What labs should be monitored when ordering Lasix (furosemide)?

Laboratory Monitoring for Furosemide (Lasix)

When ordering furosemide, you must monitor serum electrolytes (particularly potassium), CO2, creatinine, and BUN frequently during the first few months of therapy and periodically thereafter. 1

Essential Laboratory Tests

Initial and Frequent Monitoring (First Few Months)

• Serum electrolytes must be checked frequently, with particular attention to potassium levels, as hypokalemia is one of the most common adverse effects occurring in 3.6% of patients 1, 2
• Serum sodium requires close monitoring, as hyponatremia can develop and severe hyponatremia (serum sodium <120-125 mmol/L) is an absolute contraindication to continuing therapy 3, 1
• CO2 (bicarbonate) should be measured to detect hypochloremic alkalosis, which is a common electrolyte disturbance with furosemide 1
• Creatinine and BUN must be determined frequently to monitor for reversible elevations associated with dehydration and to detect worsening renal function 1

Additional Electrolytes to Monitor Periodically

• Serum calcium should be checked periodically, as furosemide may lower calcium levels and rarely cause tetany 1
• Serum magnesium requires periodic monitoring, as hypomagnesemia can develop with furosemide therapy 1
• Serum potassium is particularly critical when patients are vomiting profusely or receiving parenteral fluids, requiring more frequent determinations 1

Special Monitoring Situations

Diabetic Patients

• Blood glucose and urine glucose should be checked periodically in diabetics receiving furosemide, and even in those suspected of latent diabetes, as furosemide may increase blood glucose levels 1

Pediatric Patients (Premature Infants)

• Renal function must be monitored closely in premature infants, as furosemide may precipitate nephrocalcinosis/nephrolithiasis 1
• Renal ultrasonography should be performed in premature infants on furosemide 1

Disease-Specific Monitoring

• In cirrhosis with ascites, the European Association for the Study of the Liver recommends checking serum sodium, potassium, and creatinine every 3-7 days during initial titration 3
• In heart failure, monitoring should include daily weights targeting 0.5-1.0 kg loss per day, with electrolytes and renal function checked every 3-7 days initially 3

Monitoring Frequency Algorithm

During first few months:

• Check electrolytes (Na, K, Cl, CO2), creatinine, and BUN frequently—typically every 3-7 days during dose titration 3, 1
• Monitor more frequently if patient is vomiting, receiving parenteral fluids, or showing signs of volume depletion 1

After stabilization:

• Continue periodic monitoring of serum electrolytes, creatinine, and BUN 1
• Check calcium and magnesium levels periodically 1

Critical Signs Requiring Immediate Laboratory Assessment

Stop furosemide and check labs immediately if patient develops: 1

• Signs of fluid or electrolyte imbalance: dryness of mouth, thirst, weakness, lethargy, drowsiness, restlessness, muscle pains or cramps, muscular fatigue
• Hypotension, oliguria, tachycardia, or arrhythmia
• Gastrointestinal disturbances such as nausea and vomiting

Common Pitfalls to Avoid

• Do not assume electrolytes are stable after initial monitoring—continue periodic checks as other medications and clinical conditions can influence electrolyte levels 1
• Correct abnormalities or temporarily withdraw the drug rather than continuing therapy with uncorrected electrolyte disturbances 1
• Avoid dehydration particularly in patients with renal insufficiency, as reversible BUN elevations are associated with volume depletion 1
• Monitor more closely in high-risk situations: patients on digitalis (where hypokalemia effects are exaggerated), those receiving corticosteroids or ACTH, patients with cirrhosis, or those using laxatives chronically 1