Lockington et al1 rightly raise the issue of microbial contamination of disposable tonometer prism holders, which have been introduced because of the theoretical risk of prion transmission. They recommend the cleaning of these holders with alcohol wipes to decontaminate them between patients. They should be aware that alcohol does not inactivate prions; in fact it fixes proteins, including prions, in a viable form to inert material. Therefore, alcohol cleansing prolongs the infectivity of prions on instruments. Re-usable tonometer prism heads should never be cleaned with alcohol wipes for the same reason.

Although the disposable tonometer holders have no direct contact with patients, they should be cleaned in the same way as recommended for re-usable tonometer prisms (eg, by immediate immersion in sodium dichloroisocyanurate 1 g/l). This minimizes any theoretical risk of prion transmission.