What is the test time for the Phenol red thread test?

• A. 10 seconds
• B. 15 seconds
• C. 20 seconds
• D. 30 seconds

Answer: (B)

Phenol red thread test measures tear production by how far the tear-wetted portion of a small color-changing thread turns pink during a fixed exposure time. That exposure is standardized to 15 seconds, which provides a reliable and reproducible reading without provoking reflex tearing from excessive contact or discomfort. A shorter duration, like 10 seconds, would likely yield a smaller color change and underestimate tear production. A longer duration, such as 20 or 30 seconds, increases the chance of reflex tearing or evaporation skewing the result. Thus, the test time is 15 seconds.