When do you test digoxin (digitalis glycoside) levels?

When to Test Digoxin Levels

Digoxin levels should be tested when signs or symptoms of toxicity appear, when adding medications that may increase digoxin levels, and early during initiation of chronic therapy, with the therapeutic serum concentration maintained between 0.5 to 1.0 ng/mL. 1, 2

Specific Situations Requiring Digoxin Level Testing

• Initial monitoring: Check digoxin concentration early during chronic therapy initiation in patients with normal renal function to establish a baseline 1
• Signs and symptoms of toxicity: Immediately check levels when patients present with confusion, nausea, anorexia, visual disturbances (especially color vision changes), or cardiac arrhythmias 1, 3
• Electrolyte abnormalities: Test digoxin levels in the presence of hypokalemia, as toxicity may occur at lower serum concentrations due to increased myocardial sensitivity 4, 1
• Drug interactions: Check levels after starting medications known to increase digoxin concentrations, such as amiodarone, diltiazem, verapamil, certain antibiotics (erythromycin, clarithromycin), and quinidine 1, 5
• Renal impairment: Monitor more frequently in patients with impaired renal function, as they may take longer to reach steady state and are at higher risk for toxicity 5
• Acute poisoning: Test levels in suspected acute digoxin poisoning, especially when serum potassium exceeds 5.0 mEq/L, which is a marker of severity 3

Proper Timing and Collection of Samples

• Sample timing: Draw blood for digoxin concentration just before the next scheduled dose (trough level) 5
• If pre-dose sampling isn't possible: Sample at least 6-8 hours after the last dose to allow for adequate distribution 5
• Once-daily dosing: Be aware that concentrations will be 10-25% lower when sampled at 24 hours versus 8 hours, depending on renal function 5

Therapeutic Range and Interpretation

• Current therapeutic range: Maintain serum concentrations between 0.5 to 1.0 ng/mL for optimal clinical benefit with minimal risk of toxicity 2
• Toxicity threshold: While overt toxicity is commonly associated with levels above 2.0 ng/mL, toxicity may occur at lower levels in the presence of hypokalemia, hypomagnesemia, or hypothyroidism 2, 4
• Clinical context: Always interpret serum digoxin concentration in the overall clinical context, as an isolated measurement should not be the sole basis for dose adjustments 5

Common Pitfalls in Digoxin Monitoring

• Overreliance on serum levels: One-third of patients with clinical toxicity have concentrations below 2.0 ng/mL, so values below this threshold don't rule out toxicity 5
• Missing concurrent electrolyte testing: When monitoring digoxin, always check potassium and renal function, as hypokalemia can cause toxicity even at therapeutic digoxin levels 4, 6
• Routine annual monitoring without indication: Routine monitoring without clinical indication is considered inappropriate and often lacks concurrent assessment of more important markers like potassium and renal function 6
• Outdated therapeutic ranges: Some providers may still use older, higher therapeutic ranges (up to 2.0 ng/mL) that are now known to be associated with increased mortality 7, 2

Frequency of Monitoring

• There is no evidence that regular digoxin concentration measurements improve outcomes once a stable dose is established 1
• However, monitoring should be performed promptly when any risk factors for toxicity develop or clinical status changes 1, 2
• Patients with multiple comorbidities should be monitored more closely as they may be at greater risk for digoxin toxicity 8

Remember that digoxin has a narrow therapeutic index, and maintaining levels between 0.5-1.0 ng/mL is critical for reducing morbidity and mortality while preserving therapeutic benefit 7, 2.