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What does the Phenol Red Thread Test Actually Measure?

Tomlinson, Alan DSc, FCOptom, FAAO; Blades, Kenneth J. PhD; Pearce, E. Ian PhD

Optometry and Vision Science: March 2001 - Volume 78 - Issue 3 - p 142-146
Original Article

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.

Department of Vision Sciences, Glasgow Caledonian University, Glasgow, United Kingdom

Alan Tomlinson

Department of Vision Sciences

Glasgow Caledonian University

Cowcaddens Road

Glasgow G4 0BA

United Kingdom

e-mail: A.Tomlinson@gcal.ac.uk

Attempts to develop a simple, clinical test for the measurement of the adequacy of tear secretion date back almost 100 years. 1 Initial attempts used the absorption of tears by a filter paper strip placed in the lower fornix 2,3 for periods of up to 5 min (Schirmer test). Attempts to standardize this test 3,4 have not been entirely successful because the test suffers from high variability, poor reproducibility, 5–10 and low sensitivity for the diagnosis of dry eye. 10–14 The Schirmer test does measure reflex tear production and (possibly) the residual volume of tears in the eye, but has an significant inability to measure basal secretion rate even with the use of local anesthesia. 6,15

In 1975, Kurihashi and co-workers 5,16 introduced a new method using fine cotton threads instead of Schirmer paper strips. This method was refined by Hamano et al., 10 who used a thread impregnated with phenol red, which is pH sensitive and changes from yellow to red over the pH range of normal tears; this indicator helps to identify the length of cotton thread wetted by tears. The phenol red thread test (PRT) still requires the thread to be hooked over the lower lid, but is completed in a much shorter time (15 s) with significantly less stimulation of reflex tear production 10 than for the Schirmer test. For these reasons, it said to offer the ability to measure basal tear secretion rates. 17 This new test was found to have high repeatability 18,19 on the same groups of subjects. The absorption of tear fluid within the thread depends on the form of the cotton thread. 20

Cho and Kwong 20 reported a difference in wetting from their own cotton thread test and the PRT (Zone Quick, Showa Yakuhin Kako, Japan). Sakamoto et al. 17 found variations in measurements between different ethnic populations. The test, however, is sufficiently noninvasive to avoid tear instability during application 21 and therefore does not interfere with measurements of noninvasive tear break-up times. Initially conceived as a test for tear secretion (and probably basal secretion), recent suggestions are that the PRT measures the volume of tears in the eye and/or the residual tears located in the inferior conjunctival sac. 17,21,22 Cho and Yap 22 quote Lamberts 23 and state that the PRT measures residual volume in the inferior conjunctival sac. In a subsequent paper, Cho et al. 21 report an argument originally advanced by Sakamoto et al. 17 that because of the short test time, PRT is a measure of tear volume. No empirical support is offered for the suggestions concerning the aspect of tear physiology measured by the PRT. In this study, we attempt to resolve the question of whether the PRT measures tear secretion or volume through a comparison of the results of the PRT with those obtained for tear production and volume from fluorophotometry and by estimates of tear volume through measurement of the tear meniscus height at the inferior lid margin in a normal population.

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METHODS

Subjects

Twenty asymptomatic subjects with no history of dry eye or recent contact lens wear (10 males and 10 females; average age 30.6 ± 10.8 years) were recruited. All procedures were approved by the Ethics Committee of Glasgow Caledonian University, and all subjects gave signed, informed consent.

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Techniques

The measurements were taken in the following order on the left eye of each subject:

  • The wet length of a tear PRT (Zone Quick, Menicon) was recorded after 15 s with the thread hooked over the inferior lid in the temporal third of the eye. A single measurement was taken rather than an average of several measurements because work in our laboratory (E. I. Pearce, unpublished data) has shown no significant difference between multiple measurements taken during one measurement session.
  • Tear volume was assessed by a slitlamp measurement of tear meniscus height (TMH). 24 Three measurements were taken, and the average was recorded.
  • Tear secretion was measured from the rate of decay of fluorescence of 1 μl of 1% (w/v) fluorescein sodium instilled into the lower conjunctival sac (tear turnover rate). The rate of decay of fluorescence was measured by an automated scanning fluorophotometer (Fluorotron Master, OcuMetrics, Mountain View, CA) equipped with an anterior segment adapter. 25 The technique was modified to incorporate consistent blinking of the subject during measurements. 26 The rate of decay of fluorescence was obtained from the results of measurements taken at 1-min intervals over a 20-min period; the data were fitted with a curve using the “ANT-SEGMENT tear” software by van Best et al. 27 to give a single value of tear turnover.

Tear volume was calculated from the fluorophotometric data by back extrapolation to times zero, using the method described by Kuppens et al. 28

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RESULTS

The measurements of tear thread wetting by PRT, tear production (tear turnover rate), and tear volume by TMH and from fluorophotometric data can be seen in Table 1. All of the measured parameters were normally distributed (Shapiro-Wilk test, p > 0.05). The average value for PRT wetting in 15 s was 17.3 ± 6.7 mm, for tear turnover rate 21.4% ± 11.1% per minute was recorded; tear volume from tear meniscus height averaged 0.35 ± 0.11 mm, and tear volume as measured by fluorophotometry averaged 6.9 ± 1.5 μl.

Table 1

Table 1

The results obtained with the PRT were compared with those obtained by other techniques using the Pearson product moment correlation test. The PRT wetting in 15 s was not significantly correlated with either TMH, tear volume, or tear turnover rate (p > 0.05) (Figs. 1 through 3. Tear meniscus height did not correlate with tear volume by fluorophotometry (p > 0.05). The only significant correlation found in this study was between tear meniscus height and age (Table 2).

