By Gregg Russell, OD, FAAO (Dipl) and
Ernie Bowling, OD, MS, FAAO (Dipl)
Dry eye is one of the more frustrating ocular problems that patients endure, with symptoms that can include foreign body sensation, redness, light sensitivity, excessive watering and variable vision. Given the wide-ranging symptoms and the challenges of instituting an effective therapeutic regimen, it's no coincidence that dry eye is also frustrating for eyecare practitioners to treat. In this article, we discuss one of the tools we use to aid our diagnosis, the phenol red thread test (Zone Quick, Menicon Co., Ltd.)
When a patient's history and clinical examination point to dry eye, our next step is to assess tear volume. Most practitioners are familiar with Schirmer's tear test in which we place a strip of filter paper inside the lower eyelid, have the patient close his eyes and then wait 5 minutes before measuring the moisture on the strip. Schirmer's test can be performed with or without anesthetic, which sometimes creates confusion about how to interpret the results. Studies have shown results with Schirmer's test are not always accurate and repeatable.1,2
The phenol red thread (PRT) test is a simpler, more ergonomically friendly test. It uses a thread impregnated with phenol red dye. The pH of the tear film turns the dye from yellow to orange and helps establish the amount of basal tear production. Among the advantages of the PRT test are: a shorter test time, ease of use, greater reliability and no need for topical anesthetic.3,4
Before administering the PRT test, we recommend drying the inferior meniscus, as this has been shown to improve reproducibility.5 Then place a thread along the lateral half of the bulbar conjunctiva. Fold a small portion of the test string and insert it between the globe and the tarsal conjunctiva of the lower lid. Ask the patient to blink normally while looking straight ahead. Because the test takes only 15 seconds, we recommend testing one eye at a time. Measure the thread from the edge of the lid to the end of the color change to determine the PRT test score and the patient's basal tear production (Figure 1). Less than 9 mm of wetting is considered an abnormal result.
Figure 1. To determine the PRT test score using Zone Quick, measure the thread from the edge of the lid to the end of the color change.
Studies have confirmed the PRT test is more accurate, more reproducible and better tolerated by patients when compared to Schirmer's test.6,7 Although some researchers have questioned the efficacy of the test,8 their study involved only a small number of healthy subjects, and we believe they missed the point of the test. Normal findings are not the goal of the test. The PRT test is designed to help identify individuals with poor tear volume.
Researchers have also compared results among men and women. One study of more than 1,000 patients in the United States and Japan found that wetting lengths for men were longer than those for women.9 In addition, researchers noted a general trend toward lower tear production with increased age of study participants, although the significance of the data was not as strong.
We have found it useful to show patients the test threads, explain the meaning of the thread color and relate the patient's test results to the established test values for reduced tear volume. There is something very powerful about seeing test results instead of simply hearing a diagnosis.
We tape the threads into the patient's chart for future reference. If we learn of a significant change in the patient's history, such as medications, job status or other pertinent health events that can affect tear production and quality, we repeat the test.
The PRT test is a useful and easily employed clinical test for determining basal tear secretion. Numerous studies tout its viability, and our personal clinical experiences have borne out these data. We believe the test is a useful tool for identifying individuals with poor tear volumes and quantifying the problem. Viewed in the context of a comprehensive dry eye evaluation, the PRT test gives us real data and a visual aid for our patients, enabling us to educate our patients about their disease and our plan for managing it.Continued in page 6