Variability in doses obtained from a multi-dose ophthalmic solution: a potential source of error in the assessment of compliance

Assessment of compliance with prescribed therapy is an important aspect of patient management that can be overlooked. Compliance with topical ophthalmic medications is frequently assessed without knowledge of doses obtained per bottle.

Thirty-three normal subjects who agreed to participate were asked to instill one drop of Refresh Tears in each eye twice daily until a 3-ml bottle was empty. Each subject was also asked to record the date and time of each drop instillation using a calendar log sheet. On enrollment, the subjects were required to demonstrate proper drop instillation technique. With the use of the log sheets, doses obtained and administrations missed by each subject were tabulated. The 'expected count' per bottle was determined by counting the drops in Refresh Tears under laboratory conditions.

Of the 26 subjects, who completed the study, the mean (SD) number of doses reportedly obtained was 63.7 +/- 17.1 (range = 26 to 110). The mean number of drops counted per bottle was 63.5 +/- 1.0 (range = 62 to 65).

The range of doses reportedly obtained per bottle was quite large. This variability could represent a significant source of error if patient compliance is assessed by how long a bottle lasts.