Introduction

The Phenol Red Thread Test (PRT) is a quick, non-invasive, relatively comfortable test that grossly measures tear volume, and, as such, is a tool to consider in any dry eye workup.

Indications

Patient complaint of dry eyes

Contra-Indications

Allergy to phenol red dye

Materials

Phenol Red Thread Test Package (contains two strips)

Setup

1. Open the package, and remove one of the two strips.
2. Slightly pull down the lower lid and hook the thread (it comes pre-bent) over the lower lid margin, approximately 1/3 of the way from the lateral canthus.
3. Immediately begin timing.  After 15 seconds, pull the lower lid down and remove the thread by moving it slightly upward and away from the eye.
4. Repeat with the other eye
5. After each eye, measure and record (in mm) the amount of thread that has changed color

Results

A result of 0 mm to 10 mm may indicate a tear film deficiency

A result greater than 10 mm is considered to be normal

References

Zone Quick Phenol Red Thread (PRT) Tear Test Package Insert