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Correlation and reliability of the Zone-Quick Phenol Red Thread Tear Test to dry eye symptoms

1 May 2000


Purpose: In the past, optometry has relied on such tests as the Schirmer Test to diagnose dry eye syndrome. New methods such as the Zone-Quick Phenol Red Thread Test (PRTT) have been developed to identify dry eye patients. This project is designed to test the correlation of the PRTT to dry eye symptoms as well as test/retest reliability.

Methods: Of 120 randomly chosen young, pathology free optometry students, 54 test subjects (108 eyes) were placed into either a control group or a symptomatic group based upon their responses to a detailed dry eye questionnaire. Both eyes were tested according to PRTT protocol, resulting in a wet thread length measurement in mm. Measurements of less than 10 mm were considered a "dry" result, while lengths 20 nun or greater were considered "normal."

Results: A statistically significant difference (p=0.003) in mean wet thread length was found between the two groups. Based on mean wet thread lengths, all 27 subjects in the control group were classified as "normal" by the PRTT, while only 2 of 27 subjects in the symptomatic group were correctly classified as "'dry." Intra-trial and inter-trial correlation testing was variable, ranging from r=0.19 to r=0.76.

Conclusions: The results of this study suggest that the PRTT may be inadequate as a diagnostic test for dry eye syndrome in the normal population. Statistical analysis revealed poor repeatability between trials and inconsistent correlation between eyes. The gold standard for accurate diagnosis of dry eye syndrome remains a detailed questionnaire combined with extensive case history.


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