Normal human tear pH by direct measurement.

Tear pH was measured in 44 normal subjects by immersing the lip of a microcombination glass pH probe in the tear fluid in the inferior cul-de-sac. The normal pH range was 6.5 to 7.6; the mean value was 7.0. This method provides a rapid and accurate measure of tear pH and may better our understanding of tear physiology and tear buffering capacity.