Effects of humidity on tests of tear production

Purpose: To determine if the Schirmer test is affected by environmental humidity, to quantify any effects, and to consider the ways in which this may affect the diagnosis of dry eye disease.

Methods: All experiments took place within a controlled environment chamber at a range of 5% to 95% relative humidity (RH). In vitro testing immersed Schirmer strips at 5 mm depth in water for 5 minutes. In vivo tests were conducted using the standard clinical method. All experiments were conducted using both standard Schirmer strips and Schirmer strips sheathed in a plastic film in order to reduce the possible effects of evaporation.

Results: In vitro results showed a clear decrease in Schirmer wetting length as RH was reduced. Schirmer test strips that were encased within a plastic sheathing were less affected by changing RH. As well as wetting length, the rate of wetting of Schirmer strips was also affected by changes in RH. In vivo data showed that Schirmer results in participants with severe dry eye disease were less affected by changes in RH than "normal" participants.

Conclusions: Patients with severe dry eye disease produce Schirmer test results which are unaffected by environmental humidity. However, patients with moderate Schirmer wetting lengths may be falsely diagnosed as having dry eye disease if their test is undertaken in a low-humidity environment. This phenomenon may be overcome with the use of plastic sheathing. Previous studies investigating the effects of environmental conditions on the tear film may have been affected by this phenomenon.