Sir,

Thank you for the opportunity to respond to the issues raised in the letter by Leung and Lam and we are grateful to them for their interest and enquiry.

A single reading with both the pressure phosphene tonometer (PPT) and the Goldmann tonometer (GT) was used for two reasons. Firstly, this was considered a closer simulation of what was likely to occur in a clinical environment and secondly the risk of consequent reduction of intraocular pressure due to repeated indentations was reduced.

Leung and Lam make a very good point about the subjects' visual field status. Their visual field status was not described as the subjects were all patients sourced from a general ophthalmology clinic and could be assumed to have no or minimal visual field loss. However, those patients for whom the PPT is intended are more likely to have glaucomatous field loss.

As a result of the wide limits of agreement there seemed to be little merit in doing tests of reliability on these data. Furthermore, it needs to be noted that 31% of the subjects were unable to have their intraocular pressures measured using the PPT as they were unable to perceive a pressure phosphene.

When the data were analysed for those younger than the median vs those older, the difference between the tests and the limits of agreement were similar for the two age groups.