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

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.

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.