Toxic range: >2.5 ng/mL, but 10% of patients may show toxicity at <2 ng/mL.
Toxicity may be observed at a lower serum concentration in presence of hypokalemia, hypercalcemia, hypomagnesemia, hypoxia, and heart disease.
Increased with coadministration of
Quinidine
Verapamil
Amiodarone
Indomethacin
Cyclosporin A
Limitations
Draw blood 6 " 8 hours (or 8 " 24 hours) after last oral dose after steady state has been achieved in 1 " 2 weeks.
Pediatric toxic concentration may be higher; therapeutic index is very low (i.e., small difference between therapeutic and toxic blood concentration). However, approximately 10% of patients have serum concentration of 2 " 4 ng/mL without evidence of toxicity. On a dose of 0.25 mg/day, mean serum concentration is 1.2 ± 0.4 ng/mL; on a dose of 0.5 mg/day, mean serum concentration is 1.5 ± 0.4 ng/mL. A Digitalis leaf dose of 0.1 g/day produces the same serum concentration as 0.1 mg/day of crystalline digitoxin. There is ECG evidence of toxicity in one third to two thirds of patients, with no symptoms or signs.
False low results may be due to spironolactone.
Endogenous digoxin-like substances may produce positive test results in persons who have not received the drug, especially in
Uremia.
Severe agonal states and postmortem " therefore, a high postmortem concentration may not have been high before death and a normal postmortem concentration suggests that the antemortem concentration was not toxic.
Because most methods measure both endogenous digoxin-like substances and inactive metabolites of digoxin, therapeutic monitoring should mostly be used to assess patient compliance and to confirm drug toxicity.