Reliability of Schirmer and Phenol Red Thread test value on dry eye patients

Background: The repeatability of the Schirmer test (ST) and Phenol Red Thread test (PRT) has been investigated thoroughly on patients with normal tear function; however, it is limited to dry eye patients. Aim: This study aimed to compare the reliability of the ST and PRT on young adults with dry eyes. Setting: University of KwaZulu-Natal Eye Clinic. Methods: Forty-eight young adults (mean age 20.65 ± 1.71 years) participated in the study. Preliminary tests excluded dry eye-related abnormalities. Novesin (0.4%) was instilled into the right eye of each participant, followed by the administration of the ST and PRT, 5 min apart, then re-administered after a 10-min interval. The retest was conducted 1 week later with the PRT being performed first to minimise bias. Results: The intra-session correlation of the ST and the PRT was poor (ICC < 0.5) and the difference was statistically significant (P < 0.05). The inter-session correlation for the PRT was poor (ICC = 0.432) while the ST was moderate (ICC = 0.567) with no statistical difference observed for all parameters (P < 0.05). The Bland–Altman plots for intra- and inter-session indicate that 96% of observations fell within the 95% limits of agreement. Conclusion: The ST and PRT demonstrated good intra-session and inter-session reliability, with ST showing better inter-session agreement than the PRT. Contribution: The ST and the PRT provide consistent and reproducible results on dry eye patients when conducted on the same individuals at different time points.