A nitrazine test performed during pregnancy measures the pH of vaginal fluid to determine whether the membranes have ruptured, although a positive test is not conclusive. The pH level could be affected by blood in the mucous or bacterial vaginosis, for example, rather than amniotic fluid caused by ruptured membranes.
Other factors that can affect the outcome of a nitrazine test include semen in the vagina, alkaline urine and soap, all of which could place the pH in the range that indicates amniotic fluid. In addition, if the membranes ruptured longer than 24 hours before the test, or if only a small amount of amniotic fluid has leaked, the test could produce a false negative result. There is a 5 percent rate of false positive results using nitrazine tests, and 1 percent rate of false negative results.
Nitrazine tests are administered by placing a few drops of collected vaginal fluid onto a strip of paper that has been treated with nitrazine dye. The color of the paper strip changes depending on the pH of the sample. The color is then matched on a chart that determines the corresponding pH of the sample. Results of the test can be read immediately.