To investigate the actual adherence to treatment with preservative-free dorzolamide-timolol fixed combination (DTFC) eyedrops of primary open-angle glaucoma (POAG) patients by counting the number of unused single-dose units of DTFC.

This study included 34 POAG patients newly prescribed with preservative-free DTFC eyedrops (formulated in single-dose units). The enrolled patients were asked to bring the unused DTFC units on their next visit after 2 weeks of treatment with DTFC. On their second visit, they were asked to complete a questionnaire regarding the self-reported adherence and the number of unused DTFC single-dose units was counted. The actual adherence (%) was calculated by dividing the expected number of used DTFC units by the actual number of used DTFC units. The correlation between the self-reported adherence and the measured adherence was assessed.

Twenty-nine (93.5%) patients answered they adhered to the medication by more than 90% and 2 (6.5%) answered they instilled the eyedrops at 80-90% of the dosing schedule. However, after counting the unused DTFC single-dose units, 9 (29.0%) patients showed an actual adherence of <90%. Moreover, the actual adherence of 3 (9.7%) patients was <60%. Unexpectedly, 4 (12.9%) patients showed the actual adherence exceeding 100% (196%, 1 patient; 107-132%, 3 patients).

We demonstrated a large difference between the self-reported and the actual adherence to treatment by counting the unused single-dose units of eyedrops. Preservative-free topical anti-glaucoma medications (formulated in single-dose units) provide clinicians an opportunity to assess the actual adherence of glaucoma patients by counting the unused units of eyedrops.