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His expected digoxin clearance can be calculated from Eqn 1.1, i.e.

His average steady‐state concentration can be estimated from Eqn 1.2, i.e.

The 0.6 is an estimate of the bioavailability of digoxin tablets. The reason the measured concentration is higher than expected should be investigated. In this case, it was found that the sample had been withdrawn 2.5 hours after the dose. Digoxin is absorbed quickly but distributes slowly to the tissues. Samples taken before distribution is complete (i.e. less than 6 hours after the dose) and cannot be interpreted. As concentrations fall only by about 20% from 6 to 24 hours after the dose, samples can be taken at any time during this period.

A further (trough) sample withdrawn 24 hours after the last dose measured 2.4 μg/L (3.1 nmol L). This result is more consistent with the expected concentration but suggests that the dose is too high and may be contributing to his symptoms.