Requiring Extra Caution
In cirrhotic patients with ascites 2:
- Check for hepatic encephalopathy (generally contraindicated if overt encephalopathy present)
- Assess severity of hyponatremia (contraindicated if <120 mmol/L)
- Evaluate for renal impairment requiring specialist input
In heart failure patients 2:
- Establish baseline volume status and congestion
- Check potassium levels, as concurrent use with ACE inhibitors/ARBs increases hyperkalemia risk
- Assess for signs of cardiorenal syndrome
In patients with urinary retention risk 1:
- Evaluate for bladder emptying disorders, prostatic hyperplasia, or urethral narrowing, as furosemide can precipitate acute urinary retention
Monitoring Schedule After Initiation
- First month: Frequent clinical and biochemical monitoring is mandatory, particularly during the first weeks when diuretic-induced complications are most common 2
- Ongoing: Serum electrolytes, CO₂, creatinine, and BUN should be determined frequently during the first few months, then periodically thereafter 1
- During dose titration: Re-check blood chemistry 1-2 weeks after initiation and 1-2 weeks after final dose titration 2
Common Pitfalls to Avoid
- Starting diuretics in hypovolemic patients: This is a critical error that can precipitate acute kidney injury and thrombosis 2
- Ignoring baseline potassium: Hypokalemia develops commonly, especially with brisk diuresis, and digitalis therapy exaggerates the myocardial effects 1
- Overlooking drug interactions: Check for concurrent use of aminoglycosides (ototoxicity risk), lithium (toxicity risk), or ACE inhibitors/ARBs (severe hypotension and renal deterioration risk) 1
- Missing contraindications in cirrhosis: Severe hyponatremia, progressive renal failure, and overt hepatic encephalopathy are absolute contraindications 2