An objective assessment of the variability in number of drops per bottle of glaucoma medication

Background: The purpose of this study is to evaluate the number of eyedrops available per bottle of a variety of commonly prescribed glaucoma medications. Methods: Six bottles of each glaucoma medication were tested: three each in the vertical and horizontal orientations. Bottles were housed in a customized force gauge apparatus designed to mimic ballpoint fingertip contact with a bottle. At a standard rate, all drops were expressed from each bottle and counted with an automated drop counter. Simultaneously, bottle volume was measured and drop size and number were also estimated. The main outcome measures were: total number of drops, volume per bottle and drops per milliliter (mL) of glaucoma medication. Results: A total of 192 bottles from 32 bottle designs and manufacturers were tested. Twenty-two of the 32 bottle designs had a significantly different mean number of drops in the vertical and horizontal positions, with 10 designs have more drops dispensed in the horizontal orientation and 12 in the vertical orientation. Six of the 32 bottle designs had a significantly different mean total bottle volume in the vertical and horizontal positions, with all designs having greater volume in the vertical position. An adjusted ratio of mean number of drops/mean bottle volume demonstrated a range from 20.9 drops/mL to 40.8 drops/mL. Conclusions: There is significant variability in drops and volume available per bottle of glaucoma medication depending on both the bottle position and manufacturer. These data point to the need for circumspection in prescribing glaucoma medications and caution in evaluating therapeutic outcomes.