FIGURE 1

FIGURE 1

FIGURE 2

FIGURE 2

FIGURE 3

FIGURE 3

Table 2

Table 2

A correlation test was chosen for the analysis of data in this study rather than the alternative statistical test of agreement between methods of clinical measurement suggested by Altman and Bland. 29 The latter technique was not appropriate for this study because the measurements of the same parameter obtained by the different techniques gave values in different (not interconvertible) units. In view of the adoption of the “correlation” approach in this study, the study strictly evaluated “association” rather than “agreement” between the measurement techniques. 29

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DISCUSSION

This experiment failed to identify a significant correlate with PRT wetting. Therefore it does not offer a clear, empirical resolution of the question of what the PRT actually measures through comparison with other accepted methods of assessment of tear volume or production. Others have also not obtained a correlation between tear meniscus height and an absorption test for tear production (Schirmer test 30) in normal subjects. Scherz et al. 31 found a correlation with meniscus height and volume obtained by a fluorometer technique in normal subjects and dry eye patients, but not with the Schirmer test and tear volume. Meniscus height and kinetic Schirmer results are correlated in dry eye patients. 32 Certainly the potential for correlation is greater with a wider range of values offered by the inclusion of a dry eye group. Work on normal subjects, however, may have been expected to yield significance if a true relation existed if a large enough range of values of the parameters were included (in this study, PRT varied from 8 to 30 mm, TMH from 0.19 to 0.68 mm, and tear turnover rate from 5.75% to 52.78% per minute). A previous study incorporating dry eye patients and normal subjects did find a correlation between PRT and TMH. 33 However, their photographic technique for measuring TMH required installation of fluorescein into the eye from a strip moistened with 10 μl of saline. The addition of all, or part of, this extra fluid into an eye with between 2.74 and 7 μl 34 of preexisting tears is likely to swell the tear meniscus (80% of fluid in the eye is at this location 33) and to precipitate reflex tearing, thus rendering their results different and not comparable to this study.

The present study offers no clear evidence that the phenol red thread test measures tear volume. The comparison of the PRT wetting with volume determined with previously established methods, tear meniscus height or fluorophotometry, yields no significant correlation. It is perhaps significant that the latter two volume measures do not correlate (Table 2). Clearly these existing tests measure different aspects of tears by “meniscus swelling” (TMH) and by calculation of the tear dilution based on volume assumptions (fluorophotometric technique). Neither correlate with the “amount of residual tears” in the eye as indicated by PRT wetting length. The presumption that PRT wetting length should provide a measure of tear volume or residual tears in the lower conjunctival sac needs to be reexamined. The volume of tears in young subjects averages 7 μl. 29 The average absorption of tears by phenol red thread in 15 s has been shown to be just >0.5 μl. 20 Therefore, the proportion of total tears resident in the eye and absorbed by the phenol red thread is about 7.5%. Therefore, full elimination of tears from the eye and a consequent assessment of tear volume (or residuals) is unlikely to occur with the PRT.

It should also reassessed whether the PRT actually measures tear secretion and whether it is basal or reflex. Several studies have shown that the PRT allows a clinical differentiation of dry eye 35–37 and that the test has the ability to discriminate between an eye producing normal tears and an eye that is aqueous deficient. 35 But Holly et al. 4 defined the requirement for any strip (or thread) test that claims to measure secretion from the eye to have an absorption capacity (of the thread or strip) that exceeds the secretion (wetting) rate of the eye.

Cho and Kwong 20 have shown that the average absorption rate of the PRT in 15 s (in normal young individuals) is 0.52 μl; it can be calculated that the range of absorption of the PRT thread recorded for subjects in this study in 15 s was between 0.24 and 0.91 μl. Previous measurements of secretion rate in this time period have varied between 0.13 and 0.55 μl 34 for basal rate and between 1.4 and 10.1 μl for reflex rate. 4 Therefore the PRT could meet the criterion of Holly et al. 4 for a test to measure basal secretion rates, but it is extremely unlikely to be sufficient in its absorptive capacity to measure reflex secretion. Therefore, in view of the fact that the PRT cannot absorb all of the fluid in the eye, what drives secretion during this test? It is possible that secretion of tear fluid in the eye during the test is driven by the constancy of basal production along with a low-grade stimulus to reflex secretion as a result of thread insertion.

The results of this study suggest that the PRT is unlikely to measure tear volume of the eye or residual tears in the lower conjunctiva. It is more likely that it measures uptake of an (small) amount of fluid residing in the eye and stimulates a low degree of reflex tearing. It is not possible to say categorically whether this reflects the whole of the basal and/or a (small) part of the reflex level of tear secretion. 38 Although the PRT is more comfortable for patients than the Schirmer test, it may not offer as valid a measurement of reflex tear facility.

Because the maximum absorption of the Schirmer strip is 11.4 μl/min, 4 this strip is capable of absorbing a greater volume of tears than the normal tear volume in the eye and the amount produced by basal and reflex secretion. 34 It is possible that the PRT reflects some other aspect of tears that allows differentiation between dry eye patients and normal subjects through different absorption characteristics of the thread depending on the biophysics or composition of tears in these two groups. It has previously been shown by one of us (KB) 39 that although the relationship between the rate of thread wetting and time is a function of the thread itself, the magnitude (length) of thread wetting achieved over time is dependent on the fluid being absorbed.

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ACKNOWLEDGMENT

Received August 3, 2000; revision received December 27, 2000.

Alan Tomlinson

Department of Vision Sciences

Glasgow Caledonian University

Cowcaddens Road

Glasgow G4 0BA

United Kingdom

mail: A.Tomlinson@gcal.ac.uk

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Keywords:

phenol red thread test,; tear volume,; secretion

© 2001 American Academy of Optometry