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Digoxin is a cardiac glycoside used to treat congestive heart failure, atrial fibrillation and paroxysmal atrial tachycardia. Different products have significant bioavailability differences. Many drugs interact with digoxin, often requiring an adjustment of the digoxin dose. Clearance of digoxin is low in premature neonates, increases in full term neonates, reaches a maximum in infants and decreases slowly during childhood and adulthood. Digoxin is cleared by the kidney. The circulating half life is 1 to 1.6 days in patients with normal renal function and 3.5 to 4.5 days in anuric patients.

Although serum levels of 0.8 to 2.1 ng/mL have historically been considered therapeutic, there is considerable overlap between therapeutic and toxic concentrations. Patients may experience signs and symptoms of digitalis intoxication at serum levels less than 2.1 ng/mL. Conversely, some patients may require serum levels in excess of 2.1 ng/mL to control their arrhythmia.

A study in five hospitals demonstrated a 4.1% incidence of digoxin toxicity in inpatients (Arch Intern Med 1998; 158:2444-9). Nausea was the most common symptom followed by anorexia and diarrhea. Heart block, bradycardia, and junctional tachycardia were the most frequent ECG changes. Deteriorating renal function, hypokalemia, hypomagnesemia, and hypercalcemia are predisposing factors.

Serum digoxin measurement are not useful following the administration of digoxin immune Fab (digibind) until the antibody bound drug is cleared. Digoxin-like reactive factors are small molecular weight endogenous substances that are universally present, but usually only identified in patients with renal or liver failure, pregnant women, and newborn infants. These factors compromise the accurate measurement and clinical interpretation of digoxin.

Routine measurement of serum digoxin concentration is probably not necessary in stable patients. Digoxin measurement is necessary in the following situations:

  • After a patient has reached steady state with a new digoxin dose
  • Following significant change in renal function
  • Following addition or discontinuation of a potentially interacting drug
  • Following signs or symptoms consistent with digoxin toxicity.

Therapeutic range (trough) is 0.8-2.1 ng/mL. Trough levels >3.0 ng/mL are considered critical.

Trough specimens should be drawn 6 hours after the last dose. Specimen requirement is one SST tube of blood.

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