pH is a measure of the acid-base balance of whatever substance is being tested.
Vaginal secretions are normally slightly acid (pH <5.0).
In testing for the presence of amniotic fluid (during pregnancy, to rule in/out ruptured membranes), the more basic amniotic fluid will turn nitrazine paper a dark blue, indicating a pH of >6.0, and confirming ruptured membranes.
In testing for the presence of bacterial vaginosis, vaginal infections with anaerobic bacteria are often associated with a pH >5.0.
