A normal Phenol red thread test is greater than how many millimeters after 15 seconds?

A normal Phenol red thread test is greater than how many millimeters after 15 seconds?

• A. > 10 mm
• B. > 6 mm
• C. > 8 mm
• D. > 12 mm

Answer: (A)

The Phenol Red Thread test measures how much tear fluid is produced by seeing how far tears wet a small thread in the lower eyelid over 15 seconds. The thread changes color as tears contact it, and you measure the wet length in millimeters. In a normal eye, the thread should wet more than 10 millimeters in 15 seconds, indicating adequate aqueous tear production. Values below this suggest reduced tear production and potential dry eye, with lower thresholds (around 6–10 mm) often considered borderline and less than 6 mm indicating definite deficiency. So the normal cutoff after 15 seconds is greater than 10 mm.