Demonstration of increasing standard pH value of lacrimal fluid with increase of flow rate

. Purpose: To test the hypothesis that pH value of human tears is affected by flow rate. Methods: After stimulation with a drop of 70% alcohol into the ipsilateral nasal cavity tears from the left eyes of 30 healthy volunteers were collected with glass capillaries while flow rate of tear secretion was registered. The pCO 2 , actual and standard pH were determined in the collected samples with the Astrup method. Standard pH is defined as the pH of the sample that has been equilibrated at 37 °C with a gas mixture having pCO 2 =40 mmHg. Results: The flow rate correlated positively and significantly with both the standard pH value (r=0.503, p<0.01) and the pCO 2 (r=0.517, p<0.01) while there was no correlation between the actual pH and the flow rate. Conclusion: Loss of CO 2 from the sample is a cause of serious uncertainty in tear pH determination. A positive flow rate‐pH relationship was found for human tears when the uncertainty was removed by determination of the standard pH.