Phenol red thread test is used for dry eye:

Ans: A. In the test, volume of tears is measured as it changes color on contact with tears(Ref: Yanoff & Duker 4/e p277)Phenol red dye test measures the production of tears without topical anesthesia, as the dye changes its color to red on contact with tears.It doesn't require pH ureter for reading the result.Phenol red is a pH indicator used in cell biopsy as it changes colour from yellow to red on pH range from 6.8 to 8.2.Used to measure residual tears in inferior conjunctival sac, especially in dry eye patients.