Therapeutic drug monitoring of digoxin

Digitalis glycosides have been in use in medicine over 200 years. The main pharmacological effects include a dose-dependent increase in myocardial contractility and a negative chronotropic action. Digitalis also increases the refractory period and decreases impulse velocity in certain myocardial tissue [such as the atrioventricular (AV) node]. The electrophysiological properties of digitalis are reflected in the ECG by shortening of the QT interval. Both digoxin and digitoxin have narrow therapeutic index; thus, therapeutic drug monitoring is essential for achieving optimal efficacy as well as to avoid toxicity. The therapeutic range of digoxin usually is 0.8-2.0 ng/mL, but there is a substantial overlap between therapeutic and toxic concentrations. Moreover, mild-to-moderate renal failure may also significantly increase the risk with digoxin therapy (1). Digoxin toxicity may occur with a lower digoxin level, if hypokalemia, hypomagnesemia, or hypothyroidism coexists. Likewise, the concomitant use of drugs such as quinidine, verapamil, spironolactone, flecainide, and amiodarone can increase serum digoxin levels and increase the risk of digoxin toxicity. A recent clinical trial indicated that a beneficial effect of digoxin was observed at serum concentrations from 0.5 to 0.9 ng/mL whereas serum concentrations at or over 1.2 ng/mL appeared harmful (2).

Although digoxin concentration in serum or plasma can be detected accurately by sophisticated analytical techniques such as high-performance liquid chromatog-raphy (HPLC) combined with tandem mass spectrometry, in clinical laboratories digoxin immunoassays are the preferred method because of automation and rapid turnaround time. Immunoassays are subjected to interference by endogenous digoxin-like immunoreactive substances (DLIS) because of structural similarity with digoxin. Moreover, several Chinese medicines such as Chan Su, Lu-Shen-Wan (LSW), and oleander-containing herbs may interfere with serum digoxin measurements by immunoassays because of structural similarity of components of these alternative medicines with digoxin. Spironolactone and anti-digoxin antibodies used in treating patients overdosed with digoxin also interfere with digoxin measurement using immunoassays (3).

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