Laboratory Monitoring for Furosemide (Lasix)
Patients taking furosemide require regular monitoring of serum electrolytes (particularly potassium), CO2, creatinine, and BUN, with frequent checks during the first few months of therapy and periodically thereafter. 1
Initial Monitoring Period
During the first few months of furosemide therapy, laboratory monitoring should be performed frequently 1:
- Serum electrolytes (with particular emphasis on potassium levels)
- Serum creatinine and BUN to assess renal function
- CO2/bicarbonate to detect metabolic alkalosis
- Blood glucose in diabetic patients or those with suspected latent diabetes 1
Electrolyte determinations are particularly critical when patients are vomiting profusely or receiving parenteral fluids. 1
Ongoing Monitoring Schedule
After the initial stabilization period 1:
- Periodic monitoring of the same parameters should continue throughout therapy
- Serum calcium and magnesium levels should be checked periodically, as furosemide may lower these electrolytes (rare cases of tetany have been reported) 1
- Urine and blood glucose should be monitored periodically in diabetic patients 1
Heart Failure-Specific Monitoring
In patients with acute heart failure receiving intravenous furosemide 2:
- Regular monitoring of symptoms, urine output, renal function, and electrolytes is recommended during use of IV diuretics 2
- Blood pressure monitoring after initiation and during titration 2
For chronic heart failure patients on oral diuretics 2:
- Serum creatinine and potassium should be measured every 5-7 days after initiation of potassium-sparing diuretics (if used concomitantly) until values are stable, then every 3-6 months 2
Pediatric Considerations
In premature infants receiving furosemide 1:
- Renal function monitoring is essential
- Renal ultrasonography should be performed, as furosemide may precipitate nephrocalcinosis/nephrolithiasis 1
In children receiving diuretics for hypertension 2:
- Electrolyte monitoring should occur shortly after initiating therapy and periodically thereafter 2
Critical Electrolyte Abnormalities to Monitor
All patients should be observed for signs and symptoms of fluid or electrolyte imbalance 1:
- Hypokalemia - especially with brisk diuresis, inadequate oral intake, cirrhosis, or concomitant corticosteroid/ACTH use 1
- Hyponatremia
- Hypochloremic alkalosis
- Hypomagnesemia
- Hypocalcemia
Clinical manifestations include: dryness of mouth, thirst, weakness, lethargy, drowsiness, restlessness, muscle pains or cramps, muscular fatigue, hypotension, oliguria, tachycardia, arrhythmia, nausea, and vomiting 1
Renal Function Monitoring
Reversible elevations of BUN may occur and are associated with dehydration, which should be avoided, particularly in patients with renal insufficiency. 1 Worsening renal function during hospitalization has been associated with higher loop diuretic doses and increased in-hospital mortality 2