Time to stop counting the tablets?

We attempted to assess compliance using both a pharmacologic indicator (low-dose phenobarbital) and a return tablet count in 225 patients who were taking part in three separate studies. There were 216 patients (96%) who kept a follow-up appointment after 28 days; 161 patients appeared to have good compliance (90% to 109%) by return tablet count. Of these 161 patients, 51 (32%) had plasma phenobarbital concentrations (corrected for dose and weight) that were less than 90% of the lowest value previously found in normal volunteers, which suggested poorer compliance. When compared with the age-related volunteer values, 77 (48%) had values that were less than 90% of the lowest volunteer value. There were 6 of 10 patients with apparently excessive (greater than or equal to 110%) compliance by return tablet count and 4 of 12 who failed to return their container who also had phenobarbital concentrations that were less than 90% of the lowest volunteer value. We concluded that return tablet count grossly overestimates compliance.