Related Experiment Video
Updated: Jul 24, 2026

EPR Monitored Redox Titration of the Cofactors of Saccharomyces cerevisiae Nar1
Published on: November 26, 2014
What does the phenol red thread test actually measure?
A Tomlinson1, K J Blades, E I Pearce
1Department of Vision Sciences, Glasgow Caledonian University, United Kingdom. A.Tomlinson@gcal.ac.uk
Purpose:
This study attempts to resolve whether the phenol red thread test (PRT) is a test of tear volume or tear production through comparisons with other techniques.
Methods:
Twenty asymptomatic subjects (10 men and 10 women; average age 30.6 +/- 10.8 years) had PRT (Zone Quick, Menicon) results compared with tear turnover rate (by fluorophotometry; Fluorotron Master, OcuMetrics) and tear volumes (from tear meniscus height and back extrapolation from fluorometric data).
Results:
PRT wetting was not correlated with either tear turnover or volume (by fluorophotometry or tear meniscus height) on a Pearson product moment correlation test (p > 0.05).
Conclusions:
No clear experimental evidence in favor of the PRT being a measure of tear production or volume was found. It is probable that the PRT measures uptake of a (small) amount of fluid residing in the eye, stimulates a low degree of reflex tearing, and reflects the absorption characteristics of the thread dependent on the biophysics or composition of tears.
Related Concept Videos
Indicators
Oxidation of Phenols to Quinones
o-hydroxy phenols are oxidized to o-quinones and p-hydroxy phenols to p-quinones. Such redox reactions involve the transfer of two electrons and two protons. The reversible redox property is crucial in...
Fixation and Sectioning
The simplest type of preparation is the wet mount, in which the specimen is placed in a drop of liquid on the slide. A liquid specimen can be directly deposited on the slide using a dropper. Solid specimens, such as skin scraping, can be placed on the slide before adding a drop of liquid to prepare the wet mount. Sometimes the liquid is simply water, but stains are often added...
Redox Titration: Overview
Redox Titration: Iodimetry and Iodometry
Differential Staining Technique

