Can vaginal pH be measured from the wet mount slide?

Objectives: To assess the accuracy of vaginal pH measurement on wet mount microscopy slides compared with direct measurements on fresh vaginal fluid. We also tested whether differences in accuracy were dependent on the sampling devices used or on the diagnosis of the vaginal infections.

Study Design: Using a cotton swab, cytobrush or wooden spatula a vaginal fluid specimen was collected from 84 consecutive women attending a vulvo-vaginitis clinic. A pH strip (pH range 4-7, Merck) was brought in contact with the vaginal fluid on the sampling device and on the glass slide after adding one droplet of saline and performing microscopy by two different people unaware of the microscopy results of the clinical exam. Values were compared by Fisher exact and Student's t-tests.

Results: pH measurement from microscopy slides after the addition of saline causes systematic increases of pH leading to false positive readings. This is true for all types of disturbance of the flora and infections studied, and was seen in the abnormal as well as in the normal or intermediate pH range.

Conclusion: Vaginal pH should be measured by bringing the pH strip in direct contact with fresh vaginal fluid without first adding saline.