Aim: To evaluate the performances of the patients’ eye drop instillation and estimate the quantity of eye drop needed per month in glaucoma patients. 
Design: Cross-sectional, observational and questionnaire study. 
Methods: 137 glaucoma patients who had visual acuity better than 20/200 and had self-administered eye drops ≥ 6 months were included. All patients were informed to apply artificial tears into their eyes. Performances were directly observed and evaluated according to the following criteria: washing hands before application, applying the drops into lower conjunctival fornix, successful instillation on the first attempt, did not contaminate the tip of the bottle with eye and adnexa, and occluded their nasolacrimal duct or closing of the eyelids after application. We also interviewed about the same tasks they always do at home. Nonparametric test were used for analyses.  
Results: Median (IQR) age of the study population was 68 years (18 – 89). Median (IQR) duration since diagnosis of glaucoma was 48 (6 – 576) months. During direct observation, only 1 patient (0.7%) was able to accomplish all 5 criteria whereas 9 patients (6.6%) could not accomplish any of the criteria. Twenty-nine patients (21.2%) successfully instilled a drop in the lower fornix without touching the ocular adnexa. The overall performance under direct observation was significantly lower than the interview score (p<0.001). Younger patients (<60 years old) had higher performance under direct observation (p = 0.006) and knew the correct techniques during interview session better than the older patients (p = 0.014). Fifty-eight patients (42.4%) used more than 1 drop for each attempt. Number of eye drops used reported by the patients was significantly lower than what was directly observed (p<0.001).
Discussion: Performance of self-administered eye drop was very poor. Age affected the ability of eye drop application. Standard technique should be emphasized to improve the performance of the glaucoma treatment and prevent contamination.