The sample collected for digoxin varies with intent and state of patient health.
If toxicity is of concern, a peak 2 to 4 hr sample is indicated.
For efficacy, a just before the next dose trough is indicated.
Note that the half-life can be quite variable for digoxin, depending on patient disease. A peak and trough (2 and 12 hr) should be considered for such patients. The elimination half-life may change across time as disease progresses, indicating the need for continued monitoring.
Rubber tops may bind digoxin, and natural rubber may contain contaminants that interfere with testing; further, serum separator gels may bind digoxin. Plastic-capped tubes are recommended.