What does the phenol red thread test actually measure?

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. 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). 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). 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.