Audit exposes flawed blood sampling for "digoxin levels"

Therapeutic "digoxin level" monitoring in selected wards was audited. Time elapsing between the last dose and blood sampling was considered appropriate if greater than or equal to 6 h. If such details were not entered on the requisition, the maximum time elapsing was estimated as "appropriate" or "inappropriate" from the time samples were logged into the laboratory and the time the last dose was entered in the patient's treatment sheet. In 22 requisitions detailing sampling time, nine were considered inappropriate. In an additional 150 instances, timing was estimated as inappropriate in 45. Among the 118 requests where timing (estimated or labelled) was appropriate, available plasma digoxin concentrations yielded a mean of 1.0 nM, compared to 1.6 nM in the corresponding 54 patients with premature sampling; this difference was both clinically and statistically significant (95% confidence limits 0.8-1.2 and 1.3-1.9 nM, respectively, p less than 0.001). Premature blood sampling for digoxin levels was common and associated with higher concentrations than when appropriate. Such inappropriate timing may not have serious consequences, but digoxin levels are a matter of record and are used for teaching; due attention to timing could provide more reliable information and avoid wasting valuable resources.