How should I adjust my patient's digoxin dose, given a serum digoxin concentration of 0.6 nmol/L (target 0.64–1.3 nmol/L) while they are currently receiving 0.09375 mg daily and have no contraindicating factors such as severe renal impairment, electrolyte abnormalities, recent toxicity, or frailty?

Digoxin Dose Adjustment for Subtherapeutic Level

Increase your patient's digoxin dose to 0.125 mg daily, as the current level of 0.6 nmol/L (approximately 0.47 ng/mL when converted) is below the therapeutic target and the dose of 0.09375 mg/day is insufficient for most patients. 1

Rationale for Dose Increase

Your patient's current digoxin level requires conversion for proper interpretation:

• 0.6 nmol/L = approximately 0.47 ng/mL (using conversion factor 1 ng/mL ≈ 1.28 nmol/L)
• Target range: 0.64–1.3 nmol/L = approximately 0.5–1.0 ng/mL 1
• Current level is subtherapeutic and below the recommended target of 0.5–0.9 ng/mL for heart failure 1, 2

The dose of 0.09375 mg/day (approximately 93.75 mcg) is unusually low and falls below standard maintenance dosing recommendations. 1, 2

Specific Dosing Recommendation

Increase to 0.125 mg (125 mcg) once daily, which represents the standard low-maintenance dose for patients over 70 years, those with impaired renal function, or low lean body mass. 1, 2

Why 0.125 mg is appropriate:

• The relationship between digoxin dose and plasma concentration is linear and proportional 3
• Your patient needs approximately a 33% dose increase (from 0.09375 mg to 0.125 mg) to achieve therapeutic levels 3
• This dose is safe and well-established as the standard low-maintenance dose 1, 2
• Do not exceed 0.25 mg daily in most patients, as higher doses increase toxicity risk without improving outcomes 1

Monitoring After Dose Adjustment

Recheck digoxin level in 1–2 weeks after the dose increase, as steady-state will be achieved in approximately 5 half-lives (7–14 days depending on renal function). 1, 2

Critical monitoring parameters:

• Target serum concentration: 0.5–0.9 ng/mL (0.64–1.15 nmol/L), as concentrations above 1.0 ng/mL offer no additional benefit and may increase mortality 1, 2
• Serum potassium and magnesium must be monitored regularly, maintaining potassium 4.0–5.5 mEq/L, as deficiencies increase toxicity risk even at therapeutic levels 1, 4
• Renal function should be assessed, as declining kidney function will require dose reduction 1, 2

Important Clinical Context

When NOT to increase the dose:

• If heart rate is already well-controlled (60–100 bpm at rest, <110 bpm with activity), the current dose may be adequate regardless of the specific serum level 1
• If the patient is elderly (>70 years) with impaired renal function, 0.125 mg may still be appropriate even if levels remain at the lower end of therapeutic range 1, 5

Alternative consideration:

If your patient is under 70 years with normal renal function, the standard maintenance dose is 0.25 mg daily, and you may consider increasing directly to this dose rather than the intermediate 0.125 mg. 1, 2 However, given the current subtherapeutic level is only slightly below target, the conservative approach of 0.125 mg daily is safer. 1

Common Pitfalls to Avoid

• Do not use loading doses in stable outpatients with chronic heart failure; maintenance dosing should be started directly 1
• Do not target levels above 1.0 ng/mL, as this increases mortality risk without improving clinical outcomes 1, 2
• Do not increase dose if drug interactions are present: medications like amiodarone, verapamil, diltiazem, or clarithromycin increase digoxin levels and require dose reduction by 30–50% 1, 4
• Ensure blood sampling timing is correct: digoxin levels must be drawn at least 6–8 hours after the last dose to allow tissue-plasma equilibration 4, 2