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.